The evolution of plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s has resulted in increasing levels of complexity

Evolutionary grade

In alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.-Definition:...

, from the earliest algal mat

Algal mat

An algal mat is a layer of usually filamentous algae on marine or fresh water soft bottoms. It may be considered one of many types of microbial mats. Algae and cyanobacteria are ubiquitous, often forming within the water column and settling to the bottom. In shallow environments, they are often...

s, through bryophyte

Bryophyte

Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

s, lycopods, fern

Fern

A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...

s to the complex gymnosperm

Gymnosperm

The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos , meaning "naked seeds", after the unenclosed condition of their seeds...

s and angiosperms of today. While the groups which appeared earlier continue to thrive, especially in the environments in which they evolved, each new grade of organisation has eventually become more "successful" than its predecessors by most measures.

Probably an algal scum formed on land . In the Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 period, around , the first land plants appeared. These began to diversify in the late Silurian Period, around , and the results of their diversification are displayed in remarkable detail in an early Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 fossil assemblage from the Rhynie chert

Rhynie chert

The Rhynie chert is an Early Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness . It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away...

. This chert preserved early plants in cellular detail, petrified in volcanic springs.

By the middle of the Devonian Period most of the features recognised in plants today are present, including roots, leaves and secondary wood, and by late Devonian times seeds had evolved. Late Devonian plants had thereby reached a degree of sophistication that allowed them to form forests of tall trees.

Evolutionary innovation continued after the Devonian period. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the evolution of flowering plants in the Triassic (~), which exploded in the Cretaceous and Tertiary. The latest major group of plants to evolve were the grasses, which became important in the mid Tertiary, from around . The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low and warm, dry conditions of the tropics over the last .

Colonization of land

Land plants evolved from chlorophyte algae, perhaps as early as ; some molecular estimates place their origin even earlier, as much as . Their closest living relatives are the charophytes, specifically Charales

Charales

Charales is an order of pondweeds, freshwater algae in the division Charophyta. They are green plants believed to be the closest relatives of the green land plants. Linnaeus established the genus Chara in 1753.-Description:...

; assuming that the Charales' habit has changed little since the divergence of lineages, this means that the land plants evolved from a branched, filamentous, alga, dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools. The alga would have had a haplontic life cycle: it would only very briefly have had paired chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s (the diploid condition) when the egg

Ovum

An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

 and sperm

Sperm

The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

 first fused to form a zygote

Zygote

A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

; this would have immediately divided by meiosis

Meiosis

Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

 to produce cells with half the number of unpaired chromosomes (the haploid condition). Co-operative interactions

Symbiosis

Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 with fungi may have helped early plants adapt to the stresses of the terrestrial realm.

Plants were not the first photosynthesisers on land, though: consideration of weathering rates suggests that organisms were already living on the land , and microbial fossils have been found in freshwater lake deposits from , but the carbon isotope record suggests that they were too scarce to impact the atmospheric composition until around . These organisms, although phylogenetically diverse, were probably small and simple, forming little more than an "algal scum".

The first evidence of plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s on land comes from spores of Mid-Ordovician age (early Llanvirn, ~). These spores, known as cryptospores, were produced either singly (monads), in pairs (diads) or groups of four (tetrads), and their microstructure resembles that of modern liverwort

Marchantiophyta

The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....

 spores, suggesting they share an equivalent grade of organisation. They are composed of sporopollenin – further evidence of an embryophytic affinity. It could be that atmospheric 'poisoning' prevented eukaryotes from colonising the land prior to this, or it could simply have taken a great time for the necessary complexity to evolve.

Trilete spores similar to those of vascular plants appear soon afterwards, in Upper Ordovician rocks. Depending exactly when the tetrad splits, each of the four spores may bear a "trilete mark", a -shape, reflecting the points at which each cell squashed up against its neighbours. However, this requires that the spore walls be sturdy and resistant at an early stage. This resistance is closely associated with having a desiccation-resistant outer wall—a trait only of use when spores must survive out of water. Indeed, even those embryophytes that have returned to the water lack a resistant wall, thus don't bear trilete marks. A close examination of algal spores shows that none have trilete spores, either because their walls are not resistant enough, or in those rare cases where it is, the spores disperse before they are squashed enough to develop the mark, or don't fit into a tetrahedral tetrad.

The earliest megafossils of land plants were thalloid organisms, which dwelt in fluvial wetlands and are found to have covered most of an early Silurian flood plain. They could only survive when the land was waterlogged. There were also microbial mats.

Once plants had reached the land, there were two approaches to dealing with desiccation. The bryophytes avoid it or give in to it, restricting their ranges to moist settings, or drying out and putting their metabolism "on hold" until more water arrives. Tracheophytes resist desiccation. They all bear a waterproof outer cuticle layer wherever they are exposed to air (as do some bryophytes), to reduce water loss, but—since a total covering would cut them off from in the atmosphere—they rapidly evolved stomata, small openings to allow gas exchange. Tracheophytes also developed vascular tissue to aid in the movement of water within the organisms (see below), and moved away from a gametophyte dominated life cycle (see below). Vascular tissue also facilitated upright growth without the support of water and paved the way for the evolution of larger plants on land.

The establishment of a land-based flora caused increased accumulation of oxygen in the atmosphere, as the plants produced oxygen as a waste product. When this concentration rose above 13%, wildfires became possible. This is first recorded in the early Silurian fossil record by charcoalified plant fossils. Apart from a controversial gap in the Late Devonian, charcoal is present ever since.

Charcoalification is an important taphonomic

Taphonomy

Taphonomy is the study of decaying organisms over time and how they become fossilized . The term taphonomy was introduced to paleontology in 1940 by Russian scientist Ivan Efremov to describe the study of the transition of remains, parts, or products of organisms, from the biosphere, to the...

 mode. Wildfire drives off the volatile compounds, leaving only a shell of pure carbon. This is not a viable food source for herbivores or detritovores, so is prone to preservation; it is also robust, so can withstand pressure and display exquisite, sometimes sub-cellular, detail.

Evolution of life cycles

All multicellular plants have a life cycle comprising two generations or phases. One is termed the gametophyte

Gametophyte

A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

, has a single set of chromosomes (denoted 1N), and produces gametes (sperm and eggs). The other is termed the sporophyte

Sporophyte

All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...

, has paired chromosomes (denoted 2N), and produces spores. The gametophyte and sporophyte may appear identical – homomorphy – or may be very different – heteromorphy.

The pattern in plant evolution has been a shift from homomorphy to heteromorphy. The algal ancestors to land plants were almost certainly , being haploid for all their life cycles, with a unicellular zygote providing the 2N stage. All land plants (i.e. embryophytes) are – that is, both the haploid and diploid stages are multicellular. Two trends are apparent: bryophyte

Bryophyte

Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

s (liverwort

Marchantiophyta

The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....

s, moss

Moss

Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...

es and hornwort

Hornwort

Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found worldwide,...

s) have developed the gametophyte, with the sporophyte becoming almost entirely dependent on it; vascular plant

Vascular plant

Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...

s have developed the sporophyte, with the gametophyte being particularly reduced in the seed plants.

There are two competing theories to explain the appearance of a diplobiontic lifecycle.

The interpolation theory (also known as the antithetic or intercalary theory) holds that the sporophyte phase was a fundamentally new invention, caused by the mitotic division of a freshly germinated zygote, continuing until meiosis produces spores. This theory implies that the first sporophytes bore a very different morphology that the gametophyte they depended on. This seems to fit well with what we know of the bryophytes, in which a vegetative thalloid gametophyte is parasitised by simple sporophytes, which often comprise no more than a sporangium on a stalk. Increasing complexity of the ancestrally simple sporophyte, including the eventual acquisition of photosynthetic cells, would free it from its dependence on a gametophyte, as we see in some hornworts (Anthoceros

Anthoceros

Anthoceros is a genus of hornworts in the family Anthocerotaceae. The genus is global in its distribution. Its name means 'flower horn', and refers to the characteristic horn-shaped sporophytes that all hornworts produce...

), and eventually result in the sporophyte developing organs and vascular tissue, and becoming the dominant phase, as in the tracheophytes (vascular plants). This theory may be supported by observations that smaller Cooksonia

Cooksonia

Cooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian ; the group continues to be an important component of the flora until the Early Devonian, a total time span of...

individuals must have been supported by a gametophyte generation. The observed appearance of larger axial sizes, with room for photosynthetic tissue and thus self-sustainability, provides a possible route for the development of a self-sufficient sporophyte phase.

The alternative hypothesis is termed the transformation theory (or homologous theory). This posits that the sporophyte appeared suddenly by a delay in the occurrence of meiosis after the zygote germinated. Since the same genetic material would be employed, the haploid and diploid phases would look the same. This explains the behaviour of some algae, which produce alternating phases of identical sporophytes and gametophytes. Subsequent adaption to the desiccating land environment, which makes sexual reproduction difficult, would result in the simplification of the sexually active gametophyte, and elaboration of the sporophyte phase to better disperse the waterproof spores. The tissue of sporophytes and gametophytes preserved in the Rhynie chert is of similar complexity, which is taken to support this hypothesis.

Xylem

To photosynthesise, plants must absorb from the atmosphere. However, this comes at a price: while stomata are open to allow to enter, water can evaporate. Water is lost much faster than is absorbed, so plants need to replace it, and have developed systems to transport water from the moist soil to the site of photosynthesis. Early plants sucked water between the walls of their cells, then evolved the ability to control water loss (and acquisition) through the use of stomata. Specialised water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels.

