Dioon edule
Encyclopedia
Dioon edule is a cycad native to 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...

, also known as palma de la Virgen.

Two subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...

 are known
  • Dioon edule subsp. angustifolium
  • Dioon edule subsp. edule

Dioon Edule var. Edule

Dioon edule var. edule belongs to the Zamiaceae family within the order Cycadales. It is commonly called the Chestnut Dioon and is endemic to the eastern coast of Mexico. 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 are among the oldest seed plants and even pre-date the dinosaurs (The United States Botanical Garden). D. edule was originally described by John Lindley
John Lindley
John Lindley FRS was an English botanist, gardener and orchidologist.-Early years:Born in Catton, near Norwich, England, John Lindley was one of four children of George and Mary Lindley. George Lindley was a nurseryman and pomologist and ran a commercial nursery garden...

 in 1843. There are currently two subspecies known: D. edule subsp. edule and D. edule subsp. ausgustifolium.

Anatomy

Dioon edule is easily recognized from other Dioon species because it lacks spines on the borders of leaflet
Leaflet
A leaflet in botany is a part of a compound leaf. A leaflet may resemble an entire leaf, but it is not borne on a stem as a leaf is, but rather on a vein of the whole leaf. Compound leaves are common in many plant families...

s. Spines are present on juvenile leaves, however, they are lost when the plant matures. D. edule has a crown of pinnate leaves which measure around 135 cm along. Lateral bud
Bud
In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of the stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be specialized to develop flowers or short shoots, or may have...

ding is present and mucilage, or 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 excreted from any cut surfaces on the plant body. The plant has a large central medulla and a single vascular bundle
Vascular bundle
A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in vascular tissue, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will include supporting and protective tissues...

 containing the xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

, phloem
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...

 and cambium
Cambium (botany)
A cambium , in botany, is a tissue layer that provide undifferentiated cells for plant growth. It forms parallel rows of cells, which result in secondary tissues....

. Since the organism is slow growing and xerophilous, only a small conduction channel is needed, unlike other plants. Stomata are also present to aid in 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...

 and assimilation
Cultural assimilation
Cultural assimilation is a socio-political response to demographic multi-ethnicity that supports or promotes the assimilation of ethnic minorities into the dominant culture. The term assimilation is often used with regard to immigrants and various ethnic groups who have settled in a new land. New...

 and are associated with sporophyll
Sporophyll
A sporophyll is a leaf that bears sporangia. Both microphylls and megaphylls can be sporophylls. In heterosporous plants, sporophylls bear either megasporangia , or microsporangia...

s. D. edule has three or four large adventitious 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 surrounded by many smaller ones which harbor small nodules containing tannin
Tannin
A tannin is an astringent, bitter plant polyphenolic compound that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids.The term tannin refers to the use of...

s. Roots are composed of large amounts of corky secondary tissue made by phellogen. It is often very difficult to tell males and females apart from one another until they develop a cone. Cones generally do not show a visual difference; however, cone intervals can be used to determine the sex of the plant. Females usually have a much longer cone interval than males, typically 10–52 years as compared to 2.8-8.8 years.

Habitat

Dioon edule is endemic to the eastern coast of Mexico. It is commonly found distributed throughout the Sierra Madre Oriental
Sierra Madre Oriental
The Sierra Madre Oriental is a mountain range in northeastern Mexico.-Setting:Spanning the Sierra Madre Oriental runs from Coahuila south through Nuevo León, southwest Tamaulipas, San Luis Potosí, Querétaro, and Hidalgo to northern Puebla, where it joins with the east-west running Eje Volcánico...

 from Veracruz
Veracruz, Veracruz
Veracruz, officially known as Heroica Veracruz, is a major port city and municipality on the Gulf of Mexico in the Mexican state of Veracruz. The city is located in the central part of the state. It is located along Federal Highway 140 from the state capital Xalapa, and is the state's most...

 to the Nuevo Leon. It commonly resides in tropical 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...

 thorn forest
Thorn forest
A thorn forest is a dense, scrublike vegetation characteristic of dry subtropical and warm temperate areas with a seasonal rainfall averaging . This vegetation covers a large part of southwestern North America and southwestern Africa and smaller areas in Africa, South America, and Australia...

s and oak
Oak
An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...

 woodland
Woodland
Ecologically, a woodland is a low-density forest forming open habitats with plenty of sunlight and limited shade. Woodlands may support an understory of shrubs and herbaceous plants including grasses. Woodland may form a transition to shrubland under drier conditions or during early stages of...

s. They are usually found at an altitude of 500–1000 feet in harsh areas including exposed, shallow 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. Most of the areas in which D. edule resides are subjected to very dry climates and frequent brushfire
Brushfire
Brushfire may refer to:* Brushfire Records, a record label* Brushfire , a fictional supervillain* Brushfire Fairytales, an album by Jack Johnson* Brushfire Inc., an advertising agency in Cedar Knolls, NJ* Brush fire, a wild fire...

s which impact their survival and distribution.

