Cycads

Abstract

Present-day cycads comprise the diverse, modified remnants of a much larger group of gymnosperms which flourished in the Mesozoic Era, reaching their zenith in the Jurassic Period, about 160 million years ago. Distribution of extinct members of the Cycadales is limited to tropical and mild temperate regions. Cycads occupy a wide variety of habitats ranging from tropical rainforest to dry savanna. These plants have many xeromorphic adaptations, and in xeric habitats they may comprise a significant component of the vegetation. In mesic environments, cycads are usually restricted to the forest understorey, where they may form the principal vegetation component (Halliday and Pate 1976). In the Central American rainforests of Costa Rica and Panama, Zamia pseudoparasitica lives as an epiphyte in the canopy. Otherwise, cycads are terrestrial and have two principal growth forms. They may be arborescent (Figs. 1, 2) and achieve heights of 13–18 m (Lepidozamia hopei and Encephalartos transvenosus) or they may be small shrubs with subterranean stems like Zamia pumila. Most species are sparsely branched, and usually have one dominant axis (Fig. 1). Cycas circinalis and other members of this genus have irregular branching patterns and produce branches or “suckers” at the base of the primary axis (Fig. 2). Decapitation or wounding of the apex, as well as pronounced bending of the trunk, stimulates branching (Giddy pers. commun.). Arborescent species bear a superficial resemblance to palms, and the name “cycad” is derived from the Greek word meaning “palm-like”.