Earth Science: Seedless Plants – Ferns
The class of true ferns, with some 12,000 species, is the largest group within the spore-producing vascular plants. Its members are distributed worldwide, with the greatest diversity of species found in tropical regions.
Most ferns are herbaceous (nonwoody) inhabitants of shady forests, where they usually grow on the ground. There are also epiphytic species (see in focus), aquatic ferns, and some treelike representatives (the families Dicksoniaceae and Cyatheaceae), which can reach a height of 33 feet (1 1 m) under favorable conditions.
All ferns exhibit a heteromorphic alternation of generations-that is, they alternate regularly between sexual and asexual generations that differ in appearance. Within this cycle, the plant we recognize as a fern is the sporophyte, or the individual on which the spores form. The spores are usually found in brown-colored spore capsules (sporangia).
These are grouped together in small clumps (sori) on the underside of the fern fronds, which can be seen easily with the naked eye. They may also be covered by a protective film of tissue (indusium). As they ripen, the spores break free from the plant and are spread by the wind. After germination they form a small structure called a prothallium, which is usually soft and short-lived.
This is the fern’s gametophyte generation. The female and male sex organs (archegonia and antheridia) develop on the prothallium, and under damp conditions, tailed sperm from the antheridia are able to swim to the archegonia and fertilize the egg cells there, from which new ferns (the sporophyte generation) then develop.
In many ferns, almost the entire visible part of the plant consists of leaves or fronds, which are often intricately feathered. They usually start out tightly rolled up in a “fiddlehead” shape. This is the case, for example, with the common bracken (Pteridium aquilinum), which can grow to a height of about seven feet (two m) or more under favorable conditions.
The fronds emerge from a rhizome underground, which may reach a length of up to 164 feet (50 m). Aquatic fern leaves often have air-filled chambers, giving the plant buoyancy. Some of their leaves have evolved into rootlike structures.
EPIPHYTIC FERNS
The epiphytic species of ferns (those living on other plants) have— in addition to spore bearing fronds-sterile leaves that function as nutrient collectors. These “nest fronds” lie flat or form open funnels for collecting water and falling plant material.
This material breaks down and is metabolized by the fern. Nest fronds also form valuable humus as the old leaves decay, and their mat-like layers store moisture.
BASICS
THE REMAINS of ferns, club mosses, and whisk ferns from the Carboniferous period provide us with most our current reserves of hard coal today.