Description[ edit ] Dioons are dioecious , palmlike shrubs with cylindrical stems, usually with many leaves. The species in the genus Dioon are perennial, evergreen cycads with cylindrical stem axis is partially in the ground. The plant is thickened and made of soft wood, rarely having above ground branches. Leaf bases are persistent or shedding to leave smooth bark. The leaves are pinnate, spirally arranged, interspersed with cataphylls , with leaflets not articulated and lacking a midrib. The lower leaflets are often reduced to spines.
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Description[ edit ] Dioons are dioecious , palmlike shrubs with cylindrical stems, usually with many leaves. The species in the genus Dioon are perennial, evergreen cycads with cylindrical stem axis is partially in the ground. The plant is thickened and made of soft wood, rarely having above ground branches. Leaf bases are persistent or shedding to leave smooth bark. The leaves are pinnate, spirally arranged, interspersed with cataphylls , with leaflets not articulated and lacking a midrib.
The lower leaflets are often reduced to spines. The sporophylls are not in vertical rows in cones, and the megasporophyll apices are broadly flattened, upturned, and overlapping. However, typical specimens of other species are only between three and six feet high or remain even smaller.
Dioon can get very old, maybe even over years. Dioon edule and Dioon tomasellii have the most widespread ranges. Most species have highly limited geographical ranges. Leaves[ edit ] Leaf of Dioon spinulosum with are identified by the veining and prickly margin The leaves are paired pinnate and are spirally on the stem axis.
Some leaves are initially wrapped in protective leaves during their development, which are called cataphyll. Unlike other cycads such as Stangeria eriopus , the juvenile fins are not curled either transversely or longitudinally, but straight. The lower leaflets are often reduced to their petiole. The petioles are uncorned and thickened at the base.
The leaflets are simple, often with a prickly edge. The primary nerve consists of many forked dividing nerves, without a recognizable midrib. The nerves spring directly from the edge of the leaf rhachis and then run towards the plumage axis. The spars are hairy at least on young leaves. The hairs trichomes are colorless, branched or simple. The stomata for the gas exchange are found either only on the underside of the leaf, or in some species on both sides.
Glands are not otherwise colored and difficult to identify. The cells of the leaf epidermis are extended parallel to the plumage axis. Roots[ edit ] Like other cycads, coral-like roots are sometimes formed that grow up from the primary root and branch out quickly just below the soil surface.
The individual roots are thickened rhizomes and are inhabited by cyanobacteria of the genus Nostoc, which mainly used them for nitrogen fixation. There also seems to be a symbiotic relationship with arbuscular mycorrhizal fungi. The male cone is stalked. The male cone scales called microsporophyll, which are flattened and bent upwards, sterile at the tip.
They are arranged spirally around the pin axis. Each microsporophyll carries on its underside abaxial side a variety of pollen sacs.
These open with slots, from which the pollen then escapes. The pollen grains are streamlined and have only one germ line monosulcate. The male cones fall off after one year. The female cones are also stalked and can remain on the plant for more than a year. The female microsporophyll are more leaf-like than those of other cycads, the greatest similarity to those of the genus Cycas. The scales are flattened at the top, widened and bent up.
Each sporophyll carries two, rarely three, ovules. These are straight to the axis orthotropic , are inclined inwardly to this inverse and hang on the axis-facing side of the thickened scales. The opening at the tip of the ovules micropyle is very small, leaving only a narrow opening on the Nucellus.
The megaspore sheath thickened from 3—4. The number of archegonia varies between one and ten. The nucleus of the egg is unusually large.
The largest female cones of the genus D. The seeds are spherical, ovate or ellipsoidal and reach a diameter between about one and four centimeters. They are surrounded by a fleshy, white or cream-colored seed coat. The seed coat consists of three layers, a fleshy outer and inner layer, between which a layer of stone cells lies. The embryo is straight with two cotyledons, these are fused at the top. The embryo carrier suspensor is very long and spirally twisted.
The seeds are radiated radiosperm. The germination takes place cryptocotylar, i. Female cone of Dioon edule Male cone of Dioon mejiae cone of Dioon merolae Distribution[ edit ] Dioon species can be found in Central America between the 15th and 29th northern latitude and are part of the Neotropical floral kingdom. The distribution area includes Honduras and parts of Mexico ; it is limited by orographic conditions height structures and limited to the north to south mountain ranges in the region.
Despite the widespread distribution and ecological plasticity of the genus it is not a vegetation-determining component.
The species are found mainly in the tropical rain forest and in tropical wet forests, but also in tropical deciduous rainforests, with softwood rainforests, on dry, rocky slopes, in canyons and on dunes near the coast.
Fossil found in northern Alaska from the Eocene While the climate was still very warm at the beginning of this geological age, around 2. In the Paleocene During the Tertiary period, about 2. A spread to South America was not possible because the two continents were not connected until about 3 million years ago.
After that, the size of the seeds was the main obstacle to further spreading Dioon into the Caribbean and further south. Ecology[ edit ] Megasporophyll from Dioon edule with ripe seeds. Right: the dry brown stalk, yellow: two seeds, left: the strongly hairy, flat part of the sporophyll Pollination of Dioon by wind pollination or by animals, has not yet been determined.
The nature of the pollen light, dry, very large number rather suggests a wind pollination, also increases the number of fertilized ovules in the immediate vicinity of a pollen source. On the other hand, the inverse position of the ovules speaks against wind pollination, as the pollen is difficult to escape there and fall mainly on the sterile sections in the cone. For cycads of the genus Zamia already a pollination by the weevil Rhopalotria mollis has been detected.
Species of the genus Rhopalotria could also be found in cones of Dioon, in which there was also a large number of beetles of the genus Pharaxonotha , which feed on pollen indicating pollination by animals.
One species, D. For D. Locations are destroyed primarily by clearing and reclamation of the land. Dioon species are also very popular with international collectors. Plants are dug up and sold. The insect originally comes from Southeast Asia and infests there cycads. The pest has now been introduced to Florida , California , India and the UK, where it causes major damage to cycads. There he was also found at D. If the species were to penetrate Central America, it could drastically reduce or even destroy the populations of Dioon in a short time.
Dioon is the only genus in the tribe of the Diooeae, which together with the Encephalarteae forms the subfamily of Encephalartoideae. This classification was based on morphological considerations.
A morphological study of relationships in fossil and extant taxa revealed the following cladogram : .
Dioons: The hardy Mexican cycads
Torres Brittonia Altitude range: metres above sea level. Habitat and Ecology: Plants occur in a forest dominated by Pinus and Quercus and also in tropical semi decidious forest with soils developed on sedimentary rocks and karst limestone. Specimens are found growing on steep sandstone cliffs, along thickly vegetated watercourses. It is considered threatened in the wild.
Dioon, Cycad, Golden Dioon 'Santiago la Chiguiri'