Pachypodium
Encyclopedia
Pachypodium is a genus
of succulent spine-bearing trees and shrubs, native to Africa. It belongs to the dogbane family
, Apocynaceae
. Pachypodium comes from a Latin form from Greek
pachus (thick) and podion (foot) (or Gk podos, root form of pous, foot), hence meaning thick-footed.
s of pachycaule trunk
s and spinescence. These are the most general features of the genus
and can be considered distinguishing characteristics.
The pachycaule trunk is a morphologically enlarged trunk that stores water so as to survive seasonal drought
or intermittent periods of root desiccation in exposed, dry, and rocky conditions. Whereas there is great variation in the habit of the plant body, all Pachypodium exhibit pachycaul growth. Variation in habit can range from dwarf flattened plants to bottle shaped shrubs to dendroid-shaped trees.
The second general characteristic of Pachypodium is spinescence, or having spines. The spines come clustered in either pairs or triplets with these clusters often arranged in rings or whorls around the trunk. Spines emerge with leaves, and like leaves grow for a short period before stopping growth and hardening. Spines do not regenerate so weathering and abrasion can wear away all but the youngest spines from older specimens - leaving smooth trunks and branches.
To some extent, branch
es are a characteristic of the genus. Some caution is warranted in over-generalizing this characteristic. Pachypodium namaquanum
is often branchless. Pachypodium brevicaule
has no clear branches, and indeed may have evolved an alternative to branching in the form of nodes from which leaves, spines, and inflorescence
s emerge. In general Pachypodium have few branch
es. Since the environmental stresses and factors that contribute to branching can vary widely even in small areas, individual plants of the same species exhibit wide variation in branching morphology.
Unlike many members of the Apocynaceae
, including some members of the superficially similar Adenium, Pachypodium species do not exude a milky latex
. Rather, the sap
is always clear.
of the genus
Pachypodium varies significantly both within and between species and is highly responsive to its immediate surrounding microenvironment
. Pachypodium do not overly respond morphologically to larger vegetative zones. For example, Pachypodium can sometimes occur in prehumid vegetative zones where a taxon might find a suitable habitat on a rocky, sunny inselberg jutting above the humid canopy of the forest.
Morphologically, Pachypodium can be highly flexible in organization. Branching, if present at all, can be from either the base of the plant or at the crown. Freeform branching is a morphological adaptation to factors of the immediate microenvironment which, by their diversity, account for the wide range of habits:
Despite microenvironmental variation, Pachypodium are always succulent and always exhibit pachycaul trunks. Pachypodium are usually spinescent, but individual variation in spinescence as well as weathering and abrasion can result is plants with few if any spines.
Some species of pachypodium have developed geophytic pachycaul trunks, or trunks that are beneath the soil
's surface. These geophytic trunks are caudex
es, enlarged stems or trunks that store water. They should not be mistaken for root
s, because the enlargement occurs above the point where the roots branch off the main axis of the trunk.
The concept of "micro
-endemism
" plays an important role in this relationship between adaptation mechanisms and speciation. It suggests a certain small scale "nativeness" by virtue of originating or occurring naturally in a particular place or location
. The landscape
of Madagascar is a perfect example of "micro-endemism" for species of Pachypodium and other taxa. Three factors can be seen to attribute speciation, or the occurrence of species diversity, via adaptive mechanisms to accelerated evolution
as it occurs within the xeric landscape and climate
.
(1) The variation of geology and topology in dry climates is thought to have a greater effect upon plants than in areas with high rainfall. Xeric environments are thus more demanding of adaptive mechanisms to aid in the plant's survival than in places where rainfall is plentiful. The more the demanding, generally the more "mechanized" or "mechanisms" are needed to aid the plants' survival.
(2) The geological formations of locally xeric landscapes break up populations of organism
s, i.e. plants, into smaller groups, where each group can initially interbreed but, with time, develop new genotypes and cannot be bred with exception to natural hybridization. Localized geology
becomes harder to cross over for a given population to be "continuous" in a xeric geological landscape, because more demands are placed on the population. Therefore, populations are broken down into smaller units within this landscape. Groups of the original population become located to unique microenvironment
s within the landscape. Accordingly measures to adapt to these microenvironments become more singular to the isolated habitat. Adaptive mechanisms are employed so as to aid the survival of the plant group. This adaptation eventually, in part, leads to speciation in the habitat, or diverse species across the spectrum of the landscape.