The high levels of Silurian-Devonian times, when plants were first colonising land, meant that the need for water was relatively low . As was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant transport mechanisms evolved. As water transport mechanisms, and waterproof cuticles, evolved, plants could survive without being continually covered by a film of water. This transition from poikilohydry

Poikilohydry

Poikilohydry is a condition in organisms that lack a mechanism to prevent desiccation, such as the lichens and bryophytes. The structures supporting poikilohydry are typically small enough to reside in the diffusion boundary layer, where the cuticle or stomata do not have a sufficient guarding...

 to homoiohydry opened up new potential for colonisation. Plants then needed a robust internal structure that held long narrow channels for transporting water from the soil to all the different parts of the above-soil plant, especially to the parts where photosynthesis occurred.

During the Silurian, was readily available, so little water needed expending to acquire it. By the end of the Carboniferous, when levels had lowered to something approaching today's, around 17 times more water was lost per unit of uptake. However, even in these "easy" early days, water was at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation. This early water transport took advantage of the cohesion-tension mechanism inherent in water. Water has a tendency to diffuse to areas that are drier, and this process is accelerated when water can be wick

Capillary action

Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

ed along a fabric with small spaces. In small passages, such as that between the plant cell walls (or in tracheids), a column of water behaves like rubber – when molecules evaporate from one end, they literally pull the molecules behind them along the channels. Therefore transpiration alone provided the driving force for water transport in early plants. However, without dedicated transport vessels, the cohesion-tension mechanism cannot transport water more than about 2 cm, severely limiting the size of the earliest plants. This process demands a steady supply of water from one end, to maintain the chains; to avoid exhausing it, plants developed a waterproof cuticle

Plant cuticle

Plant cuticles are a protective waxy covering produced only by the epidermal cells of leaves, young shoots and all other aerial plant organs without periderm...

. Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue. However, dehydration at times was inevitable; early plants cope with this by having a lot of water stored between their cell walls, and when it comes to it sticking out the tough times by putting life "on hold" until more water is supplied.

To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system. During the early Silurian, they developed specialized cells, which were lignified

Lignin

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

 (or bore similar chemical compounds) to avoid implosion; this process coincided with cell death, allowing their innards to be emptied and water to be passed through them. These wider, dead, empty cells were a million times more conductive than the inter-cell method, giving the potential for transport over longer distances, and higher diffusion rates.

The first macrofossils to bear water-transport tubes in situ are the early Devonian pretracheophytes Aglaophyton

Aglaophyton

Aglaophyton major was the sporophyte generation of a diplohaplontic, pre-vascular, axial, free-sporing land plant of the Lower Devonian . It had anatomical features intermediate between those of the bryophytes and vascular plants or tracheophytes.A. major was first described by Kidston and Lang in...

and Horneophyton

Horneophyton

Horneophyton, a member of the Horneophytopsida, was an early plant which may form a "missing link" between the hornworts and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert.-Description:...

, which have structures very similar to the hydroid

Hydroid

-Marine biology:Hydroids are a life stage for most animals of class Hydrozoa, small predators related to jellyfish.-Botany:In mosses, hydroids form the innermost layer of the stem of long, colourless, thin walled cells of small diameter.The cells are dead and lack protoplasm.They function as water...

s of modern mosses.
Plants continued to innovate new ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport. Bands on the walls of tubes, in fact apparent from the early Silurian onwards, are an early improvisation to aid the easy flow of water. Banded tubes, as well as tubes with pitted ornamentation on their walls, were lignified and, when they form single celled conduits, are considered to be tracheids. These, the "next generation" of transport cell design, have a more rigid structure than hydroids, allowing them to cope with higher levels of water pressure. Tracheids may have a single evolutionary origin, possibly within the hornworts, uniting all tracheophytes (but they may have evolved more than once).

Water transport requires regulation, and dynamic control is provided by stoma

Stoma

In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

ta.
By adjusting the amount of gas exchange, they can restrict the amount of water lost through transpiration. This is an important role where water supply is not constant, and indeed stomata appear to have evolved before tracheids, being present in the non-vascular hornworts.

An endodermis

Endodermis

The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic substances to restrict apoplastic flow of water to the inside...

 probably evolved during the Silu-Devonian, but the first fossil evidence for such a structure is Carboniferous. This structure in the roots covers the water transport tissue and regulates ion exchange (and prevents unwanted pathogens etc. from entering the water transport system). The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver.

Once plants had evolved this level of controlled water transport, they were truly homoiohydric, able to extract water from their environment through root-like organs rather than relying on a film of surface moisture, enabling them to grow to much greater size. As a result of their independence from their surroundings, they lost their ability to survive desiccation – a costly trait to retain.

During the Devonian, maximum xylem diameter increased with time, with the minimum diameter remaining pretty constant. By the middle Devonian, the tracheid diameter of some plant lineages had plateaued. Wider tracheids allow water to be transported faster, but the overall transport rate depends also on the overall cross-sectional area of the xylem bundle itself. The increase in vascular bundle thickness further seems to correlate with the width of plant axes, and plant height; it is also closely related to the appearance of leaves and increased stomatal density, both of which would increase the demand for water.

While wider tracheids with robust walls make it possible to achieve higher water transport pressures, this increases the problem of cavitation. Cavitation occurs when a bubble of air forms within a vessel, breaking the bonds between chains of water molecules and preventing them from pulling more water up with their cohesive tension. A tracheid, once cavitated, cannot have its embolism removed and return to service (except in a few advanced angiosperms which have developed a mechanism of doing so). Therefore it is well worth plants' while to avoid cavitation occurring. For this reason, pits in tracheid walls have very small diameters, to prevent air entering and allowing bubbles to nucleate. Freeze-thaw cycles are a major cause of cavitation. Damage to a tracheid's wall almost inevitably leads to air leaking in and cavitation, hence the importance of many tracheids working in parallel.

Cavitation is hard to avoid, but once it has occurred plants have a range of mechanisms to contain the damage.
Small pits link adjacent conduits to allow fluid to flow between them, but not air – although ironically these pits, which prevent the spread of embolisms, are also a major cause of them. These pitted surfaces further reduce the flow of water through the xylem by as much as 30%. Conifers, by the Jurassic, developed an ingenious improvement, using valve-like structures to isolate cavitated elements. These torus-margo structures have a blob floating in the middle of a donut; when one side depressurises the blob is sucked into the torus and blocks further flow. Other plants simply accept cavitation; for instance, oaks grow a ring of wide vessels at the start of each spring, none of which survive the winter frosts. Maples use root pressure each spring to force sap upwards from the roots, squeezing out any air bubbles.

Growing to height also employed another trait of tracheids – the support offered by their lignified walls. Defunct tracheids were retained to form a strong, woody stem, produced in most instances by a secondary xylem. However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough sclerenchyma on the outer rim of the stems. Even when tracheids do take a structural role, they are supported by sclerenchymatic tissue.

Tracheids end with walls, which impose a great deal of resistance on flow; vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel. The function of end walls, which were the default state in the Devonian, was probably to avoid embolism

Embolism

In medicine, an embolism is the event of lodging of an embolus into a narrow capillary vessel of an arterial bed which causes a blockage in a distant part of the body.Embolization is...

s. An embolism is where an air bubble is created in a tracheid. This may happen as a result of freezing, or by gases dissolving out of solution. Once an embolism is formed, it usually cannot be removed (but see later); the affected cell cannot pull water up, and is rendered useless.

End walls excluded, the tracheids of prevascular plants were able to operate under the same hydraulic conductivity as those of the first vascular plant, Cooksonia.

The size of tracheids is limited as they comprise a single cell; this limits their length, which in turn limits their maximum useful diameter to 80 μm. Conductivity grows with the fourth power of diameter, so increased diameter has huge rewards; vessel elements, consisting of a number of cells, joined at their ends, overcame this limit and allowed larger tubes to form, reaching diameters of up to 500 μm, and lengths of up to 10 m.

Vessels first evolved during the dry, low periods of the late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in the mid Cretaceous in angiosperms and gnetophytes.
Vessels allow the same cross-sectional area of wood to transport around a hundred times more water than tracheids! This allowed plants to fill more of their stems with structural fibres, and also opened a new niche to vines, which could transport water without being as thick as the tree they grew on. Despite these advantages, tracheid-based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation.

Leaves

Leaves

Leaves

-History:Vocalist Arnar Gudjonsson was formerly the guitarist with Mower, and he was joined by Hallur Hallsson , Arnar Ólafsson , Bjarni Grímsson , and Andri Ásgrímsson . Late in 2001 they played with Emiliana Torrini and drew early praise from the New York Times...

 today are, in almost all instances, an adaptation to increase the amount of sunlight that can be captured for photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. Leaves certainly evolved more than once, and probably originated as spiny outgrowths to protect early plants from herbivory.

Leaves are the primary photosynthetic organs of a plant. Based on their structure, they are classified into two types - microphylls, that lack complex venation patterns and megaphylls, that are large and with a complex venation. It has been proposed that these structures arose independently. Megaphylls, according to the Telome hypothesis, have evolved from plants that showed a three dimensional branching architecture, through three transformations—planation, which involved formation of a planar

Planar

In computer graphics, planar is the method of representing pixel colours with several bitplanes of RAM. Each bit in a bitplane is related to one pixel on the screen...

 architecture, webbing, or formation of the outgrowths between the planar branches and fusion, where these webbed outgrowths fused to form a proper leaf

Leaf

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

 lamina. All three steps happened multiple times in the evolution of today's leaves.

It has been proposed that the before the evolution of leaves

Leaves

-History:Vocalist Arnar Gudjonsson was formerly the guitarist with Mower, and he was joined by Hallur Hallsson , Arnar Ólafsson , Bjarni Grímsson , and Andri Ásgrímsson . Late in 2001 they played with Emiliana Torrini and drew early praise from the New York Times...

