These are mostly located in tropical and subtropical regions. They appeared at least 250 MYA during the Permian and were exceedingly common at that time. Now, 11 genera still exist containing 140 species.

Zamia pumila (shown above), is found in the US in Floridian sandy woods and is the only native US cycad

Most are fairly large, some reaching 50+ feet in height. The trunk is covered with the bases of leaves that have dropped. Living leaves are concentrated on the top as a crown. Although the central area is just pith, there is an outer ring of vascular tissue of 2nd origin.

The trees can be toxic, as with many of these early plants, and may contain both neurotoxic as well as carcinogenic compounds.

AGAIN AS WITH A NUMBER OF EARLY PLANTS, THEY SUPPORT CYANOBACTERIA WHICH ALLOWS THE PRODUCTION OF FIXED-N, CRITICAL IN THESE LOW NUTRIENT SANDY SOILS.

Reproductively, all are diecious; that is they have both male and female trees. The Sporangia are attached and clustered into large conelike structures borne near the apex of the plant. Although both male & female cones are large, the ovulate one is especially so.

There is a suggestion of early insect dependence for pollination. Beetles of specific groups associate with the cones, especially male, carrying their whole life cycle within its confines. It may be speculated that this association is quite old..

The male gametophyte grows a pollen tube, through which the large, highly flagellated sperm ( thousands of flagella) swims to the egg.