(3) Taxa tend to develop specialized xeromorphoric structures at some architectural level in arid
, geological and topological
landscapes, where a strategy of a "flexible" and "strict" architectural, organizational morphology at various levels of structure for Pachypodium becomes advantageous to succeeding in the isolated, specialized landscape. This strategy is seen in the manifest flexible variations of habit in species of Pachypodium while all the same they are "strictly" xeromorphic pachycaule trunks meant to conserve water for dry periods. At another level of structure, namely that of organs, we can see that dew
and fog
dripping spines are examples of a xeromorphic adaptive mechanized organ responding to microenvironments.
These newly created species from within the xeromorphic landscape take on different characters as responses to the habitat
. For instance, there is an advantage to morphologically developing into bottle-shaped "shrubs" where the plants exist in open, sunny microenvironments on top of porous sandstone
. Little competition exists for height within the habitat. Likewise, where competition for resources is more competitive—both in the number of species and the height of surrounding plants—there are times when it is to the advantage of a plant to develop into arborescent, dendroid “trees.” This development is because these particular Pachypodium must compete with other plants for resources in a dry deciduous forest, composed of, perhaps, arborescent Aloe
, members of the Didiereaceae
genera--Alluaudia
, Alluaudiopsis, Decaryia, and Didierea; all endemic to Madagascar--and Uncarina species, for instance.
The adaptive mechanism in a morphological form and an ecological response to habitats are typically manifested together at once for the genus Pachypodium.
Examining Pachypodium reveals characteristics of various organs that adapt to the microenvironment. These adaptations, variations on habit, trunks
, branches, branchlets, spine
s, leaves
, or flowers, are plentiful in demonstrating how Pachypodium as a genus fosters greater variation in its speciation. The manner in which speciation occurs in Pachypodium, therefore, is apparent: adaptive mechanisms on a morphological level respond to the microenvironment of Pachypodium habitat. The genus' unique organizational, architectural morphology shapes plants that are highly, adaptively responsive to their immediate, surrounding, microenvironments. The duplicity of an adaptive mechanism that is at once "strict" and "flexible" at differing levels of plant physiology, or structure, has granted Pachypodium the ability to evolve within the landscape into variations that fulfill an ecological
niche as various species.
The hypothesis
of micro-endemism, therefore, states that speciation occurs in small specific habitats as aided by adaptive mechanism occurring in geological, topographical, and climatic isolation. Geologically and topographically, plant populations in xeric climates are broken down into smaller groups. The microclimate responds to the given location transforming it into a habitat. Isolated , the duplicity of organization in Pachypodium form through geology and location significant variation where over evolutionary time a new species might develop, if not have developed. The development of new species is through, in part, the adaptive mechanisms of pachycaule and spinescence as well as strict and flexible structural organization at various levels of plant physiology.
, where isolated landscape
s and micro
-environmental conditions have produced highly specialized species. The species count continues to grow as Pachypodium menabeum has been resurrected from invalid taxonomy and Pachypodium makayense added newly to the list. One can speculate that in regions such as Madagascar
, there might still be unidentified species that are confined to a single rocky outcrop or an inselberg.
that can be considered succulent plant
s: Adenium, Pachypodium, and Plumeria
. The first two genera (Pachypodium and Adenium) are generally assumed to have closed association with each other. Studies; however, of these two genera reveal that they are not as intimately close as once thought.
However, a study of key characteristics of the taxon and a cladistic study of the subfamily Apocynoideae and the family Asclepiadaceae
(before its merging with the Apocynaceae
), demonstrates that this closed association is not warranted. True, both are succulent plants and pachycaule. According to Leeuwenberg however, Adenium is maintained in the subtribe Neriinae, placed underneath the tribe
Wrightieae whereas Pachypodium is placed beside the in the subtribe Pachypodiinae, within the tribe Echiteae. Though related, these taxa means that the two are not intimately related.
and continental Southern Africa
, i.e. Angola
, Botswana
, Mozambique
, Namibia
, South Africa
, Swaziland
and Zimbabwe
.
In elevation, Pachypodium in both mainland Africa
and Madagascar
grow between an altitude of sea level, where some species
grow in sand dunes, such as Pachypodium geayi
, to 1600 m (5200 feet) for Pachypodium lealii
in southern Africa and 1900 m (6200 feet) for Pachypodium brevicaule
in Madagascar.
In continental southern Africa, the extreme temperature
s range from -10 °C (14 °F) in some locations to as much as 45 °C (113 °F). Whereas in Madagascar, with not such a great temperature amplitude, the temperature ranges from -6 °C (21 °F) to 40 °C (104 °F).