, plants had the photosynthetic apparatus on the stems. Today's megaphyll leaves probably became commonplace some 360mya, about 40my after the simple leafless plants had colonized the land in the early Devonian period. This spread has been linked to the fall in the atmospheric carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 concentrations in the Late Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 era associated with a rise in density of stomata on leaf surface. This must have allowed for better transpiration

Transpiration

Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...

 rates and gas exchange. Large leaves with less stomata would have gotten heated up in the sun's heat, but an increased stomatal density allowed for a better-cooled leaf, thus making its spread feasible.

The rhyniophytes of the Rhynie chert comprised nothing more than slender, unornamented axes. The early to middle Devonian trimerophytes, therefore, are the first evidence we have of anything that could be considered leafy. This group of vascular plants are recognisable by their masses of terminal sporangia, which adorn the ends of axes which may bifurcate or trifurcate. Some organisms, such as Psilophyton

Psilophyton

Psilophyton is a genus of extinct vascular plants. Described in 1859, it was one of the first fossil plants to be found which was of Devonian age . Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China...

, bore s. These are small, spiny outgrowths of the stem, lacking their own vascular supply.

Around the same time, the zosterophyllophytes were becoming important. This group is recognisable by their kidney-shaped sporangia, which grew on short lateral branches close to the main axes. They sometimes branched in a distinctive H-shape. The majority of this group bore pronounced spines on their axes. However, none of these had a vascular trace, and the first evidence of vascularised enations occurs in the Rhynie genus Asteroxylon

Asteroxylon

Asteroxylon is an extinct genus of plants of the Division Lycopodiophyta known from anatomically preserved specimens in an Early Devonian deposit of chert at Rhynie, Aberdeenshire in North-East Scotland that has been dated at...

. The spines of Asteroxylon had a primitive vasuclar supply – at the very least, leaf traces could be seen departing from the central protostele towards each individual "leaf". A fossil known as Baragwanathia

Baragwanathia

Baragwanathia is a genus of extinct plants of the division Lycopodiophyta of Late Silurian to Early Devonian age , fossils of which have been found in Australia, Canada and China.-Description:...

appears in the fossil record slightly earlier, in the late Silurian. In this organism, these leaf traces continue into the leaf to form their mid-vein. One theory, the "enation theory", holds that the leaves developed by outgrowths of the protostele connecting with existing enations, but it is also possible that microphylls evolved by a branching axis forming "webbing".

Asteroxylon and Baragwanathia are widely regarded as primitive lycopods. The lycopods are still today, familiar as the quillwort

Quillwort

Isoëtes, also written Isoetes and commonly known as the quillworts, is a genus of plants in the class Isoetopsida and order Isoetales. They are considered "fern allies". There are about 140-150 species, with a cosmopolitan distribution but often scarce to rare...

 Isoetes and the club mosses. Lycopods bear distinctive microphylls – leaves with a single vascular trace. Microphylls could grow to some size – the Lepidodendrales

Lepidodendrales

Lepidodendrales were primitive, vascular, arborescent plants related to the lycopsids . They thrived during the Carboniferous period, and some reached heights of over 30 meters, with trunks often more than one meter in diameter...

 boasted microphylls over a meter in length – but almost all just bear the one vascular bundle. (An exception is the branching Selaginella).

The more familiar leaves, megaphylls, are thought to have separate origins – indeed, they appeared four times independently, in the ferns, horsetails, progymnosperms, and seed plants. They appear to have originated from dichotomising branches, which first overlapped (or "overtopped") one another, and eventually developed "webbing" and evolved into gradually more leaf-like structures. So megaphylls, by this "teleome theory", are composed of a group of webbed branches – hence the "leaf gap" left where the leaf's vascular bundle leaves that of the main branch resembles two axes splitting. In each of the four groups to evolve megaphylls, their leaves first evolved during the late Devonian to early Carboniferous, diversifying rapidly until the designs settled down in the mid Carboniferous.

The cessation of further diversification can be attributed to developmental constraints, but why did it take so long for leaves to evolve in the first place? Plants had been on the land for at least 50 million years before megaphylls became significant. However, small, rare mesophylls are known from the early Devonian genus Eophyllophyton

Eophyllophyton

Eophyllophyton bellum is the oldest known plant bearing megaphyllous leaves....

– so development could not have been a barrier to their appearance. The best explanation so far incorporates observations that atmospheric was declining rapidly during this time – falling by around 90% during the Devonian. This corresponded with an increase in stomatal density by 100 times. Stomata allow water to evaporate from leaves, which causes them to curve. It appears that the low stomatal density in the early Devonian meant that evaporation was limited, and leaves would overheat if they grew to any size. The stomatal density could not increase, as the primitive steles and limited root systems would not be able to supply water quickly enough to match the rate of transpiration.

Clearly, leaves are not always beneficial, as illustrated by the frequent occurrence of secondary loss of leaves, famously exemplified by cacti

Cacti

-See also:* RRDtool The underlying software upon which Cacti is built* MRTG The original Multi Router Traffic Grapher from which RRDtool was "extracted".* Munin -External links:******...

 and the "whisk fern" Psilotum

Psilotum

Psilotum is a genus of fern-like vascular plants, one of two genera in the family Psilotaceae, order Psilotales, and class Psilotopsida...

.

Secondary evolution can also disguise the true evolutionary origin of some leaves. Some genera of ferns display complex leaves which are attached to the pseudostele by an outgrowth of the vascular bundle, leaving no leaf gap. Further, horsetail (Equisetum) leaves bear only a single vein, and appear for all the world to be microphyllous; however, in the light of the fossil record and molecular evidence, we conclude that their forbears bore leaves with complex venation, and the current state is a result of secondary simplification.

Deciduous

Deciduous

Deciduous means "falling off at maturity" or "tending to fall off", and is typically used in reference to trees or shrubs that lose their leaves seasonally, and to the shedding of other plant structures such as petals after flowering or fruit when ripe...

 trees deal with another disadvantage to having leaves. The popular belief that plants shed their leaves when the days get too short is misguided; evergreens prospered in the Arctic circle

Arctic Circle

The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. For Epoch 2011, it is the parallel of latitude that runs north of the Equator....

 during the most recent greenhouse earth. The generally accepted reason for shedding leaves during winter is to cope with the weather – the force of wind and weight of snow are much more comfortably weathered without leaves to increase surface area. Seasonal leaf loss has evolved independently several times and is exhibited in the ginkgoales, pinophyta

Pinophyta

The conifers, division Pinophyta, also known as division Coniferophyta or Coniferae, are one of 13 or 14 division level taxa within the Kingdom Plantae. Pinophytes are gymnosperms. They are cone-bearing seed plants with vascular tissue; all extant conifers are woody plants, the great majority being...

 and angiosperms. Leaf loss may also have arisen as a response to pressure from insects; it may have been less costly to lose leaves entirely during the winter or dry season than to continue investing resources in their repair.

Factors influencing leaf architectures

Various physical and physiological forces like light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 intensity, humidity

Humidity

Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

, temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, wind speeds etc. are thought to have influenced evolution of leaf shape and size. It is observed that high trees rarely have large leaves, owing to the obstruction they generate for winds. This obstruction can eventually lead to the tearing of leaves, if they are large. Similarly, trees that grow in temperate

Temperate

In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 or taiga

Taiga

Taiga , also known as the boreal forest, is a biome characterized by coniferous forests.Taiga is the world's largest terrestrial biome. In North America it covers most of inland Canada and Alaska as well as parts of the extreme northern continental United States and is known as the Northwoods...

 regions have pointed leaves, presumably to prevent nucleation of ice onto the leaf surface and reduce water loss due to transpiration. Herbivory, not only by large mammals, but also small insects has been implicated as a driving force in leaf evolution, an example being plants of the genus Aciphylla, that are commonly found in New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

. The now extinct Moas fed upon these plants, and its seen that the leaves have spines on their bodies, which probably functioned to discourage the moas from feeding on them. Other members of Aciphylla that did not co-exist with the moas, do not have these spines.

At the genetic level, developmental studies have shown that repression of the KNOX genes is required for initiation of the leaf

Leaf

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

 primordium

Primordium

A primordium , in embryology, is defined as an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells...

. This is brought about by ARP genes, which encode transcription factors. Genes of this type have been found in many plants studied till now, and the mechanism i.e. repression of KNOX genes in leaf primordia, seems to be quite conserved. Interestingly, expression of KNOX genes in leaves produces complex leaves. It is speculated that the ARP function arose quite early in vascular plant

Vascular plant

Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...

 evolution, because members of the primitive group Lycophytes also have a functionally similar gene Other players that have a conserved role in defining leaf primordia are the phytohormone auxin

Auxin

Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

, gibberelin and cytokinin

Cytokinin

Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence...

.

One interesting feature of a plant is its phyllotaxy. The arrangement of leaves on the plant body is such that the plant can maximally harvest light under the given constraints, and hence, one might expect the trait to be genetically robust. However, it may not be so. In maize

Maize

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

, a mutation in only one gene called abphyl (ABNORMAL PHYLLOTAXY) was enough to change the phyllotaxy of the leaves. It implies that sometimes, mutational tweaking of a single locus on the genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 is enough to generate diversity. The abphyl gene was later on shown to encode a cytokinin

Cytokinin

Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence...

 response regulator protein.

