Other species of equisetum have only one kind of shoot—a tall, hard, leafless, green shoot with the spike at its summit. Equisetum stems are full of silex, and they are sometimes used for scouring floors and utensils; hence the common name "scouring rush."
Isoëtes (Pteridophyta)
Isoëtes or quillwort is usually found in water or damp soil on
the edges of ponds and lakes. The general habit of the plant is
seen in Fig. 300, a. It consists
of a short, perennial stem bearing
numerous erect, quill-like
leaves with broad sheathing bases.
The plants are commonly mistaken
for young grasses.
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Fig. 300.—Isoëtes, showing habit of plant at a; b, base of leaf, showing sporangium, velum, and ligule.
Isoëtes bears two kinds of spores, large roughened ones, the macrospores, and small ones or microspores. Both kinds are formed in sporangia borne in an excavation in the expanded base of the leaf. The macrospores are formed on the outer and the microspores on the inner leaves. A sporangium in the base of a leaf is shown at b. It is partially covered by a thin membrane, the velum. The minute triangular appendage at the upper end of the sporangium is called the ligule.
The spores are liberated by the decay of the sporangia. They form rudimentary prothallia of two kinds. The microspores produce prothallia with antheridia, while the macrospores produce prothallia with archegonia. Fertilization takes place as in the mosses or liverworts, and the fertilized egg-cell, by continued growth, gives rise again to the isoëtes plant.
Club-Mosses (Pteridophyta)
The club-mosses are low trailing plants of moss-like looks and
habit, although more closely allied to ferns than to true mosses.
Except one genus in Florida, all our club mosses belong to the
