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The Cycadophyta (cycads) are "gymnosperms", along with the conifers, Ginkgo, and Gnetales ("gymnosperms" is a term commonly used to refer to the seed plants that remain when you take away the diverse and often flashier angiosperms, or flowering plants; it does not refer to a monophyletic group). The exact relationship of cycads to other living gymnosperms based on molecular data remains controversial. Cycads superficially resemble palms or large ferns, but are not closely related to them.
Based on the fossil record, cycads appeared in the Pennsylvanian, approximately 300 million years ago, and dominated the Mesozoic forests along with the conifers and Ginkgoales. Modern cycads, however, are largely confined to the tropics and subtropics of both the Old and New Worlds, with the genus Cycas having the widest distribution and extending as far north as Japan. Today many species (and even some genera) of cycads are facing extinction in the wild.
The number of cycad species described gradually increased as exploration of cycad habitats
spread, with seven species described by 1800, 33 by 1850, and 85 by 1900. The number of genera recognized also rose to six in 1850 and 10 by 1900. There are now thought to be around 300 species and between 10 and 12 genera in three to five families.
One unusual feature of cycads is their development of specialized roots (known as coralloid roots) that are inhabited by nitrogen-fixing cyanobacteria of the genus Nostoc. As far as is known, cyanobacteria form symbiotic relationships with all cycads.
Most, if not at all, cycads are insect pollinated.
(Hill et al. 2003 and references therein; Chaw et al. 2005 and references therein)
A rich and authoritative online resource with a wealth of information about cycads (including a list of key references and a page for each living cycad species with a description and information about geographic distribution, habitat, and conservation status) is The Cycad Pages. Another excellent resource is the website for the IUCN/SSC Cycad Specialist Group.