Once the leaf primordial cells are established from the SAM cells, the new axes

Axes

Axes may refer to:* Axes, woodworking hand tools* The plural of axis* Axes , a 2005 rock album by the British band Electrelane* X and Y axes, or X, Y, and Z axes, perpendicular lines used in the Cartesian coordinate system...

 for leaf growth are defined, one important (and more studied) among them being the abaxial-adaxial (lower-upper surface) axes. The genes involved in defining this, and the other axes seem to be more or less conserved among higher plants. Proteins of the HD-ZIPIII family have been implicated in defining the adaxial identity. These proteins deviate some cells in the leaf primordium

Primordium

A primordium , in embryology, is defined as an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells...

 from the default abaxial state, and make them adaxial. It is believed that in early plants with leaves, the leaves just had one type of surface - the abaxial one. This is the underside of today's leaves. The definition of the adaxial identity occurred some 200 million years after the abaxial identity was established. One can thus imagine the early leaves as an intermediate stage in evolution of today's leaves, having just arisen from spiny stem-like outgrowths of their leafless ancestors, covered with stomata all over, and not optimized as much for light harvesting

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

.

How the infinite variety of plant leaves is generated is a subject of intense research. Some common themes have emerged. One of the most significant is the involvement of KNOX genes in generating compound leaves, as in tomato

Tomato

The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...

 (see above). But this again is not universal. For example, pea

Pea

A pea is most commonly the small spherical seed or the seed-pod of the pod fruit Pisum sativum. Each pod contains several peas. Peapods are botanically a fruit, since they contain seeds developed from the ovary of a flower. However, peas are considered to be a vegetable in cooking...

 uses a different mechanism for doing the same thing. Mutations in genes affecting leaf curvature

Curvature

In mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...

 can also change leaf form, by changing the leaf from flat, to a crinky shape, like the shape of cabbage

Cabbage

Cabbage is a popular cultivar of the species Brassica oleracea Linne of the Family Brassicaceae and is a leafy green vegetable...

 leaves. There also exist different morphogen

Morphogen

A morphogen is a substance governing the pattern of tissue development, and the positions of the various specialized cell types within a tissue...

 gradients in a developing leaf which define the leaf's axis. Changes in these morphogen gradients may also affect the leaf form. Another very important class of regulators of leaf development are the microRNAs, whose role in this process has just begun to be documented. The coming years should see a rapid development in comparative studies on leaf development, with many EST

Expressed sequence tag

An expressed sequence tag or EST is a short sub-sequence of a cDNA sequence. They may be used to identify gene transcripts, and are instrumental in gene discovery and gene sequence determination. The identification of ESTs has proceeded rapidly, with approximately 65.9 million ESTs now available in...

 sequences involved in the process coming online.

Tree form

The early Devonian landscape was devoid of vegetation taller than waist height. Without the evolution of a robust vascular system, taller heights could not be attained. There was, however, a constant evolutionary pressure to attain greater height. The most obvious advantage is the harvesting of more sunlight for photosynthesis – by overshadowing competitors – but a further advantage is present in spore distribution, as spores (and, later, seeds) can be blown greater distances if they start higher. This may be demonstrated by Prototaxites

Prototaxites

The genus Prototaxites describes terrestrial organisms known only from fossils dating from the Silu-Devonian, approximately 420 to 370 million years ago. Prototaxites formed large trunk-like structures up to wide, reaching in height, made up of interwoven tubes just in diameter...

, thought to be a late Silurian fungus reaching eight metres in height.

To attain , early plants had to develop woody tissue that provided support and water transport. To understand wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

, we must know a little of vascular behaviour. The stele of plants undergoing "secondary growth" is surrounded by the vascular cambium

Vascular cambium

The vascular cambium is a part of the morphology of plants. It consists of cells that are partly specialized, for the tissues that transport water solutions, but have not reached any of the final forms that occur in their branch of the specialization graph...

, a ring of cells which produces more xylem (on the inside) and phloem (on the outside). Since xylem cells comprise dead, lignified tissue, subsequent rings of xylem are added to those already present, forming wood.

The first plants to develop this secondary growth, and a woody habit, were apparently the ferns, and as early as the middle Devonian one species, Wattieza

Wattieza

Wattieza was a genus of prehistoric trees that existed in the mid-Devonian that belong to the cladoxylopsids, close relatives of the modern ferns and horsetails...

, had already reached heights of 8 m and a tree-like habit.

Other clades did not take long to develop a tree-like stature; the late Devonian Archaeopteris

Archaeopteris

Archaeopteris is an extinct genus of tree-like plants with fern-like leaves. A useful index fossil, this tree is found in strata dating from the Upper Devonian to Lower Carboniferous , and has a global distribution....

, a precursor

Progymnosperm

The progymnosperms are an extinct group of woody, spore-bearing plants that is presumed to have evolved from the "trimerophytes", and eventually gave rise to the gymnosperms. They have been treated formally at the rank of division Progymnospermophyta or class Progymnospermopsida...

 to gymnosperm

Gymnosperm

The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos , meaning "naked seeds", after the unenclosed condition of their seeds...

s which evolved from the trimerophytes, reached 30 m in height. These progymnosperms were the first plants to develop true wood, grown from a bifacial cambium, of which the first appearance is in the mid Devonian Rellimia. True wood is only thought to have evolved once, giving rise to the concept of a "lignophyte" clade.

These Archaeopteris forests were soon supplemented by lycopods, in the form of lepidodendrales

Lepidodendrales

Lepidodendrales were primitive, vascular, arborescent plants related to the lycopsids . They thrived during the Carboniferous period, and some reached heights of over 30 meters, with trunks often more than one meter in diameter...

, which topped 50m in height and 2m across at the base. These lycopods rose to dominate late Devonian and Carboniferous coal deposits. Lepidodendrales differ from modern trees in exhibiting determinate growth: after building up a reserve of nutrients at a low height, the plants would "bolt" to a genetically determined height, branch at that level, spread their spores and die. They consisted of "cheap" wood to allow their rapid growth, with at least half of their stems comprising a pith-filled cavity. Their wood was also generated by a unifacial vascular cambium – it did not produce new phloem, meaning that the trunks could not grow wider over time.

The horsetail

Horsetail

Equisetum is the only living genus in the Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.Equisetum is a "living fossil", as it is the only living genus of the entire class Equisetopsida, which for over one hundred million years was much more diverse and...

 Calamites

Calamites

Calamites is a genus of extinct arborescent horsetails to which the modern horsetails are closely related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to heights of more than 30 meters...

was next on the scene, appearing in the Carboniferous

Carboniferous

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

. Unlike the modern horsetail Equisetum, Calamites

Calamites

Calamites is a genus of extinct arborescent horsetails to which the modern horsetails are closely related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to heights of more than 30 meters...

had a unifacial vascular cambium, allowing them to develop wood and grow to heights in excess of 10 m. They also branched multiple times.

While the form of early trees was similar to that of today's, the groups containing all modern trees had yet to evolve.

The dominant groups today are the gymnosperms, which include the coniferous trees, and the angiosperms, which contain all fruiting and flowering trees. It was long thought that the angiosperms arose from within the gymnosperms, but recent molecular evidence suggests that their living representatives form two distinct groups. The molecular data has yet to be fully reconciled with morphological data, but it is becoming accepted that the morphological support for paraphyly is not especially strong.
This would lead to the conclusion that both groups arose from within the pteridosperms, probably as early as the Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

.

The angiosperms and their ancestors played a very small role until they diversified during the Cretaceous. They started out as small, damp-loving organisms in the understory, and have been diversifying ever since the mid-Cretaceous, to become the dominant member of non- forests today.

Roots

The roots (bottom image) of lepidodendrales are thought to be functionally equivalent to the stems (top), as the similar appearance of "leaf scars" and "root scars" on these specimens from different species demonstrates.

Roots are important to plants for two main reasons: Firstly, they provide anchorage to the substrate; more importantly, they provide a source of water and nutrients from the soil. Roots allowed plants to grow taller and faster.

The onset of roots also had effects on a global scale. By disturbing the soil, and promoting its acidification (by taking up nutrients such as nitrate and phosphate), they enabled it to weather more deeply, promoting the draw-down of with huge implications for climate. These effects may have been so profound they led to a mass extinction

Late Devonian extinction

The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...

.

But how and when did roots evolve in the first place? While there are traces of root-like impressions in fossil soils in the late Silurian, body fossils show the earliest plants to be devoid of roots. Many had tendrils which sprawled along or beneath the ground, with upright axes or dotted here and there, and some even had non-photosynthetic subterranean branches which lacked stomata. The distinction between root and specialised branch is developmental; true roots follow a different developmental trajectory to stems. Further, roots differ in their branching pattern, and in possession of a root cap

Root cap

The root cap is a section of tissue at the tip of a plant root. It is also called calyptra. Root caps contain statocytes which are involved in gravity perception in plants. If the cap is carefully removed the root will grow randomly. The root cap protects the growing tip in plants...

. So while Silu-Devonian plants such as Rhynia

Rhynia

Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Lower Devonian that had anatomical features more advanced than those of the bryophytes, and was basal to modern vascular plants or eutracheophytes.-Description:R...

and Horneophyton

Horneophyton

Horneophyton, a member of the Horneophytopsida, was an early plant which may form a "missing link" between the hornworts and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert.-Description:...

possessed the physiological equivalent of roots, roots – defined as organs differentiated from stems – did not arrive until later. Unfortunately, roots are rarely preserved in the fossil record, and our understanding of their evolutionary origin is sparse.

Rhizoids – small structures performing the same role as roots, usually a cell in diameter – probably evolved very early, perhaps even before plants colonised the land; they are recognised in the Characeae, an algal sister group to land plants. That said, rhizoids probably evolved more than once; the rhizines of lichen

Lichen

Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

s, for example, perform a similar role. Even some animals (Lamellibrachia

Lamellibrachia

Lamellibrachia is a genus of tube worms related to the giant tube worm, Riftia pachyptila. It lives at deep-sea cold seeps where hydrocarbons are leaking out of the seafloor. It is entirely reliant on internal, sulfide-oxidizing bacterial symbionts for its nutrition.L...