A generalization about precipitation
regimes for both southern Africa and Madagascar does not have much meaning because the habitats of Pachypodium vary so greatly with a moisture regime. In some places, Pachypodium receive annually from as little as 75 mm (2.95 inches) from the southern part of Africa to a high level of 1985 mm (78.15 inches). A precipitation regime for a species of Pachypodium, therefore, depends upon a habitat's location relative to the influences of the Atlantic
and Indian
Oceans and the various mountain ranges of southern continental Africa and of Madagascar.
The genus grows in areas where there are significant periods of dry months
that range from five months to ten months. It would seem likely that the Atlantic and India Oceans pay a major role in the creation of weather conducive to rainfall, not to mention mountain
ranges
. For example, the Madagascar dry deciduous forests
with their long dry season and severe limestone ridden soils provide one ideal setting for pachypodium.
Pachypodium grows in various types of substrate
s. Some species only grow in one substrate whereas other will grow in several. The degree to which a taxon can grow in a given substrate seems to determine how specialized its habitat
is within the landscape
and climate
s. On outcrops, steep hills, and inselbergs, the plants are subjected to fluctuating moisture
, high wind
s, and temperature
extremes. Only plants with special adaptations to exposure and extreme drought can survive, let alone thrive, on these exposed geological habitats. Pachypodium root in cleft, fissures, and crevices of those rocky formations. The non-succulent roots penetrate deeply into the acuminated soil
, mineral
, and humus in these crevices. Moisture is able to seep deep into these crevices. Very little transpiration
occurs. In this manner, rocky substrates provide moisture in the habitat. This saturation of crevices can only occur, however, if there is not a considerable runoff from the rock's surface and if there is abundant fine soil in the cracks that, in turn, retain water. The substrate
, therefore, plays a critical role in the creation of micro-environmental "arid islands."
Sand readily store water because it is taken up easily and there is less evaporation except for the top layer. Very deep sand; however, has the problem of seepage. Yet in moderation shallow and deep sand substrates have water available to Pachypodium. With shallow sand substrates, Pachypodium grow on sand dunes near the sea. Where water is in deep sandy substrate, Pachypodium grow on sand "over" laterite
red soil. Laterite soil is a largely impermeable soil that traps water for the use of the flora that include Pachypodium.
Extinction
of identified species seems yet unlikely, as the collection of seed and the cultivation of the plant safeguard the genus.
Pachypodium demonstrates the typical process of a taxon becoming a new genus. Initially debate occurred over if Pachypodium belonged to the genus Echites or if it constituted a separate genus. Pachypodium were first published as a unique genus, separate from Echites, by Leandley in 1830.
Then the debate centered on the nomenclature
of species
uniquely found in continental Southern Africa
. That changed when, in 1892, Baker
contributed the first species accepted into the genus from Madagascar
. The degree of speciation
then turned to Madagascar
, where the count of species far exceeds those on the mainland.
In 1907, Costantin
and Bois constructed the first monograph
, of Pachypodium, in which they enumerated 17 species, where ten were from Madagascar
and seven were from continental southern Africa.
records of Pachypodium known. Yet certain conclusions can be drawn from the geology of the landscape in Madagascar as to the past natural history
of Pachypodium.
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...
of succulent spine-bearing trees and shrubs, native to Africa. It belongs to the dogbane family
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...
, Apocynaceae
Apocynaceae
The Apocynaceae or dogbane family is a family of flowering plants that includes trees, shrubs, herbs, and lianas.Many species are tall trees found in tropical rainforests, and most are from the tropics and subtropics, but some grow in tropical dry, xeric environments. There are also perennial herbs...
. Pachypodium comes from a Latin form from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
pachus (thick) and podion (foot) (or Gk podos, root form of pous, foot), hence meaning thick-footed.
Genus characteristics
All Pachypodium are succulent plants that exhibit, to varying degrees, the morphological characteristicCharacteristic
Characteristic may refer to:In physics and engineering, any characteristic curve that shows the relationship between certain input and output parameters, for example:...
s of pachycaule trunk
Trunk (botany)
In botany, trunk refers to the main wooden axis of a tree that supports the branches and is supported by and directly attached to the roots. The trunk is covered by the bark, which is an important diagnostic feature in tree identification, and which often differs markedly from the bottom of the...
s and spinescence. These are the most general features of 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...
and can be considered distinguishing characteristics.
The pachycaule trunk is a morphologically enlarged trunk that stores water so as to survive seasonal drought
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
or intermittent periods of root desiccation in exposed, dry, and rocky conditions. Whereas there is great variation in the habit of the plant body, all Pachypodium exhibit pachycaul growth. Variation in habit can range from dwarf flattened plants to bottle shaped shrubs to dendroid-shaped trees.
