WHISK FERN

any member of the genus Psilotum in the division Psilotophyta. A whisk fern has water- and food-conducting tissues but lacks true leaves and roots. Photosynthesis occurs in the aerial stems, and water and mineral absorption in the horizontal, underground, rootlike stems (rhizomes). There are two phases in the life cycle of a whisk fern. The large asexual plants (sporophytes) produce spores that develop into very small, colourless sexual plants (gametophytes), which grow on tree trunks or develop underground. Eggs and sperm are produced in special structures on their surfaces. Union of these gametes initiates the sporophyte phase. The genus Psilotum contains two species of tropical plants with whisklike, woody green stems and scalelike leaves. One species, P. nudum, grows as far north as Florida and is cultivated as a greenhouse plant.