The cycads resemble ferns with their frond-like leaves.  However, they do produce seeds on specialized leaves.  Some taxa like Cycas produce megasporophylls that look like slightly modified vegetative leaves.  Others like Zamia and Dioon produce highly modified megasporophylls in simple strobili (cones).    

The descriptions of Bold et al. (1987) and Bierhorst (1971) follow the classical view that the cycads are the most primitive of the living seed plants.  Current analyses suggest the same thing.  For example, the Tree of Life Project illustrates the cycads as a surviving seed fern.  Tudge (2000) has a similar (but more truncate) scheme.  The fossil history of the group suggests that this could be true.  Cycads appear in the Carboniferous and became very abundant during the Mesozoic.  The few survivors today are dioecious and rarely dominant in those environments where they occur.  The molecular  evidence of Chaw et al. (1999) also is consistent with the seed fern connection in that that the cycads are basal in their gymnosperm clade.  Pearson (1995) departs from the classical view in that he shows the cycads as a major group in the line that leads to the flowering plants and not basal in the gymnosperm groups..   

Descriptions of the groups and the taxonomic system follows Bold et al. (1987) and Bierhorst (1971).

I. PHYLUM CHARACTERISTICS

A. Structure

Habit: The cycads are fern-like seed plants with barrel-shaped stems and large frond-like leaves. All extant species are dioecious.

Pollen: Pollen walls with 1 suture and no saccus. The microgametophyte has a single prothallial cell, a stalk cell, 2 multiflagellate sperms and a tube cell.

Microstrobilus: Simple strobilus with numerous abaxial microsporangia on the microsporophylls.

Seeds: Usually large with 3 integument layers around a massive nucellus. Archaegonia (each with 2 neck canal cells) develop at the micropylar end of the megagametophyte. The embryo is dicotyledonous.

Megastrobilus: Simple, usually compact and cone-like.

Stems: Generally barrel-shaped (although some can grow to 18m tall), fleshy with weak secondary growth. Stems are primarily cortex with mucilage canals.

Leaves: Large and frond-like with sunken stomata.

Roots: Roots are knotty (coralloid) and fleshy; some with symbiotic blue-green algae.

B. Ecology: All extant plants occur in tropical or subtropical. Their fossil history dates from the late Carboniferous to the present. They were very abundant during the Mesozoic.

II. TAXONOMY: As I have defined this phylum, the CYCADOPHYTA has a single class, CYCADOPSIDA, with a single order, CYCADALES.

examples: Cycas, Dioon, Zamia.