The second general characteristic of Pachypodium is spinescence, or having spines. The spines come clustered in either pairs or triplets with these clusters often arranged in rings or whorls around the trunk. Spines emerge with leaves, and like leaves grow for a short period before stopping growth and hardening. Spines do not regenerate so weathering and abrasion can wear away all but the youngest spines from older specimens - leaving smooth trunks and branches.
To some extent, branch
Branch
A branch or tree branch is a woody structural member connected to but not part of the central trunk of a tree...
es are a characteristic of the genus. Some caution is warranted in over-generalizing this characteristic. Pachypodium namaquanum
Pachypodium namaquanum
Pachypodium namaquanum Welw. is a succulent single-stemmed plant growing to 4 metres tall in the arid, rocky mountains of the Richtersveld in the Northern Cape and southern Namibia. The warty trunk, thickset at the base and tapering to the top, is densely covered in sharp spines. Where damaged,...
is often branchless. Pachypodium brevicaule
Pachypodium brevicaule
Pachypodium brevicaule is a species of plant that belongs to the dogbane family Apocynaceae, which is now amplified by the inclusion of the milkweed family Asclepiadaceae -- an important union to botanists and horticulturalists interested in the alliance succulents.-Distribution:This plant is...
has no clear branches, and indeed may have evolved an alternative to branching in the form of nodes from which leaves, spines, and 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...
s emerge. In general Pachypodium have few branch
Branch
A branch or tree branch is a woody structural member connected to but not part of the central trunk of a tree...
es. Since the environmental stresses and factors that contribute to branching can vary widely even in small areas, individual plants of the same species exhibit wide variation in branching morphology.
Unlike many members of the Apocynaceae
Apocynaceae
The Apocynaceae or dogbane family is a family of flowering plants that includes trees, shrubs, herbs, and lianas.Many species are tall trees found in tropical rainforests, and most are from the tropics and subtropics, but some grow in tropical dry, xeric environments. There are also perennial herbs...
, including some members of the superficially similar Adenium, Pachypodium species do not exude a milky latex
Latex
Latex is the stable dispersion of polymer microparticles in an aqueous medium. Latexes may be natural or synthetic.Latex as found in nature is a milky fluid found in 10% of all flowering plants . It is a complex emulsion consisting of proteins, alkaloids, starches, sugars, oils, tannins, resins,...
. Rather, the sap
Sap
Sap may refer to:* Plant sap, the fluid transported in xylem cells or phloem sieve tube elements of a plant* Sap , a village in the Dunajská Streda District of Slovakia...
is always clear.
Morphology
The morphologyMorphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
of 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...
Pachypodium varies significantly both within and between species and is highly responsive to its immediate surrounding microenvironment
Microenvironment
Microenvironment carries different meanings depending on the context.* Medical: a small or relatively small usually distinctly specialized and effectively isolated habitat or environment...
. Pachypodium do not overly respond morphologically to larger vegetative zones. For example, Pachypodium can sometimes occur in prehumid vegetative zones where a taxon might find a suitable habitat on a rocky, sunny inselberg jutting above the humid canopy of the forest.
Morphologically, Pachypodium can be highly flexible in organization. Branching, if present at all, can be from either the base of the plant or at the crown. Freeform branching is a morphological adaptation to factors of the immediate microenvironment which, by their diversity, account for the wide range of habits:
- flattened dwarf species less than 8 cm tall but reaching 40 cm in diameter
- bottle- or oval-shaped shrubs to 4 m tall
- both branching and unbranched cigar- and cactus-like trees to 5 m tall.
Despite microenvironmental variation, Pachypodium are always succulent and always exhibit pachycaul trunks. Pachypodium are usually spinescent, but individual variation in spinescence as well as weathering and abrasion can result is plants with few if any spines.
Adaptive features
Variation among Pachypodium species is significant but all Pachypodium are succulent plants inhabiting seasonally or chronically dry landscapes. The genus employs two morphological adaptations to these xeric, isolated, habitats: Pachycaul trunks and spinescence.Pachycaul Trunks
Pachypodium trunks and branches are thickened with water-storing tissue. Plants must rely on the food and water stored in their thickened trunks during seasonal or intermittent drought when leaves have been shed and no water is available from the substrate. In addition to the lower surface-to-volume ratio which aides in water retention, the thickened trunks and branches can also possess photosynthetic surface tissue to allow nutrient synthesis even when leaves are not present.Some species of pachypodium have developed geophytic pachycaul trunks, or trunks that are beneath the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
's surface. These geophytic trunks are caudex
Caudex
A caudex is a form of stem morphology appearing as a thickened, short, perennial stem that is either underground or near ground level . It may be swollen for the purpose of water storage, especially in xerophytes...
es, enlarged stems or trunks that store water. They should not be mistaken for root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s, because the enlargement occurs above the point where the roots branch off the main axis of the trunk.
