Vegetative morphology

All living cycads are dioecious, long-lived, slow-growing woody perennials (also see Foster and Gifford, 1989; Norstog and Nichols, 1997).

Stems are pachycaul (short and thick) with a broad pith and cortex and manoxylic wood (a wood type that contains abundant parenchyma, typical of cycads), and may be subterranean or aerial. Internal stem structure is characterized by a eustele with endarch protoxylem, where a small amount of manoxylic wood is produced from a bifacial vascular cambium. Leaf vasculature traces in the stem are girdling; that is, traces arise from the stele at a point opposite the point of leaf attachment and dichotomously branch, with the two branches girdling the stele as they transverse the cortex and meeting again before entering the petiole. This condition is known in no other extant group of seed plants. Axillary buds are absent, and vegetative branching is either dichotomous or adventitious.

Cycad roots are heteromorphic, with contractile and coralloid roots in addition to normally functioning roots. Contractile tissue is present in roots and, to a lesser extent, stems, especially in juvenile plants (Stevenson, 1980). Coralloid roots are highly modified roots, with apogeotropic growth and extensive dichotomous branching, with the branches shortened, thickened and modified to internally accommodate symbiotic cyanobacteria (Nathanielsz and Staff, 1975).

Leaves are large, spirally arranged, pinnate, bipinnate or bipinnatifid, exstipulate or stipulate or with a stipular hood, loosely pubescent at least when young, and usually arranged in crowns on the stem-apex. The leaves are often scleromorphic, owing to the strong fibres, thick cuticle and thick hypo-dermis. Leaf-bases may be persistent or abscisent, depending on species. Leaves are interspersed with scale-leaves (cataphylls), except in Stangeria and Bowenia.

The pachycaul habit of modern-day cycads is thought by some to be a Tertiary development, many Mesozoic cycads having dense wood and a leptocaul habit (Delevoryas, 1993).

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