) have root-like structures!

More advanced structures are common in the Rhynie chert, and many other fossils of comparable early Devonian age bear structures that look like, and acted like, roots. The rhyniophytes bore fine rhizoids, and the trimerophytes and herbaceous lycopods of the chert bore root-like structure penetrating a few centimetres into the soil. However, none of these fossils display all the features borne by modern roots. Roots and root-like structures became increasingly more common and deeper penetrating during the Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 period, with lycopod trees forming roots around 20 cm long during the Eifelian and Givetian. These were joined by progymnosperms, which rooted up to about a metre deep, during the ensuing Frasnian stage. True gymnosperms and zygopterid ferns also formed shallow rooting systems during the Famennian period.

The rhizomorphs of the lycopods provide a slightly different approach to rooting. They were equivalent to stems, with organs equivalent to leaves performing the role of rootlets. A similar construction is observed in the extant lycopod Isoetes, and this appears to be evidence that roots evolved independently at least twice, in the lycophytes and other plants.

A vascular system is indispensable to a rooted plants, as non-photosynthesising roots need a supply of sugars, and a vascular system is required to transport water and nutrients from the roots to the rest of the plant. These plants are little more advanced than their Silurian forbears, without a dedicated root system; however, the flat-lying axes can be clearly seen to have growths similar to the rhizoids of bryophytes today.

By the mid-to-late Devonian, most groups of plants had independently developed a rooting system of some nature. As roots became larger, they could support larger trees, and the soil was weathered to a greater depth. This deeper weathering had effects not only on the aforementioned drawdown of , but also opened up new habitats for colonisation by fungi and animals.

Roots today have developed to the physical limits. They penetrate many metres of soil to tap the water table. The narrowest roots are a mere 40 μm in diameter, and could not physically transport water if they were any narrower.
The earliest fossil roots recovered, by contrast, narrowed from 3 mm to under 700 μm in diameter; of course, taphonomy is the ultimate control of what thickness we can see.

Arbuscular mycorrhizae

The efficiency of many plants' roots is increased via a relationship with a fungal partner. The most common are arbuscular mycorrhizae (AM), literally "tree-like fungal roots". These comprise fungi which invade some root cells, filling the cell membrane with their e. They feed on the plant's sugars, but return nutrients generated or extracted from the soil (especially phosphate), which the plant would otherwise have no access to.

This symbiosis appears to have evolved early in plant history. AM are found in all plant groups, and 80% of extant vascular plants, suggesting an early ancestry; a "plant"-fungus symbiosis may even have been the step that enabled them to colonise the land,. Such fungi increase the productivity even of simple plants such as liverworts. Indeed, AM are abundant in the Rhynie chert; the association occurred even before there were true roots to colonise, and some have suggested that roots evolved to provide a more comfortable habitat for mycorrhizal fungi.

Seeds

Early land plants reproduced in the fashion of ferns: spores germinated into small gametophytes, which produced sperm. These would swim across moist soils to find the female organs (archegonia) on the same or another gametophyte, where they would fuse with an ovule to produce an embryo, which would germinate into a sporophyte.

Heterosporic organisms, as their name suggests, bear spores of two sizes – microspores and megaspores. These would germinate to form microgametophytes and megagametophytes, respectively. This system paved the way for seeds: taken to the extreme, the megasporangia could bear only a single megaspore tetrad, and to complete the transition to true seeds, three of the megaspores in the original tetrad could be aborted, leaving one megaspore per megasporangium.

The transition to seeds continued with this megaspore being "boxed in" to its sporangium while it germinates. Then, the megagametophyte is contained within a waterproof integument, which forms the bulk of the seed. The microgametophyte – a pollen grain which has germinated from a microspore – is employed for dispersal, only releasing its desiccation-prone sperm when it reaches a receptive megagametophyte.

Lycopods go a fair way down the path to seeds without ever crossing the threshold. Fossil lycopod megaspores reaching 1 cm in diameter, and surrounded by vegetative tissue, are known – these even germinate into a megagametophyte in situ. However, they fall short of being seeds, since the nucellus, an inner spore-covering layer, does not completely enclose the spore. A very small slit remains, meaning that the seed is still exposed to the atmosphere. This has two consequences – firstly, it means it is not fully resistant to desiccation, and secondly, sperm do not have to "burrow" to access the archegonia of the megaspore.

The first "spermatophytes" (literally:seed plants) – that is, the first plants to bear true seeds – are called pteridosperms: literally, "seed ferns", so called because their foliage consisted of fern-like fronds, although they were not closely related to ferns. The oldest fossil evidence of seed plants is of Late Devonian age and they appear to have evolved out of an earlier group known as the progymnosperms. These early seed plants ranged from trees to small, rambling shrubs; like most early progymnosperms, they were woody plants with fern-like foliage. They all bore ovules, but no cones, fruit or similar. While it is difficult to track the early evolution of seeds, we can trace the lineage of the seed ferns from the simple trimerophytes through homosporous Aneurophytes.

This seed model is shared by basically all gymnosperms (literally: "naked seeds"), most of which encase their seeds in a woody or fleshy (the yew

Taxus

Taxus is a genus of yews, small coniferous trees or shrubs in the yew family Taxaceae. They are relatively slow-growing and can be very long-lived, and reach heights of 1-40 m, with trunk diameters of up to 4 m...

, for example) cone, but none of which fully enclose their seeds. The angiosperms ("vessel seeds") are the only group to fully enclose the seed, in a carpel.

Fully enclosed seeds opened up a new pathway for plants to follow: that of seed dormancy

Seed dormancy

Seed dormancy is a condition of plant seeds that prevents germination when the seeds are under optimal environmental conditions for germination. Living, non dormant seeds germinate when soil temperatures and moisture conditions are suited for cellular processes and division; dormant seeds do...

. The embryo, completely isolated from the external atmosphere and hence protected from desiccation, could survive some years of drought before germinating.
Gymnosperm seeds from the late Carboniferous have been found to contain embryos, suggesting a lengthy gap between fertilisation and germination. This period is associated with the entry into a greenhouse earth period, with an associated increase in aridity. This suggests that dormancy arose as a response to drier climatic conditions, where it became advantageous to wait for a moist period before germinating. This evolutionary breakthrough appears to have opened a floodgate: previously inhospitable areas, such as dry mountain slopes, could now be tolerated, and were soon covered by trees.

Seeds offered further advantages to their bearers: they increased the success rate of fertilised gametophytes, and because a nutrient store could be "packaged" in with the embryo, the seeds could germinate rapidly in inhospitable environments, reaching a size where it could fend for itself more quickly. For example, without an endosperm, seedlings growing in arid environments would not have the reserves to grow roots deep enough to reach the water table before they expired from dehydration. Likewise, seeds germinating in a gloomy understory require an additional reserve of energy to quickly grow high enough to capture sufficient light for self-sustenance.
A combination of these advantages gave seed plants the ecological edge over the previously dominant genus Archaeopteris, thus increasing the biodiversity of early forests.

Flowers

For a more ecological discussion on the evolution of flowers, go to Flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

Flowers are modified leaves possessed only by the angiosperms, which are relatively late to appear in the fossil record; the group originated and diversified during the Early Cretaceous and became ecologically significant thereafter. Flower-like structures first appear in the fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 records some ~130 mya, in the Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 era.
Colourful and/or pungent structures surround the cones of plants such as cycads and gnetales, making a strict definition of the term "flower" elusive.

The main function of a flower is reproduction, which, before the evolution of the flower and angiosperms, was the job of microsporophylls and megasporophylls. A flower can be considered a powerful evolutionary innovation

Innovation

Innovation is the creation of better or more effective products, processes, technologies, or ideas that are accepted by markets, governments, and society...

, because its presence allowed the plant world to access new means and mechanisms for reproduction.
The flowering plants have long been assumed to have evolved from within the gymnosperms; according to the traditional morphological view, they are closely allied to the gnetales. However, as noted above, recent molecular evidence is at odds to this hypothesis, and further suggests that gnetales are more closely related to some gymnosperm groups than angiosperms, and that gymnosperms form a distinct clade to the angiosperms, the two clades diverging some .
The relationship of stem groups to the angiosperms is important in determining the evolution of flowers. stem groups provide an insight into the state of earlier "forks" on the path to the current state. Convergence increases the risk of misidentifying stem groups. Since the protection of the megagametophyte is evolutionarily desirable, probably many separate groups evolved protective encasements independently. In flowers, this protection takes the form of a carpel, evolved from a leaf and recruited into a protective role, shielding the ovules. These ovules are further protected by a double-walled .

Penetration of these protective layers needs something more than a free-floating microgametophyte. Angiosperms have pollen grains comprising just three cells. One cell is responsible for drilling down through the integuments, and creating a conduit for the two sperm cells to flow down. The megagametophyte has just seven cells; of these, one fuses with a sperm cell, forming the nucleus of the egg itself, and another joins with the other sperm, and dedicates itself to forming a nutrient-rich endosperm

Endosperm

Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...

. The other cells take auxiliary roles. This process of "double fertilisation" is unique and common to all angiosperms.

In the fossil record, there are three intriguing groups which bore flower-like structures. The first is the Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 pteridosperm Glossopteris

Glossopteris

Glossopteris is the largest and best-known genus of the extinct order of seed ferns known as Glossopteridales ....

, which already bore recurved leaves resembling carpels. The Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 Caytonia is more flower-like still, with enclosed ovules – but only a single integument. Further, details of their pollen and stamens set them apart from true flowering plants.