Spinescence
The various species of Pachypodium are more or less heavily spined. Species from more arid regions have evolved denser and longer spines. Fog condenses on their spines in the form of dew, which drips down to the ground and increases the amount of moisture that's available to their often shallow roots.The concept of "micro
Micro
Micro is a prefix in the metric system denoting a factor of 10-6 . Confirmed in 1960, the prefix comes from the Greek , meaning "small".The symbol for the prefix is the Greek letter μ...
-endemism
Endemic (ecology)
Endemism is the ecological state of being unique to a defined geographic location, such as an island, nation or other defined zone, or habitat type; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere. For example, all species of lemur are endemic to the...
" plays an important role in this relationship between adaptation mechanisms and speciation. It suggests a certain small scale "nativeness" by virtue of originating or occurring naturally in a particular place or location
Location (geography)
The terms location and place in geography are used to identify a point or an area on the Earth's surface or elsewhere. The term 'location' generally implies a higher degree of can certainty than "place" which often has an ambiguous boundary relying more on human/social attributes of place identity...
. The landscape
Landscape
Landscape comprises the visible features of an area of land, including the physical elements of landforms such as mountains, hills, water bodies such as rivers, lakes, ponds and the sea, living elements of land cover including indigenous vegetation, human elements including different forms of...
of Madagascar is a perfect example of "micro-endemism" for species of Pachypodium and other taxa. Three factors can be seen to attribute speciation, or the occurrence of species diversity, via adaptive mechanisms to accelerated 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...
as it occurs within the xeric landscape and climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
.
(1) The variation of geology and topology in dry climates is thought to have a greater effect upon plants than in areas with high rainfall. Xeric environments are thus more demanding of adaptive mechanisms to aid in the plant's survival than in places where rainfall is plentiful. The more the demanding, generally the more "mechanized" or "mechanisms" are needed to aid the plants' survival.
(2) The geological formations of locally xeric landscapes break up populations of organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s, i.e. plants, into smaller groups, where each group can initially interbreed but, with time, develop new genotypes and cannot be bred with exception to natural hybridization. Localized geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
becomes harder to cross over for a given population to be "continuous" in a xeric geological landscape, because more demands are placed on the population. Therefore, populations are broken down into smaller units within this landscape. Groups of the original population become located to unique microenvironment
Microenvironment
Microenvironment carries different meanings depending on the context.* Medical: a small or relatively small usually distinctly specialized and effectively isolated habitat or environment...
s within the landscape. Accordingly measures to adapt to these microenvironments become more singular to the isolated habitat. Adaptive mechanisms are employed so as to aid the survival of the plant group. This adaptation eventually, in part, leads to speciation in the habitat, or diverse species across the spectrum of the landscape.
(3) Taxa tend to develop specialized xeromorphoric structures at some architectural level in arid
Arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...
, geological and topological
Topology
Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...
landscapes, where a strategy of a "flexible" and "strict" architectural, organizational morphology at various levels of structure for Pachypodium becomes advantageous to succeeding in the isolated, specialized landscape. This strategy is seen in the manifest flexible variations of habit in species of Pachypodium while all the same they are "strictly" xeromorphic pachycaule trunks meant to conserve water for dry periods. At another level of structure, namely that of organs, we can see that dew
Dew
[Image:Dew on a flower.jpg|right|220px|thumb|Some dew on an iris in Sequoia National Park]]Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening...
and fog
Fog
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...
dripping spines are examples of a xeromorphic adaptive mechanized organ responding to microenvironments.
These newly created species from within the xeromorphic landscape take on different characters as responses to the habitat
Habitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...
. For instance, there is an advantage to morphologically developing into bottle-shaped "shrubs" where the plants exist in open, sunny microenvironments on top of porous sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
. Little competition exists for height within the habitat. Likewise, where competition for resources is more competitive—both in the number of species and the height of surrounding plants—there are times when it is to the advantage of a plant to develop into arborescent, dendroid “trees.” This development is because these particular Pachypodium must compete with other plants for resources in a dry deciduous forest, composed of, perhaps, arborescent Aloe
Aloe
Aloe , also Aloë, is a genus containing about 500 species of flowering succulent plants. The most common and well known of these is Aloe vera, or "true aloe"....
