All living cycads are slow-growing, long-lived, dioecious, pachycaul, woody perennials. The stems have a broad, starch-rich cortex, and may be subterranean or aerial in different species. Leaf vasculature traces in the stems are girdling (i.e. traces arise from the stele at a point opposite the point of leaf attachment and encircle the stem). Axillary buds are absent, and any branching occurring is either dichotomous (by equal division of the growing apex) or adventitious (arising from undifferentiated cortical tissues). Leaves are spirally arranged in crowns on the stem apex, pinnate (bipinnate in Bowenia), and lacking stipules (stipules or stipular hoods are present in some genera not occurring in Australia). Leaves are pubescent when young. Leaves are interspersed with reduced scale-leaves, known as cataphylls (except in Stangeriaceae). Sporophylls of both sexes are simple and aggregated into cones (except in Cycadaceae). Male sporophylls carry numerous sporangia on the under- or abaxial surface, and sporangia open by slits to shed pollen. Pollen is weakly ornamented, cymbiform, monosulcate, and bilaterally symmetrical. Male gametophytes are large, multiflagellate and motile. Female sporophylls are simple and entire (dissected in Cycadaceae). Seeds are large, with a 2 layered testa; an inner woody layer, and an outer, often distinctly coloured, fleshy layer. Endosperm is haploid, derived from the female gametophyte. The embryo is straight, with 2 cotyledons which are usually united at the tips, and germination is cryptocotular. Cycad roots are heteromorphic, with contractile and coralloid roots in addition to normally functioning roots. Contractile roots are present in all cycads (above), particularly in juvenile plants. These serve to draw the sensitive growing apex of seedlings below the soil surface, affording protection from the drought and fires that are a frequent feature of many cycad habitats. Coralloid roots are highly modified roots, with apogeotropic growth and extensive dichotomous branching, with the branches shortened, thickened, and modified to internally accomodate symbiotic cyanobacteria. These bacteria fix atmospheric nitrogen and contribute to the nutrient needs of the plant, providing an advantage in the nutritionally deficient soils occurring in many cycad habitats. The cycads are a distinct monophyletic (natural) group, defined by the presence of cycasin, the girdling leaf traces, the absence of axillary buds, the simple megasporophylls, and the primary thickening meristem which gives rise to the pachycaul habit. The three cycad families commonly recognised are distinguished as follows:
,==CYCADACEAE================= Cycas
|
| ,======= Encephalartos
| ,===|
| ,===| `======= Lepidozamia
ANCESTOR<=| | |
| | `=========== Macrozamia
| |
| |=============== Bowenia
| |
| |=============== Stangeria
`==ZAMIACEAE==|
|=============== Dioon
|
| ,=========== Ceratozamia
`===|
| ,======= Microcycas
`===|
| ,=== Chigua
`===|
`=== Zamia
|
