Cycads

Cycads are woody plants which produce seeds. Although they may have a general appearance which is readily identifiable by most people, they are usually linked to palms or ferns, when in fact they are not related to either. Cycads are actually a unique assemblage of plants and although they are grouped with the gymnosperms they are in point of fact unrelated to any other group of living plants.

Cycads certainly have a distinctive appearance which is related to their primitiveness or antiquity. Within the living seed plants they are nearly unique in that they produce motile sperm cells, and thus are an important link to the earliest of the ancient seed plants. Cycads flourished in eons past and reached their peak in the Mesozoic Era some 150 million years ago. While the fossil story may still be unclear, it is certain that cycads were more varied and profuse in earlier times and more widely distributed. Today cycads may be regarded as relicts consisting of small populations distributed disjunctly on many continents.

Living cycads are found in the tropical, subtropical and warm temperate regions of both the north and south hemispheres. While substantial numbers exist on the continents of Africa, Australia and South America they are also prominent in Central America (which has the greatest diversity) and the Caribbean Islands. Cycas is the most widespread genus, with representatives occurring as far north as Japan and others being scattered throughout various Pacific islands, China and India to Madagascar and the east coast of Africa.

Cycads are basically woody plants which have roots, a stem, leaves and reproductive structures known as cones.

The main roots of cycads are thickened and fleshy and as they may have storage capacities they are often termed tuberous. Along with the fleshy stem they may have contractile properties which serve to regulate the level of the stem in the ground. Specialised, upright-growing, branched roots, known as coralloid roots, are also produced by all species. These roots contain symbiotic blue-green algae which can fix nitrogen from the atmosphere.

The stems of cycads may be completely subterranean or emerge from the ground and be trunk-like. Soil depth may influence this development and in shallow, stony soils, species which normally have subterranean stems may develop an above-ground trunk.

The leaves of most cycads are once-divided (pinnate) and often develop an attractive palm-like crown. Those of Stangeria bear a strong resemblance to the fronds of a fern, whereas in the Australian genus Bowenia the leaves are twice-divided (bipinnate).

Cycads reproduce when mature by the production of cones. A plant is either male or female and the cones of each sex are usually quite different in size and shape and to a much lesser extent colour. Specialised woody growths on the cones, called sporophylls, bear the sexual parts with those of the male cone producing pollen and on a female cone they bear large ovules which if fertilised develop into seeds. The seeds of cycads are relatively large and have an outer layer (sarcotesta) which is often colourful.