, members of the Didiereaceae
Didiereaceae
Didiereaceae is a small family of just four genera and 11 species of flowering plants endemic to south and southwest Madagascar, where they form an important component of the Madagascar spiny forests.-Description:...
genera--Alluaudia
Alluaudia
Alluaudia is a genus of six species of flowering plants endemic to Madagascar, where they form an important component of the Madagascar spiny forests. They are spiny succulent shrubs and trees from 2–20 m tall, with leaves that are deciduous in the long dry season...
, Alluaudiopsis, Decaryia, and Didierea; all endemic to Madagascar--and Uncarina species, for instance.
The adaptive mechanism in a morphological form and an ecological response to habitats are typically manifested together at once for the genus Pachypodium.
Examining Pachypodium reveals characteristics of various organs that adapt to the microenvironment. These adaptations, variations on habit, trunks
Trunk (botany)
In botany, trunk refers to the main wooden axis of a tree that supports the branches and is supported by and directly attached to the roots. The trunk is covered by the bark, which is an important diagnostic feature in tree identification, and which often differs markedly from the bottom of the...
, branches, branchlets, spine
Thorns, spines, and prickles
In botanical morphology, thorns, spines, and prickles are hard structures with sharp, or at least pointed, ends. In spite of this common feature, they differ in their growth and development on the plant; they are modified versions of different plant organs, stems, stipules, leaf veins, or hairs...
s, 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...
, or flowers, are plentiful in demonstrating how Pachypodium as a genus fosters greater variation in its speciation. The manner in which speciation occurs in Pachypodium, therefore, is apparent: adaptive mechanisms on a morphological level respond to the microenvironment of Pachypodium habitat. The genus' unique organizational, architectural morphology shapes plants that are highly, adaptively responsive to their immediate, surrounding, microenvironments. The duplicity of an adaptive mechanism that is at once "strict" and "flexible" at differing levels of plant physiology, or structure, has granted Pachypodium the ability to evolve within the landscape into variations that fulfill an ecological
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
niche as various species.
The hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
of micro-endemism, therefore, states that speciation occurs in small specific habitats as aided by adaptive mechanism occurring in geological, topographical, and climatic isolation. Geologically and topographically, plant populations in xeric climates are broken down into smaller groups. The microclimate responds to the given location transforming it into a habitat. Isolated , the duplicity of organization in Pachypodium form through geology and location significant variation where over evolutionary time a new species might develop, if not have developed. The development of new species is through, in part, the adaptive mechanisms of pachycaule and spinescence as well as strict and flexible structural organization at various levels of plant physiology.
Number of species
There are now 25 known species, of which 20 come from MadagascarMadagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, where isolated landscape
Landscape
Landscape comprises the visible features of an area of land, including the physical elements of landforms such as mountains, hills, water bodies such as rivers, lakes, ponds and the sea, living elements of land cover including indigenous vegetation, human elements including different forms of...
s and micro
Micro
Micro is a prefix in the metric system denoting a factor of 10-6 . Confirmed in 1960, the prefix comes from the Greek , meaning "small".The symbol for the prefix is the Greek letter μ...
-environmental conditions have produced highly specialized species. The species count continues to grow as Pachypodium menabeum has been resurrected from invalid taxonomy and Pachypodium makayense added newly to the list. One can speculate that in regions such as Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, there might still be unidentified species that are confined to a single rocky outcrop or an inselberg.
Affinities within the Apocynaceae
The family Apocynaceae before it included Asclepiadaceae had 3 generaGenus
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...
that can be considered succulent 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: Adenium, Pachypodium, and Plumeria
Plumeria
Plumeria is a genus of flowering plants of the family that includes Dogbane: the Apocynaceae. It contains 7-8 species of mainly deciduous shrubs and small trees...
. The first two genera (Pachypodium and Adenium) are generally assumed to have closed association with each other. Studies; however, of these two genera reveal that they are not as intimately close as once thought.
However, a study of key characteristics of the taxon and a cladistic study of the subfamily Apocynoideae and the family Asclepiadaceae
Asclepiadaceae
According to APG II, the Asclepiadaceae is a former plant family now treated as a subfamily in the Apocynaceae...
(before its merging with the Apocynaceae
Apocynaceae
The Apocynaceae or dogbane family is a family of flowering plants that includes trees, shrubs, herbs, and lianas.Many species are tall trees found in tropical rainforests, and most are from the tropics and subtropics, but some grow in tropical dry, xeric environments. There are also perennial herbs...
), demonstrates that this closed association is not warranted. True, both are succulent plants and pachycaule. According to Leeuwenberg however, Adenium is maintained in the subtribe Neriinae, placed underneath the tribe
Tribe
A tribe, viewed historically or developmentally, consists of a social group existing before the development of, or outside of, states.Many anthropologists use the term tribal society to refer to societies organized largely on the basis of kinship, especially corporate descent groups .Some theorists...