The Bennettitales

Bennettitales

Bennettitales is an extinct order of seed plants that first appeared in the Triassic period and became extinct toward the end of the Cretaceous...

 bore remarkably flower-like organs, protected by whorls of s which may have played a similar role to the petals and sepals of true flowers; however, these flower-like structures evolved independently, as the Bennettitales are more closely related to cycad

Cycad

Cycads are seed plants typically characterized by a stout and woody trunk with a crown of large, hard and stiff, evergreen leaves. They usually have pinnate leaves. The individual plants are either all male or all female . Cycads vary in size from having a trunk that is only a few centimeters...

s and ginkgo

Ginkgo

Ginkgo , also spelled gingko and known as the Maidenhair Tree, is a unique species of tree with no close living relatives...

s than to the angiosperms.

However, no true flowers are found in any groups save those extant today. Most morphological and molecular analyses place Amborella

Amborella

Amborella is a genus of rare understory shrubs or small trees endemic to the island of New Caledonia. The genus consists of only a single species, Amborella trichopoda, and is the only member of the family Amborellaceae. Wood of Amborella lacks the vessels characteristic of most flowering plants...

, the nymphaeales

Nymphaeales

Nymphaeales is an order of plants, which consists of water lilies and other aquatic plants.This order is considered to be a basal, or early diverging, group of angiosperms...

 and Austrobaileya

Austrobaileya

Austrobaileya is a genus of flowering plants native to Queensland state in northeastern Australia, consisting of two species of evergreen lianas, A. maculata and A. scandens. Austrobaileya is the sole genus in the family Austrobaileyaceae.Austrobaileya are woody vines. Its main stem is loosely...

ceae in a basal clade dubbed "ANA". This clade appear to have diverged in the early Cretaceous, around – around the same time as the earliest fossil angiosperm, and just after the first angiosperm-like pollen, 136 million years ago.
The magnoliids diverged soon after, and a rapid radiation had produced eudicots and monocots by . By the end of the Cretaceous , over 50% of today's angiosperm orders had evolved, and the clade accounted for 70% of global species.
It was around this time that flowering trees became dominant over conifers

The features of the basal "ANA" groups suggest that angiosperms originated in dark, damp, frequently disturbed areas. It appears that the angiosperms remained constrained to such habitats throughout the Cretaceous – occupying the niche of small herbs early in the successional series. This may have restricted their initial significance, but given them the flexibility that accounted for the rapidity of their later diversifications in other habitats.

Origins of the flower

The family Amborellaceae

Amborellaceae

Amborella is a genus of rare understory shrubs or small trees endemic to the island of New Caledonia. The genus consists of only a single species, Amborella trichopoda, and is the only member of the family Amborellaceae. Wood of Amborella lacks the vessels characteristic of most flowering plants...

 is regarded as the sister family of all living flowering plants. That means members of this family were most likely the first flowering plants.

It seems that on the level of the organ, the leaf

Leaf

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

 may be the ancestor of the flower, or at least some floral organs. When we mutate some crucial genes involved in flower development, we end up with a cluster of leaf-like structures. Thus, sometime in history, the developmental program leading to formation of a leaf must have been altered to generate a flower. There probably also exists an overall robust framework within which the floral diversity has been generated. An example of that is a gene called LEAFY

Leafy

LEAFY is a plant gene that causes groups of undifferentiated cells called meristems to develop into flowers instead of shoots....

 (LFY), which is involved in flower development in Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

. The homologs

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 of this gene are found in angiosperms as diverse as tomato

Tomato

The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...

, snapdragon

SnapDragon

SnapDragon is a contemporary jazz band based in San Antonio, Texas. The group released their debut CD, "Stealing a Moment" on Humbug Records in 2008, and the 10-song CD soon broke the Top 100 in U.S radio airplay...

, pea

Pea

A pea is most commonly the small spherical seed or the seed-pod of the pod fruit Pisum sativum. Each pod contains several peas. Peapods are botanically a fruit, since they contain seeds developed from the ovary of a flower. However, peas are considered to be a vegetable in cooking...

, maize

Maize

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

 and even gymnosperms. Expression of Arabidopsis thaliana LFY in distant plants like poplar

Poplar

Populus is a genus of 25–35 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar , aspen, and cottonwood....

 and citrus

Citrus

Citrus is a common term and genus of flowering plants in the rue family, Rutaceae. Citrus is believed to have originated in the part of Southeast Asia bordered by Northeastern India, Myanmar and the Yunnan province of China...

 also results in flower-production in these plants. The LFY gene regulates the expression of some gene belonging to the MADS-box

MADS-box

The MADS box is a conserved sequence motif found in genes which comprise the MADS-box gene family. The MADS box encodes the DNA-binding MADS domain. The MADS domain binds to DNA sequences of high similarity to the motif CC[A/T]6GG termed the CArG-box. MADS-domain proteins are generally...

 family. These genes, in turn, act as direct controllers of flower development.

Evolution of the MADS-box family

The members of the MADS-box

MADS-box

The MADS box is a conserved sequence motif found in genes which comprise the MADS-box gene family. The MADS box encodes the DNA-binding MADS domain. The MADS domain binds to DNA sequences of high similarity to the motif CC[A/T]6GG termed the CArG-box. MADS-domain proteins are generally...

 family of transcription factors play a very important and evolutionarily conserved role in flower development. According to the ABC Model of flower development

The ABC Model of Flower Development

The ABC model of flower development in angiosperms was formulated by Enrico Coen and Elliot Meyerowitz in 1991. This model is built on the observation of mutants with defects in floral organ development...

, three zones - A,B and C - are generated within the developing flower primordium, by the action of some transcription factors, that are members of the MADS-box

MADS-box

The MADS box is a conserved sequence motif found in genes which comprise the MADS-box gene family. The MADS box encodes the DNA-binding MADS domain. The MADS domain binds to DNA sequences of high similarity to the motif CC[A/T]6GG termed the CArG-box. MADS-domain proteins are generally...

 family. Among these, the functions of the B and C domain genes have been evolutionarily more conserved than the A domain gene. Many of these genes have arisen through gene duplication

Gene duplication

Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...

s of ancestral members of this family. Quite a few of them show redundant functions.

The evolution of the MADS-box

MADS-box

The MADS box is a conserved sequence motif found in genes which comprise the MADS-box gene family. The MADS box encodes the DNA-binding MADS domain. The MADS domain binds to DNA sequences of high similarity to the motif CC[A/T]6GG termed the CArG-box. MADS-domain proteins are generally...

 family has been extensively studied. These genes are present even in pteridophytes, but the spread and diversity is many times higher in angiosperms. There appears to be quite a bit of pattern into how this family has evolved. Consider the evolution of the C-region gene AGAMOUS

Agamous

AGAMOUS is a gene and transcription factor for Arabidopsis thaliana. The TAIR ATG number is AT4G18960.-External links:*...

 (AG). It is expressed in today's flowers in the stamens, and the carpel, which are reproductive organs. It's ancestor in gymnosperms also has the same expression pattern. Here, it is expressed in the strobili

Strobilus

A strobilus is a structure present on many land plant species consisting of sporangia-bearing structures densely aggregated along a stem. Strobili are often called cones, but many botanists restrict the use of the term cone to the woody seed strobili of conifers...

, an organ that produces pollens or ovules. Similarly, the B-genes' (AP3 and PI) ancestors are expressed only in the male organs in gymnosperms. Their descendants in the modern angiosperms also are expressed only in the stamens, the male reproductive organ. Thus, the same, then-existing components were used by the plants in a novel manner to generate the first flower. This is a recurring pattern in evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

.

Factors influencing floral diversity

There is enormous variation in the developmental programs of plants. For example, grasses possess unique floral structures. The carpels and stamens are surrounded by scale-like lodicules and two bracts the lemma and the palea. Genetic evidence and morphology suggest that lodicules are homologous to eudicot petals. The palea and lemma may be homologous to sepals in other groups, or may be unique grass structures. The genetic evidence is not clear.

Variation in floral structure is typically due to slight changes in the MADS-box genes and their expression pattern.

Another example is that of Linaria vulgaris

Linaria vulgaris

Linaria vulgaris is a species of toadflax , native to most of Europe and northern Asia, from the United Kingdom south to Spain in the west, and east to eastern Siberia and western China...

, which has two kinds of flower symmetries-radial and bilateral. These symmetries are due to epigenetic changes in just one gene called CYCLOIDEA.
Arabidopsis thaliana has a gene called AGAMOUS

Agamous

AGAMOUS is a gene and transcription factor for Arabidopsis thaliana. The TAIR ATG number is AT4G18960.-External links:*...

that plays an important role in defining how many petals and sepals and other organs are generated. Mutations in this gene give rise to the floral meristem

Meristem

A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....

 obtaining an indeterminate fate, and many floral organs keep on getting produced. We have flowers like rose

Rose

A rose is a woody perennial of the genus Rosa, within the family Rosaceae. There are over 100 species. They form a group of erect shrubs, and climbing or trailing plants, with stems that are often armed with sharp prickles. Flowers are large and showy, in colours ranging from white through yellows...

s, carnations and morning glory

Morning glory

Morning glory is a common name for over 1,000 species of flowering plants in the family Convolvulaceae, whose current taxonomy and systematics is in flux...

, for example, that have very dense floral organs. These flowers have been selected by horticulturists since long for increased number of petals. Researchers have found that the morphology of these flowers is because of strong mutations in the AGAMOUS homolog in these plants, which leads to them making a large number of petals and sepals. Several studies on diverse plants like petunia

Petunia

Petunia is a widely cultivated genus of flowering plants of South American origin, closely related with tobacco, cape gooseberries, tomatoes, deadly nightshades, potatoes and chili peppers; in the family Solanaceae. The popular flower derived its name from French, which took the word petun, meaning...

, tomato

Tomato

The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...

