Zamia is a genus of cycad of the family Zamiaceae, native to North America from the United States (in Georgia and Florida) throughout the West Indies, Central America, and South America as far south as Bolivia. The genus is considered to be the most ecologically and morphologically diverse of the cycads, and is estimated to have originated about 68.3 million years ago.
Description
The genus comprises deciduous shrubs with aerial or subterranean circular stems, often superficially resembling palms. They produce spirally arranged, pinnate leaves which are pubescent, at least when young, having branched and simple, transparent and coloured hairs. The articulated leaflets lack a midrib, and are broad with subparallel dichotomous venation. Lower leaflets are not reduced to spines, though the petioles often have prickles. The emerging leaves of many Zamia species are striking, some emerging with a reddish or bronze cast (Z. roezlii being an example). Z. picta is even more distinctive, being the only truly variegated cycad (having whitish/yellow speckles on the leaves). Like all Zamiaceae, Zamia plants have "coralloid" (coral-shaped) roots which host nitrogen-fixing cyanobacteria. Stems are 3 to 25 centimetres (1.2 to 9.8 in) in diameter, and when arboreal, up to 5 metres (16 ft) tall. Leaves vary from 2 to up to 15 in number, and have an even number (5 to 60 pairs) of leaflets. petioles (leaf stalks) and rachis (leaf axis, to which leaflets are attached) may be smooth, or have prickles on the petiole and lower part of the rachis. Leaflets are linear to ovate and may be directly attached to the rachis, or may be on short petiolules. The edges of leaflets may be smooth, or may be toothed partly or around the whole leaflet. The morphology of leaflets is highly varied between even closely related species of Zamia, and within species and even within populations of a species. Studies have found that the amount of sun a plant is exposed to is responsible for differences in leaflet length, width, surface area, width ratio, shape, and density and thickness of leaflets. There is also significant morphological variation in leaflets between male and female plants in some species.
Reproduction Zamia sporophylls are born in vertical rows in cones, and the megasporophyll apices are faceted or flattened, not spinose. The fleshy seeds are subglobular to oblong or ellipsoidal, and are red, orange, yellow or rarely white. The endosperm is haploid, derived from the female gametophyte. The embryo is straight, with two cotyledons that are usually united at the tips and a very long, spirally twisted suspensor. The sperm of members from the genus are large, as is typical of cycads, and Z. roezlii is an example; its sperm are approximately 0.4 mm long and can be seen by the unaided eye. Seed-dispersal in Zamia is poorly documented for most species, but there are reports of birds and/or small to medium-sized mammals dispersing the seeds of a few Zamia species.
Pollination by insects It was long believed that Zamia plants, like all cycads, relied completely on wind pollination. In the 1980s it was discovered that at least some Zamia species were pollinated by beetles of the Pharaxonotha and Rhopalotria genera. Further studies have found that pollination by beetles is widespread in Zamia and other cycads. Fossils are now understood to indicate that cycads and beetles have been associated since the early Jurassic, about 200 million years ago. Pollen occurs in sticky clumps in the male cones. The clumps are broken up when Pharaxonotha beetles feed on them, scattering pollen grains throughout the interiors of the cone. Both Pharaxonotha and Rhopalotria adults, if present, become covered with pollen grains and then move to a female cone. As the beetles crawl around in female cones, a droplet at the tip of each micropyle (an opening into the ovule which will produce a seed) can capture a pollen grain as a beetle crawls past.
Thermogenesis Cycads, including Zamia species, are thermogenic, capable of raising the temperature of tissues in reproductive cones. Thermogenesis has been measured in 17 Zamia species, with the temperature of male (pollen) and female (ovule) cones (strobili) varying daily (Zamia plants are dioecious, with male and female cones on separate plants). Male cones ready to shed pollen heat up early in the evening, and then cool down, while receptive female cones heat up about three hours later and then cool down. Thermogenesis in reproductive cones typically occurs daily for about two weeks. The reproductive cones of cycads, including Zamia, emit combinations of volatiles that produce a distinctive odor. The odors may be distinctive for each species of Zamia, or for each species of pollinator. Male and female cones of a particular Zamia species emit nearly identical odors, although the odor produced by female cones is usually weaker than that produced by the male cones of the same species. The emission of odors is enhanced by the thermogenetic heating of the cones, and the movement of pollinating beetles between cones correlates with changes in temperature and odor level. Pharaxonotha and Rhopalotria beetles associated with Zamia species have receptors for infrared radiation on their antennae, by which they appear to detect the elevated temperature of the cones on Zamia plants. Beetles have been observed moving to cones on male plants when the cone's temperature is elevated, crawling into openings in the cone and emerging covered with pollen, and then moving to the cones on female plants when their temperature is elevated, again crawling into openings in the cone, presumably transferring the pollen.
Habitats With one exception, Zamia species are found only in the Neotropical realm in the Americas. The exception is Z. integrifolia, which has a range that extends into the Nearctic realm in northern Florida, and formerly into the southeastern corner of Georgia.
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