Wrightieae whereas Pachypodium is placed beside the in the subtribe Pachypodiinae, within the tribe Echiteae. Though related, these taxa means that the two are not intimately related.
Distribution
Pachypodium are native to MadagascarMadagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
and continental Southern Africa
Southern Africa
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...
, i.e. Angola
Angola
Angola, officially the Republic of Angola , is a country in south-central Africa bordered by Namibia on the south, the Democratic Republic of the Congo on the north, and Zambia on the east; its west coast is on the Atlantic Ocean with Luanda as its capital city...
, Botswana
Botswana
Botswana, officially the Republic of Botswana , is a landlocked country located in Southern Africa. The citizens are referred to as "Batswana" . Formerly the British protectorate of Bechuanaland, Botswana adopted its new name after becoming independent within the Commonwealth on 30 September 1966...
, Mozambique
Mozambique
Mozambique, officially the Republic of Mozambique , is a country in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi and Zambia to the northwest, Zimbabwe to the west and Swaziland and South Africa to the southwest...
, Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, Swaziland
Swaziland
Swaziland, officially the Kingdom of Swaziland , and sometimes called Ngwane or Swatini, is a landlocked country in Southern Africa, bordered to the north, south and west by South Africa, and to the east by Mozambique...
and Zimbabwe
Zimbabwe
Zimbabwe is a landlocked country located in the southern part of the African continent, between the Zambezi and Limpopo rivers. It is bordered by South Africa to the south, Botswana to the southwest, Zambia and a tip of Namibia to the northwest and Mozambique to the east. Zimbabwe has three...
.
Habitat
In elevation, Pachypodium in both mainland Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
and Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
grow between an altitude of sea level, where some species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
grow in sand dunes, such as Pachypodium geayi
Pachypodium geayi
Pachypodium geayi is a species of Pachypodium originated from Southwest Madagascar. It has a metallic grey pachycaule trunk. Leaves are thin and grey-green, with a bright pink mid-rib. The plant has white flowers. Pachypodium geayi is one of the largest of the Madagascar species....
, to 1600 m (5200 feet) for Pachypodium lealii
Pachypodium lealii
The Bottle tree is a species of plant included in the genus Pachypodium. The scientific name derives from the 19th century Portuguese geologist Fernando da Costa Leal, who described the Bottle tree during an exploration in southern Angola.This species can be either a shrub or a tree up to 6 meters...
in southern Africa and 1900 m (6200 feet) for Pachypodium brevicaule
Pachypodium brevicaule
Pachypodium brevicaule is a species of plant that belongs to the dogbane family Apocynaceae, which is now amplified by the inclusion of the milkweed family Asclepiadaceae -- an important union to botanists and horticulturalists interested in the alliance succulents.-Distribution:This plant is...
in Madagascar.
In continental southern Africa, the extreme 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...
s range from -10 °C (14 °F) in some locations to as much as 45 °C (113 °F). Whereas in Madagascar, with not such a great temperature amplitude, the temperature ranges from -6 °C (21 °F) to 40 °C (104 °F).
A generalization about precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
regimes for both southern Africa and Madagascar does not have much meaning because the habitats of Pachypodium vary so greatly with a moisture regime. In some places, Pachypodium receive annually from as little as 75 mm (2.95 inches) from the southern part of Africa to a high level of 1985 mm (78.15 inches). A precipitation regime for a species of Pachypodium, therefore, depends upon a habitat's location relative to the influences of the Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
and Indian
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
Oceans and the various mountain ranges of southern continental Africa and of Madagascar.
The genus grows in areas where there are significant periods of dry months
Dry season
The dry season is a term commonly used when describing the weather in the tropics. The weather in the tropics is dominated by the tropical rain belt, which oscillates from the northern to the southern tropics over the course of the year...
that range from five months to ten months. It would seem likely that the Atlantic and India Oceans pay a major role in the creation of weather conducive to rainfall, not to mention mountain
Mountain
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...
ranges
Mountain range
A mountain range is a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, formation, and age; a component part of a mountain system or of a mountain chain...
. For example, the Madagascar dry deciduous forests
Madagascar dry deciduous forests
The Madagascar dry deciduous forests represent a tropical dry forest ecoregion generally situated in the western part of Madagascar. The area has high numbers of endemic plant and animal species but has suffered large-scale clearance for agriculture...
with their long dry season and severe limestone ridden soils provide one ideal setting for pachypodium.
