Psilophytes (Whisk Fern) in the Christopher B. Smith Preserve
Psilophyte Characteristics: Whisk ferns have alternating generations of gametophytes and sporophytes. We normally see the sporophytes. They grow 30 cm and taller. A whisk fern is the only living vascular plant that lacks both roots and leaves. Instead of roots, it has horizontally creeping stems, called rhizomes.The erect portion of each stem bears pairs of outgrowths that look like miniature leaves, but lack vascular tissue. Above the outgrowths are spore-producing organs.
Following spore dispersal, gametophtes emerge from the spores. Gametophytes are very small, usually less than two mm long. They are subterranian and saprophitic. When the gametophyte reaches sexual maturity, an egg and sperm cells are produced. Self and cross-fertilization occur. Once a sperm unites with an egg cell, a new sporophyte is created.
Recent developmental and molecular studies support the idea that whisk ferns may have had fern-like ancestors. Today, whisk ferns grow in tropical and subtropical regions and live on rich soil or as epiphytes. There are only two genera in the world and only one in North America, Psilotum.
Interactions in the Smith Preserve: Like other plants, psilophytes convert energy of sunlight to energy other organisms can use. The conversion process is photosynthesis and the energy produced is distributed to animals through the food web. Also during photosynthesis, whisk ferns produce oxygen. In addition, psilophytes provide habitat for other organisms. Terrestrial whisk fern rhizomes hold soil in place which prevents soil erosion. Epiphytic whisk ferns typically live at the base of trees, where their rhizomes help trap and hold forest debris that eventually becomes soil. The gametophyte stage of psilophytes forms a symbiotic relationship with soil fungi.
Phylum Family Species Name Common Name Psilophyta Psilotaceae Psilotum nudum Whisk Fern
Psilotum nudum is a native member of Family Psilotaceae. The common name "whisk fern" originated from its use in making small brooms. The genus name, Psilotum, originates from the Greek word "psilos" which means naked. The species name, nudum, is Latin for "naked or nude." Psilotum nudum lacks leaves. It also lacks roots. Instead of roots, a whisk fern has subterranean stems (rhizomes) that anchor the plant and serve as an organ of absorption of water and nutrients. As can be seen in these photographs, whisk fern has upright stem that are cylindrical and ridged. The photograph at left shows scaly leaf-like structures called enations on the stem.
The plant shown in these photographs is the sporophyte generation of the whisk fern. As is shown above, whisk fern sporophytes have dichotomous branching (forked branching) and spherical structures called synangia. The synangia form in the axils of the enations. The synangia produce spores. Once the synangia release the spores into the wind, the spores are distributed. Once on the ground, spores germinate, becoming the gametophyte generation of the plant. The gametophtye stage is branched dichotomously, and lives underground, where it maintains a symbiotic relationship with soil fungi.
The whisk fern in these photographs was growing at the base of a tree in the hammock portion of the Preserve as an epiphyte. But whisk ferns can also grow in moist, rich soil as independent terrestrial plants.
Psilotum is a prehistoric genus that has changed very little over time.
© Photographs and text by Susan Leach Snyder (Conservancy of Southwest Florida Volunteer), unless otherwise credited above.
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