Cycadophyta

Cycads resemble palm trees with fleshy stems and leathery featherlike leaves. The 10–11 genera and 305 living species are distributed throughout the world but are concentrated in equatorial regions. Cycads are typically short and squat, although the Australian cycad Macrozamia hopei may reach 19 metres (62 feet) in height. Given their attractive foliage and sometimes colourful cones, the plants are used in gardens in warmer latitudes and some may even thrive indoors.

Cycads are dioecious, meaning an individual only produces male or female cones. All genera bear microstrobili consisting of an axis with microsporophylls inserted in a close helical arrangement. The microsporophylls are reduced leaves with abaxial sporangia. In the genus Cycas, ovules are borne among the edges of the stalk of a reduced leaf with a bladelike region still present. Those modified leaves, or megasporophylls, are clustered at the apex of the plant but not arranged in a cone. All other genera of cycads, however, have megastrobili, with the megasporophylls reduced and not leaflike in appearance. Each megasporophyll has a stalk with an expanded distal portion, on the inner face of each of which develop two seeds.

Cycad leaves are compound, with thick leathery leaflets borne in a pinnate (featherlike) arrangement on a main axis. Produced among the normal photosynthetic leaves are stiff scalelike leaves called cataphylls that contribute to the persistent “armour” on the trunk surfaces.

Cycad stems feature a large fleshy pith surrounded by a cylinder of xylem and phloem. The plants have less secondary vascular tissue than conifers, which makes the wood less dense. Many cycad species host cyanobacteria (also known as blue-green algae) in nodules in the roots and may form coralline masses on the ground surface known as coralloid roots. It is thought that those bacteria fix atmospheric nitrogen into a form usable by the plant.