Pachypodium grows in various types of substrate
Substrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
s. Some species only grow in one substrate whereas other will grow in several. The degree to which a taxon can grow in a given substrate seems to determine how specialized its habitat
Habitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...
is within the landscape
Landscape
Landscape comprises the visible features of an area of land, including the physical elements of landforms such as mountains, hills, water bodies such as rivers, lakes, ponds and the sea, living elements of land cover including indigenous vegetation, human elements including different forms of...
and climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
s. On outcrops, steep hills, and inselbergs, the plants are subjected to fluctuating moisture
Moisture
Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...
, high wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
s, and 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...
extremes. Only plants with special adaptations to exposure and extreme drought can survive, let alone thrive, on these exposed geological habitats. Pachypodium root in cleft, fissures, and crevices of those rocky formations. The non-succulent roots penetrate deeply into the acuminated soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
, mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
, and humus in these crevices. Moisture is able to seep deep into these crevices. Very little 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...
occurs. In this manner, rocky substrates provide moisture in the habitat. This saturation of crevices can only occur, however, if there is not a considerable runoff from the rock's surface and if there is abundant fine soil in the cracks that, in turn, retain water. The substrate
Substrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
, therefore, plays a critical role in the creation of micro-environmental "arid islands."
Sand readily store water because it is taken up easily and there is less evaporation except for the top layer. Very deep sand; however, has the problem of seepage. Yet in moderation shallow and deep sand substrates have water available to Pachypodium. With shallow sand substrates, Pachypodium grow on sand dunes near the sea. Where water is in deep sandy substrate, Pachypodium grow on sand "over" laterite
Laterite
Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock...
red soil. Laterite soil is a largely impermeable soil that traps water for the use of the flora that include Pachypodium.
Protection status
Internationally Pachypodium are protected under the CITES treaty. According to it, members of this genus cannot be collected from endemic, native locations within the landscape. They are not easily, readily imported and exported between nations either. The protection afforded by the CITES treaty responses to two issues:- The esteem the genus has within Collector's and Nursery Trade. As highly esteemed plants, succulent enthusiast desire to collect more and more species and cultivarCultivarA cultivar'Cultivar has two meanings as explained under Formal definition. When used in reference to a taxon, the word does not apply to an individual plant but to all those plants sharing the unique characteristics that define the cultivar. is a plant or group of plants selected for desirable...
s. In the case of Pachypodium, seedSeedA seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
, seedlings, and even mature, nursery-grown specimen plants are fortunately available readily in Nursery Trade.
- Destruction of the genus's endemic habitatsHabitat (ecology)A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...
, e.g. through agricultureAgricultureAgriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
.
Extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
of identified species seems yet unlikely, as the collection of seed and the cultivation of the plant safeguard the genus.
History of the genus
The early history of the genusGenus
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...
Pachypodium demonstrates the typical process of a taxon becoming a new genus. Initially debate occurred over if Pachypodium belonged to the genus Echites or if it constituted a separate genus. Pachypodium were first published as a unique genus, separate from Echites, by Leandley in 1830.
Then the debate centered on the nomenclature
Botanical nomenclature
Botanical nomenclature is the formal, scientific naming of plants. It is related to, but distinct from taxonomy. Plant taxonomy is concerned with grouping and classifying plants; botanical nomenclature then provides names for the results of this process. The starting point for modern botanical...
of species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
uniquely found in continental Southern Africa
Southern Africa
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...
. That changed when, in 1892, Baker
John Gilbert Baker
John Gilbert Baker was an English botanist.Baker was born in Guisborough, the son of John and Mary Baker and educated at Quaker schools in Ackworth and York....
contributed the first species accepted into the genus from Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
. The degree of speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
then turned to Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, where the count of species far exceeds those on the mainland.
In 1907, Costantin
Julien Noël Costantin
Julien Noël Costantin was a French botanist and mycologist who was a native of Paris.He studied at École Normale Supérieure de la rue d'Ulm. In 1881 he received his license in natural history and two years later earned his doctorate...
and Bois constructed the first monograph
Monograph
A monograph is a work of writing upon a single subject, usually by a single author.It is often a scholarly essay or learned treatise, and may be released in the manner of a book or journal article. It is by definition a single document that forms a complete text in itself...
, of Pachypodium, in which they enumerated 17 species, where ten were from Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
and seven were from continental southern Africa.
Natural history
There is no fossilFossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
records of Pachypodium known. Yet certain conclusions can be drawn from the geology of the landscape in Madagascar as to the past natural history
Natural history
Natural history is the scientific research of plants or animals, leaning more towards observational rather than experimental methods of study, and encompasses more research published in magazines than in academic journals. Grouped among the natural sciences, natural history is the systematic study...
of Pachypodium.