The whisk fern (Psilotum spp., family Psilotaceae) splays its leafless, whisk-like branches upward, and is a living fossil from the time before the dinosaurs. It can grow as an epiphyte in moist climates or as a terrestrial plant in drier areas. Found in the tropics from around the world, the whisk fern is descended from the first vascular land plants, the Rhyniophytes, which appeared about 400 million years ago. It is not a true fern, unlike the popular Boston fern, but both the whisk fern and true ferns are ancient plants when compared to the flowering plants or angiosperms.
The leaved genus Tmesipteis (family Tmesipteridaceae) and Psilotum are the only representatives of the division Psilophyta (order Psilopsida). The principal usefulness of Psilotum to humans lies in their limited decorative use, and in scientific study as a living example of a very ancient land plant.
The primitive nature of the whisk fern is underscored by its having flagellated sperm, unlike the more advanced flowering plants, the angiosperms. The simple branched stems of Psilotum recalls the structure of the rhyniophytes, and the whisk fern is unique among living vascular plants in its lack of roots and leaves.
In place of roots the whisk fern has rhizomes, that is, modified underground stems. In Psilotum nudum the rhizome occurs with a mutualistic fungus in a type of mycorrhiza useful for obtaining necessary nutrients. Because Psilotum is without leaves, the interior parts of the stem conduct food and water, known as the vascular cylinder. In Psilotum the vascular cylinder lacks a central part made of large, open-looking cells, called pith. The lack of these cells defines the type of vascular cylinder known as a protostele.
The lack of seeds in the reproductive cycle of the whisk fern is another example of its ancient evolutionary origins. In place of the pollen and ovule of angiosperms, Psilotum has multicellular male and female gametophytes, and the whisk fern has spores which give rise to the gametophytes.
The gametophyte is the stage of the plant life cycle which has a haploid complement of chromosomes (1n). The gametophytes of flowering plants are extremely reduced in size. The pollen grain and the seven-celled ovule are hidden within the unpollinated ovary. However, in ancient plants such as the whisk fern, the gametophyte is relatively large. The gametophyte of Psilotum even has vascular tissue and a distinct area of food—and water—conducting tissues, unlike the gametophytes of more ancient plants, such as moss and liverworts. The cigar-shaped gametophytes also grow underground, unlike the gametophytes of many other plants, where they are nourished by an endophytic fungus. Scientists have now learned how to germinate the spores of some species of Psilotum in the laboratory, allowing for a more complete study of their gametophytes.