, Impatiens

Impatiens

Impatiens is a genus of about 850–1,000 species of flowering plants, widely distributed throughout the Northern Hemisphere and tropics. Together with the puzzling Hydrocera triflora, this genus makes up the family Balsaminaceae...

, maize

Maize

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

 etc. have suggested that the enormous diversity of flowers is a result of small changes in genes

Gênes

Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...

 controlling their development.

Some of these changes also cause changes in expression patterns of the developmental genes, resulting in different phenotypes. The Floral Genome Project

Floral Genome Project

The Floral Genome Project is a collaborative research cooperation primarily between Penn State University, University of Florida, and Cornell University. The initial funding came from a grant of $7.4 million from the National Science Foundation. The Floral Genome Project was initiated to bridge the...

 looked at the EST

Expressed sequence tag

An expressed sequence tag or EST is a short sub-sequence of a cDNA sequence. They may be used to identify gene transcripts, and are instrumental in gene discovery and gene sequence determination. The identification of ESTs has proceeded rapidly, with approximately 65.9 million ESTs now available in...

 data from various tissues of many flowering plants. The researchers confirmed that the ABC Model of flower development is not conserved across all angiosperms. Sometimes expression domains change, as in the case of many monocots, and also in some basal angiosperms like Amborella

Amborella

Amborella is a genus of rare understory shrubs or small trees endemic to the island of New Caledonia. The genus consists of only a single species, Amborella trichopoda, and is the only member of the family Amborellaceae. Wood of Amborella lacks the vessels characteristic of most flowering plants...

. Different models of flower development like the The fading boundaries model, or the Overlapping-boundaries model which propose non-rigid domains of expression, may explain these architectures. There is a possibility that from the basal to the modern angiosperms, the domains of floral architecture have gotten more and more fixed through evolution.

Flowering time

Another floral feature that has been a subject of natural selection

Natural selection

Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

 is flowering time. Some plants flower early in their life cycle, others require a period of vernalization

Vernalization

Vernalization is the acquisition of a plant's ability to flower or germinate in the spring by exposure to the prolonged cold of winter...

 before flowering. This decision is based on factors like temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, light intensity

Light intensity

Several measures of light are commonly known as intensity. These are obtained by dividing either a power or a luminous flux by a solid angle, a planar area, or a combination of the two...

, presence of pollinators and other environmental signals. We know that genes like CONSTANS (CO), Flowering Locus C

Flowering Locus C

Flowering Locus C is a MADS-box gene that in late-flowering ecotypes of the plant Arabidopsis thaliana is responsible for vernalization. In a new seedling FLC is expressed, which prevents flowering. Upon exposure to cold, less FLC is expressed , and flowering becomes possible...

(FLC) and FRIGIDA regulate integration of environmental signals into the pathway for flower development. Variations in these loci have been associated with flowering time variations between plants. For example, Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

ecotypes that grow in the cold, temperate

Temperate

In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 regions require prolonged vernalization before they flower, while the tropical varieties, and the most common lab strains, don't. We now know that this variation is due to mutations in the FLC and FRIGIDA genes, rendering them non-functional.

Quite a few players in this process are conserved across all the plants studied. Sometimes though, despite genetic conservation, the mechanism of action turns out to be different. For example, rice

Rice

Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

 is a short-day plant, while Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

is a long-day plant. Now, in both plants, the proteins CO and FLOWERING LOCUS T (FT) are present. But in Arabidopsis thaliana, CO enhances FT production, while in rice, the CO homolog represses FT production, resulting in completely opposite downstream effects.

Theories of flower evolution

There are many theories that propose how flowers evolved. Some of them are described below.

The Anthophyte Theory was based upon the observation that a gymnospermic group Gnetales has a flower-like ovule

Ovule

Ovule means "small egg". In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...

. It has partially developed vessels

Vessel element

A vessel element is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are typically found in the angiosperms but absent from most gymnosperms such as the conifers....

 as found in the angiosperms, and the megasporangium is covered by three envelopes, like the ovary

Ovary

The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

 structure of angiosperm flowers. However, many other lines of evidence show that Gnetales is not related to angiosperms.
The Mostly Male Theory has a more genetic basis. Proponents of this theory point out that the gymnosperms have two very similar copies of the gene LFY while angiosperms just one. Molecular clock

Molecular clock

The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...

 analysis has shown that the other LFY paralog was lost in angiosperms around the same time as flower fossils become abundant, suggesting that this event might have led to floral evolution. According to this theory, loss of one of the LFY paralog led to flowers that were more male, with the ovules being expressed ectopically. These ovules initially performed the function of attracting pollinators, but sometime later, may have been integrated into the core flower.

Evolution of photosynthetic pathways

The C₄ metabolic pathway

C4 carbon fixation

C4 carbon fixation is one of three biochemical mechanisms, along with and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in contrast to the 3-carbon molecule products in plants. fixation is an...

 is a valuable recent innovation in plants.

Photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 is not quite as simple as adding water to to produce sugars and oxygen. A complex chemical pathway is involved, facilitated along the way by a range of enzymes and co-enzymes. The enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 RuBisCO

RuBisCO

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...

 is responsible for "fixing" – that is, it attaches it to a carbon-based molecule to form a sugar, which can be used by the plant, releasing an oxygen molecule along the way. However, the enzyme is notoriously inefficient, and just as effectively will also fix oxygen instead of in a process called photorespiration

Photorespiration

Photorespiration, or "'photo-respiration'", is a process in plant metabolism by which RuBP has oxygen added to it by the enzyme , instead of carbon dioxide during normal photosynthesis. This is the beginning step of the Calvin-Benson cycle...

. This is energetically costly as the plant has to use energy to turn the products of photorepsiration back into a form that can react with .

Concentrating carbon

C₄ plants evolved carbon concentrating mechanisms. These work by increasing the concentration of around RuBisCO, thereby facilitating photosynthesis and decreasing photorespiration. The process of concentrating around RuBisCO requires more energy than allowing gases to diffuse

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

, but under certain conditions – i.e. warm temperatures (>25°C), low concentrations, or high oxygen concentrations – pays off in terms of the decreased loss of sugars through photorespiration.

One type of C₄ metabolism employs a so-called Kranz anatomy. This transports through an outer mesophyll layer, via a range of organic molecules, to the central bundle sheath cells, where the is released. In this way, is concentrated near the site of RuBisCO operation. Because RuBisCO is operating in an environment with much more than it otherwise would be, it performs more efficiently.

A second mechanism, CAM photosynthesis, temporally separates photosynthesis from the action of RuBisCO. RuBisCO only operates during the day, when stomata are sealed and is provided by the breakdown of the chemical malate

Malate

Malate is the ionized form of malic acid. It is an important chemical compound in biochemistry. In the C4 carbon fixation process, malate is a source of CO2 in the Calvin cycle....

. More is then harvested from the atmosphere when stomata open, during the cool, moist nights, reducing water loss.

Evolutionary record

These two pathways, with the same effect on RuBisCO, evolved a number of times independently – indeed, C₄ alone arose in 18 different plant families

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

. The C₄ construction is most famously used by a subset of grasses, while CAM is employed by many succulents and cacti

Cacti

-See also:* RRDtool The underlying software upon which Cacti is built* MRTG The original Multi Router Traffic Grapher from which RRDtool was "extracted".* Munin -External links:******...

. The trait appears to have emerged during the Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

, around ; however, they did not become ecologically significant until the Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

, . Remarkably, some charcoalified fossils preserve tissue organised into the Kranz anatomy, with intact bundle sheath cells, allowing the presence C₄ metabolism to be identified without doubt at this time. In deducing their distribution and significance, we resort to the use of isotopic markers.
C₃ plants preferentially use the lighter of two isotopes of carbon in the atmosphere, ¹²C, which is more readily involved in the chemical pathways involved in its fixation. Because C₄ metabolism involves a further chemical step, this effect is accentuated. Plant material can be analysed

Mass spectrometry

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

 to deduce the ratio of the heavier ¹³C to ¹²C. This ratio is denoted . C₃ plants are on average around 14‰ (parts per thousand) lighter than the atmospheric ratio, while C₄ plants are about 28‰ lighter. The of CAM plants depends on the percentage of carbon fixed at night relative to what is fixed in the day, being closer to C₃ plants if they fix most carbon in the day and closer to C₄ plants if they fix all their carbon at night.

It's troublesome procuring original fossil material in sufficient quantity to analyse the grass itself, but fortunately we have a good proxy: horses. Horses were globally widespread in the period of interest, and browsed almost exclusively on grasses. There's an old phrase in isotope palæontology, "you are what you eat (plus a little bit)" – this refers to the fact that organisms reflect the isotopic composition of whatever they eat, plus a small adjustment factor. There is a good record of horse teeth throughout the globe, and their has been measured. The record shows a sharp negative inflection around , during the Messinian, and this is interpreted as the rise of C₄ plants on a global scale.

When is C₄ an advantage?

While C₄ enhances the efficiency of RuBisCO, the concentration of carbon is highly energy intensive. This means that C₄ plants only have an advantage over C₃ organisms in certain conditions: namely, high temperatures and low rainfall. C₄ plants also need high levels of sunlight to thrive. Models suggest that without wildfires removing shade-casting trees and shrubs, there would be no space for C₄ plants. But wildfires have occurred for 400 million years – why did C₄ take so long to arise, and then appear independently so many times? The Carboniferous period (~) had notoriously high oxygen levels – almost enough to allow spontaneous combustion

Spontaneous combustion

Spontaneous combustion is the self-ignition of a mass, for example, a pile of oily rags. Allegedly, humans can also ignite and burn without an obvious cause; this phenomenon is known as spontaneous human combustion....

 – and very low , but there is no C₄ isotopic signature to be found. And there doesn't seem to be a sudden trigger for the Miocene rise.