Ecology and Distribution

Disturbances alter the population dynamics
Population dynamics
Population dynamics is the branch of life sciences that studies short-term and long-term changes in the size and age composition of populations, and the biological and environmental processes influencing those changes...

 of Dioon edule population
Population biology
Population biology is a study of populations of organisms, especially the regulation of population size, life history traits such as clutch size, and extinction...

s. If a population is heavily dominated by adult plants, the disturbance will be minimal as compared to the population consisting of younger plants. This is due to their ability to survive harsh conditions. D. edule exemplifies a Type III survivorship curve
Survivorship curve
A survivorship curve is a graph showing the number or proportion of individuals surviving at each age for a given species or group...

 and clumped distributions. It is found mostly on shallow, rocky soils due to competition with other, faster growing species for water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...

s and seed dispersal
Seed dispersal
Seed dispersal is the movement or transport of seeds away from the parent plant. Plants have limited mobility and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic vectors. Seeds can be dispersed away from the parent plant...

. In response, they have developed a selective advantage for growing in these harsh climates including vigorous tap roots for anchorage and water conduction. Furthermore, it is harder for rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s to reach the plant and seed
Seed
A 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...

s if they are established in difficult locations to reach. In addition to competition
Competition (biology)
Competition is an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource used by both is required. Competition both within and between species is an important topic in ecology, especially community ecology...

, causal agents of D. edule's random, clumped distribution include predation
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

, variations in 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...

 and 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...

, and measures of seed dispersal.

Death rates among seedling
Seedling
thumb|Monocot and dicot seedlingsA seedling is a young plant sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle , the hypocotyl , and the cotyledons...

s are considerable, which is detrimental to a slow growing plant such as the Dioon edule. Seeds must combat harsh, dry climates and frequent bushfires. Most mature plants are heavily armored and able to withstand these fires. Smaller seeds and juvenile plants are not so fortunate. However, the release of nutrients and 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...

s encourages females to develop cones during the following season. In addition to climate, seeds must also avert rodent predators. This is reasonably unusual because cycad seeds and foliage are poison
Poison
In the context of biology, poisons are substances that can cause disturbances to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism....

ous to most mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, excluding these several species of rodents. Young leaves of D. edule are also eaten by the Eumaeus debora butterfly
Butterfly
A butterfly is a mainly day-flying insect of the order Lepidoptera, which includes the butterflies and moths. Like other holometabolous insects, the butterfly's life cycle consists of four parts: egg, larva, pupa and adult. Most species are diurnal. Butterflies have large, often brightly coloured...

 larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e.

One of the ways in which cycads are identified includes leaflet characteristics, which were not heavily studied when cycads were first identified. With subsequent studies, it was found that leaflet width was related to geographic dispersal. Wider leaves typical of Dioon edule subsp. edule are found in the south whereas the narrower leaves of D. edule subsp. augustifolium are found to the north. Evidence suggests D. edule susbp. edule was much more widespread than it is today and became fragmented thus allowing for genetic isolation. It is also speculated that the narrower leaves are endemic in the northern subspecies due to less temperate and humid environmental conditions in these areas.

In addition, endomycorrhizae are present in roots and allow for increased absorption of nutrients and water. Symbiotic blue-green algae are also found in the coralloid roots. They assist the plant by fixing nitrogen thus giving the plant essential nutrients in exchange for protection. Cycads in general have significant roles in the nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 budget of their respective ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s.

Endangerment

Both subspecies of Dioon edule are on the verge of becoming endangered species
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...

. They are threatened by human-caused habitat destruction
Habitat destruction
Habitat destruction is the process in which natural habitat is rendered functionally unable to support the species present. In this process, the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction by human activity mainly for the purpose of...

 and collection
Overexploitation
Overexploitation, also called overharvesting, refers to harvesting a renewable resource to the point of diminishing returns. Sustained overexploitation can lead to the destruction of the resource...

 for horticultural
Horticulture
Horticulture is the industry and science of plant cultivation including the process of preparing soil for the planting of seeds, tubers, or cuttings. Horticulturists work and conduct research in the disciplines of plant propagation and cultivation, crop production, plant breeding and genetic...

 and medicinal
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 purposes. Bouncing back from these disturbances is extremely difficult for slow growing plants such as D. edule. The United States Botanical Garden suggests that plant material should be conserved
Conservation biology
Conservation biology is the scientific study of the nature and status of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction...

, seed bank
Seed bank
Seed bank may refer to:*Seedbank, a repository of preserved seeds*The store of viable plant seed in an ecosystem; for example:** Soil seed bank, the viable seed present in the soil;...

s should be established and legislation
Legislation
Legislation is law which has been promulgated by a legislature or other governing body, or the process of making it...

 regarding trading cycads and protection of their habitats should be implemented and enforced (The United States Botanical Garden).
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