|
An order of tropical and subtropical gymnosperms containing one family, the Cycadaceae, divided into some 9 genera and about 100 species. Cycads have a thick, generally unbranched, stem bearing a terminal rosette of large palmlike leaves. Scale leaves are also present on the upper part of the stem and apex. Cycads are very slow-growing and vary considerably in height between species: some have a large proportion of the stem underground and reach a height of only a few metres, while others attain heights up to 15 m. Most of the mechanical support of the stem is provided by the sclerenchymatous leaf bases. The starchy nature of the stem (and of the seeds) has led to the use of certain cycads, the sago palms, as a food source. Cycads are dioecious and bear exceptionally large female cones and ovules. The egg cell of the archegonium is larger than that found in any other group of plants. Cycads first appeared in the late Palaeozoic and are thought to be most closely related to the Pteridospermales.
|
