Psilotum is a genus that has only two species. They are Psilotum nudum and Psilotum flaccidum. The word Psilotum is derived from the Greek word Psilos which means naked or bare which is in regard to its lack of true leaves on the stem. It belongs to the division Pteridophyta which essentially comprises of plants that reproduce by means of spores rather than seeds.
Psilotum nudum is more commonly known as the Whisk Fern, and is also known by the scientific name Psilotum triquetrum. On the other hand, Psilotum flaccidum, known more as a Hawaiian fern ally is also known by the scientific name Psilotum complanatum. Both these species differ in their distribution and general description.
• General description
Distribution: Psilotum nudum is a terrestrial plant that grows in humus, among rocks and at the base of trees. At times, it may adapt itself to grow as an epiphyte on trees. This species of Psilotum is found in both hemispheres in the tropics and subtropics, but not temperate regions. Psilotum flaccidum is restricted to a few Pacific Islands, Jamaica and Hawaii. Mexico and Florida are also regions of P. flaccidum occurance. It is an epiphyte which hangs from trees.
Common features: Both Psilotum species are slender and medium sized. They do not possess leaves but rather have scale leaves present on the stem. Roots are absent. In place of a root system, there is an underground or subterranean stem or rhizome that possess rhizoids and serve as organs of absorption / food intake. From this rhizome, the stems arise. The stems of the plant are either upright (P. nudum) or pendulous (P. flaccidum). The stems which are ridged (2-3 ridged), are dichotomously branched or forked. In the case of P. nudum, the branches or branchlets tend towards being cylindrical whereas, in P. flaccisum, the stems are flat in cross section.
The scale-like foliar appendages on the stem lack veins and the mesophyll is not differentiated as in higher plants. The sterile scale leaves are awl-shaped, whereas the fertile leaves are bifid in appearance.
Reproductive features: Psilotum species reproduce by means of vegetative reproduction (gemmae or brood bodies)and by sexual reproduction through the formation of spores and sporangia.
Psilotum is a prehistoric genus with little to no change in its physical and demographic patterns. Among the vascular plants, they are one of the oldest and the simplest. Yet their presence and contribution to the world of plants have helped with understanding the evolutionary pattern of Pteridophytes and higher vascular plants.
