Earth Science: Biology – Plants and Fungi – Whisk ferns, club mosses, and horsetails
The spore-producing vascular plants, Pteridophyta, includes the so-called true ferns, whisk ferns, club mosses, and horsetails. Their life cycle involves spore-based reproduction and the alternation of generations.
In ferns, the sporophyte is an independent plant, representing the dominant of the two reproductive phases known as the alternation of generations.
In the true ferns, club mosses, and horsetails, the sporophyte consists of a central shoot, leaves, and roots, while among the whisk ferns, no true roots are present.
Whisk ferns
Whisk ferns, found in the tropics or subtropics, consist of only one order (Psilotales) with two small families. Most whisk ferns possess only tiny, scalelike leaves, so that photosynthesis occurs predominantly in the central shoots.
Their short-stemmed, rounded spore capsules (sporangia) are prominent, forming on the upper parts of the shoots, which usually have forking branches.
Club mosses
Club mosses, which enjoyed a golden age some 300 million years ago in the Upper Carboniferous period, include just over a thousand
species. They have small or very narrow leaves (microphylls), usually in a spiral arrangement around the central shoot.
The sporangia are typically gathered in thick bundles on the tips of the shoots. Club mosses can be found almost all over the world. Most grow on the ground—sometimes producing yard-long creeping shoots—but other species are epiphytes, growing on other plants.
Since club moss spores will often not germinate for several years, and then take up to 15 years to reach full maturity, many species are rare and are protected in several countries.
Horsetails
Horsetails also reached the high point of their development in the Carboniferous period. At that time the group had more species such as woody horsetails, which could reach a height of 98.4 feet (30 m). Today, only one genus remains with a few herbaceous species that form non- branching fertile sprouts from a long stem emerging in springtime.
These die off after producing spores. In contrast, the leafy summer sprouts that appear later are sterile, serving only to carry out metabolic processes and store energy.
THE RESURRECTION PLANT
An unusual plant from this group is Selagmella lepydophylla, a species of club moss that is often called the “resurrection plant.” While most of its relatives grow in moist tropical forests, these plants can be found in extremely dry areas, such as parts of California and Mexico.
In times of drought, they curl up into a ball and can survive for years in a dormant state. When rain falls, the stems rapidly absorb moisture from the air and spread out green shoots again.