Cycad

Cycads are seed plants with a stout, woody, and usually unbranched cylindrical trunk, and a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow slowly^[3] and have long lifespans. Because of their superficial resemblance to palms or ferns, they are sometimes mistaken for them, but they are not closely related to either group. Cycads are gymnosperms (naked-seeded), meaning their unfertilized seeds are open to the air to be directly fertilized by pollination, as contrasted with angiosperms, which have enclosed seeds with more complex fertilization arrangements. Cycads have very specialized pollinators, usually a specific beetle, and more rarely a thrips or a moth.^[4]

The leaves are pinnate (shaped like feathers), with a central leaf stalk from which parallel ribs emerge from each side of the stalk, perpendicular to it. The leaves are typically either compound, or have margins so deeply cut as to appear compound. The Australian genus Bowenia and some Asian species like Cycas multipinnata, C. micholitzii and C. debaoensis, have bipinnate leaves, the leaflets each having subleaflets.^[5] The apex of the stem is protected by modified leaves called cataphylls.^[6]

Cycads superficially resemble palms in foliage and plant structure, occur in similar climates, and are often mistaken for them. However, they are so distantly related that they are classified in different phyla. Their similarities are caused by convergent evolution. Differences between cycads and palms include the cones (strobili) of cycads: they are gymnosperms, whereas palms are flowering plants and bear fruit. Both groups' mature foliage look similar, but young emerging cycad leaves – before they unfold and shift into place in the rosette crown – resemble a fiddlehead fern; in contrast, new leaves of palms are just miniature versions of a mature frond. Another difference is in the stem: Both phyla show scarring on their stems – below the rosette, where leaves used to attach – but the scars on a cycad's trunks are helically arranged and small; the scars on palm trunks are a circle, that wraps around the whole stem. The stems of cycads are generally rougher and shorter than those of palms.^[7]

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  Growth habit
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  Rosette of pinnate leaves around a cylindrical trunk
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  Leaves and strobilus of Encephalartos sclavoi
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  Bowenia spectabilis: plant with single leaf

The living cycads are found across much of the subtropical and tropical parts of the world, with a few in temperate regions such as in Australia.^[20] The greatest diversity is in the Americas, but they also grow in China, South and Southeast Asia, the Pacific islands, and southern and tropical Africa.^[21][19] Some are xerophytes that can survive in desert or semi-desert climates,^[22] others in wet rain forest conditions,^[23] and some in both.^[24]

Cycads accommodate nitrogen-fixing cyanobacteria in their coralloid roots.^[25] The cyanobacteria produce a neurotoxin, BMAA, that accumulates in the plants' seeds, possibly as a defense against herbivores.^[26][27]

Cycads have acquired a family of genes (fitD) from a microbe, most likely a fungus, via horizontal gene transfer, which gives them the ability to produce an insecticidal toxin.^[28]

Nuts of Cycas orientis (nyathu) are eaten by the Yolngu in Australia's Arnhem Land. They are harvested during the dry season and soaked in water overnight before being ground into a paste, wrapped under bark, and cooked on an open fire.^[29] A pair of namele cycad leaves, representing peace, appears on the Flag of Vanuatu.^[30][31] Cycads are used as narcotics in Mexico, where they are among the substances called "peyote", while in South Africa, Encephalartos is used for the same purpose. In both regions, collecting for the drugs market is harming wild cycad populations.^[32] Cycads all over the world are in decline, with four species on the brink of extinction and seven species having fewer than 100 plants left in the wild.^[33][34][35] The Fossil Cycad National Monument was a protected area in South Dakota from 1922, containing many fossils of Cycadeoidea.^[36] After vandalism of the exposed fossils, the status was withdrawn in 1957.^[37]