During the Micoene, the atmosphere and climate was relatively stable. If anything, increased gradually from before settling down to concentrations similar to the Holocene. This suggests that it did not have a key role in invoking C₄ evolution. Grasses themselves (the group which would give rise to the most occurrences of C₄) had probably been around for 60 million years or more, so had had plenty of time to evolve C₄, which in any case is present in a diverse range of groups and thus evolved independently. There is a strong signal of climate change in South Asia; increasing aridity – hence increasing fire frequency and intensity – may have led to an increase in the importance of grasslands. However, this is difficult to reconcile with the North American record. It is possible that the signal is entirely biological, forced by the fire- (and elephant?)- driven acceleration of grass evolution – which, both by increasing weathering and incorporating more carbon into sediments, reduced atmospheric levels. Finally, there is evidence that the onset of C₄ from is a biased signal, which only holds true for North America, from where most samples originate; emerging evidence suggests that grasslands evolved to a dominant state at least 15Ma earlier in South America.

Evolution of secondary metabolism

Although we know many secondary metabolites produced by plants, the extent of the same is still unfathomable. Secondary metabolites are essentially low molecular weight compounds, sometimes having complex structures. They function in processes as diverse as immunity

Immunity (medical)

Immunity is a biological term that describes a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of wide...

, anti-herbivory, pollinator

Pollinator

A pollinator is the biotic agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain...

 attraction, communication

Communication

Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...

 between plants, maintaining symbiotic associations with soil flora, enhancing the rate of fertilization etc., and hence are significant from the evo-devo perspective. The structural and functional diversity of these secondary metabolites across the plant kingdom is vast; it is estimated that hundreds of thousands of enzymes might be involved in this process in the entire of the plant kingdom, with about 15–25% of the genome coding for these enzymes, and every species having its unique arsenal of secondary metabolites. Many of these metabolites are of enormous medical significance to humans.

What is the purpose of having so many secondary metabolites being produced, with a significant chunk of the metabolome

Metabolome

Metabolome refers to the complete set of small-molecule metabolites to be found within a biological sample, such as a single organism...

 devoted to this activity? It is hypothesized that most of these chemicals help in generating immunity, and in consequence, the diversity of these metabolites is a result of a constant war between plants and their parasites. There is evidence that this may be true in many cases. The big question here is the reproductive cost involved in maintaining such an impressive inventory. Various models have been suggested that probe into this aspect of the question, but a consensus on the extent of the cost is lacking. We still cannot predict whether a plant with more secondary metabolites would be better off than other plants in its vicinity.

Secondary metabolite production seems to have arisen quite early during evolution. Even bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 possess the ability to make these compounds. But they assume more significant roles in life from fungi onwards to plants. In plants they seem to have spread out using different mechanisms like gene duplications, evolution of novel genes etc. Furthermore, studies have shown that diversity in some of these compounds may be positively selected for.

Although the role of novel gene evolution in the evolution of secondary metabolism cannot be denied, there are several examples where new metabolites have been formed by small changes in the reaction. For example, cyanogen glycosides have been proposed to have evolved multiple times in different plant lineages. There are several such instances of convergent evolution

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

. For example, we now know that enzymes for synthesis of limonene

Limonene

Limonene is a colourless liquid hydrocarbon classified as a cyclic terpene. The more common D isomer possesses a strong smell of oranges. It is used in chemical synthesis as a precursor to carvone and as a renewably-based solvent in cleaning products....

 – a terpene

Terpene

Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...

 – are more similar between angiosperms and gymnosperms than to their own terpene synthesis enzymes. This suggests independent evolution of the limonene biosynthetic pathway in these two lineages.

Mechanisms and players in evolution of plant form

While environmental factors are significantly responsible for evolutionary change, they act merely as agents for natural selection

Natural selection

Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

. Change is inherently brought about via phenomena at the genetic level - mutations, chromosomal rearrangements and epigenetic changes. While the general types of mutations hold true across the living world, in plants, some other mechanisms have been implicated as highly significant.

Polyploidy

Polyploidy

Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...

 is a very common feature in plants. It is believed that at least half (and probably all) plants are or have been polyploids. Polyploidy leads to genome doubling, thus generating functional redundancy in most genes. The duplicated genes may attain new function, either by changes in expression pattern or changes in activity. Polyploidy and gene duplication

Gene duplication

Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...

 are believed to be among the most powerful forces in evolution of plant form. It is not know though, why genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 doubling is such a frequent process in plants. One probable reason is the production of large amounts of secondary metabolites in plant cells. Some of them might interfere in the normal process of chromosomal segregation, leading to polypoidy.

In recent times, plants have been shown to possess significant microRNA families, which are conserved across many plant lineages. In comparison to animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s, while the number of plant miRNA families are lesser than animals, the size of each family is much larger. The miRNA

Mirna

Mirna may refer to:geographical entities* Mirna , a river in Istria, Croatia* Mirna , a river in Slovenia, tributary of the river Sava* Mirna , a settlement in the municipality of Mirna in Southeastern Sloveniapeople...

 genes are also much more spread out in the genome than those in animals, where we find them clustered. It has been proposed that these miRNA families have expanded by duplications of chromosomal regions. Many miRNA genes involved in regulation of plant development have been found to be quite conserved between plants studied.

Domestication

Domestication

Domestication or taming is the process whereby a population of animals or plants, through a process of selection, becomes accustomed to human provision and control. In the Convention on Biological Diversity a domesticated species is defined as a 'species in which the evolutionary process has been...

 of plants like maize

Maize

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

, rice

Rice

Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

, barley

Barley

Barley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods...

, wheat

Wheat

Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...

 etc. has also been a significant driving force in their evolution. Some studies have tried to look at the origins of the maize

Maize

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

 plant and it turns out that maize is a domesticated derivative of a wild plant from Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

 called teosinte. Teosinte

Teosinte

Zea is a genus of grasses in the family Poaceae. Several species are commonly known as teosintes and are found in Mexico, Guatemala, and Nicaragua....

 belongs to the genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Zea, just as maize, but bears very small inflorescence

Inflorescence

An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Strictly, it is the part of the shoot of seed plants where flowers are formed and which is accordingly modified...

, 5-10 hard cobs and a highly branched and spread out stem.
Interestingly, crosses between a particular teosinte variety and maize yields fertile offsprings that are intermediate in phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 between maize and teosinte. QTL analysis has also revealed some loci that when mutated in maize yield a teosinte-like stem or teosinte-like cobs. Molecular clock

Molecular clock

The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...

 analysis of these genes estimates their origins to some 9000 years ago, well in accordance with other records of maize domestication. It is believed that a small group of farmers must have selected some maize-like natural mutant of teosinte some 9000 years ago in Mexico, and subjected it to continuous selection to yield the maize plant as we know today.

Another interesting case is that of cauliflower

Cauliflower

Cauliflower is one of several vegetables in the species Brassica oleracea, in the family Brassicaceae. It is an annual plant that reproduces by seed...

. The edible cauliflower is a domesticated version of the wild plant Brassica oleracea

Brassica oleracea

Brassica oleracea, or wild cabbage, is a species of Brassica native to coastal southern and western Europe, where its tolerance of salt and lime and its intolerance of competition from other plants typically restrict its natural occurrence to limestone sea cliffs, like the chalk cliffs on both...

, which does not possess the dense undifferentiated inflorescence

Inflorescence

An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Strictly, it is the part of the shoot of seed plants where flowers are formed and which is accordingly modified...

 called the curd, that cauliflower possesses.
Cauliflower possesses a single mutation in a gene called CAL, controlling meristem

Meristem

A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....

 differentiation into inflorescence

Inflorescence

An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Strictly, it is the part of the shoot of seed plants where flowers are formed and which is accordingly modified...

. This causes the cells at the floral meristem to gain an undifferentiated identity, and instead of growing into a flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

, they grow into a lump of undifferentiated cells. This mutation has been selected through domestication at least since the Greek

Greeks

The Greeks, also known as the Hellenes , are a nation and ethnic group native to Greece, Cyprus and neighboring regions. They also form a significant diaspora, with Greek communities established around the world....

 empire.

See also

Plants:

• Plant
  Plant
  Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
  
  
• Flora
  Flora
  Flora is the plant life occurring in a particular region or time, generally the naturally occurring or indigenous—native plant life. The corresponding term for animals is fauna.-Etymology:...
  
  

General evolution:

• Evolutionary history of life
  Evolutionary history of life
  The evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life on Earth first originated until the present day. Earth formed about 4.5 Ga and life appeared on its surface within one billion years...
  
  
• Timeline of plant evolution
  Timeline of plant evolution
  This article attempts to place key plant innovations in a geological context. It concerns itself only with novel adaptations and events that had a major ecological significance, not those that are of solely anthropological interest...
  
  
• Timeline of evolution
  Timeline of evolution
  This timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
  
  

Study of plants:

• Paleobotany
  Paleobotany
  Paleobotany, also spelled as palaeobotany , is the branch of paleontology or paleobiology dealing with the recovery and identification of plant remains from geological contexts, and their use for the biological reconstruction of past environments , and both the evolutionary history of plants, with a...
  
  
• Plant evolutionary developmental biology
  Plant evolutionary developmental biology
  Evolutionary developmental biology refers to the study of developmental programs and patterns from an evolutionary perspective. It seeks to understand the various influences shaping the form and nature of life on the planet. Evo-devo arose as a separate branch of science rather recently. An early...
  
  
• Cryptospores
  Cryptospores
  Cryptospores are fossilised primitive plant spores that first appear in the fossil record during the late Ordovician to early Silurian period.-Occurrence:Cryptospores are generally found in non-marine rocks and decrease in abundance with distance offshore...
  
  

Plant interactions:

• Evolution of herbivory