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.
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 the club-mosses belong to the genus Lycopodium. They grow mostly in woods, having 1-nerved evergreen leaves arranged in four or more ranks. Some of them make long strands, as the ground pine, and are much used for Christmas decorations. The spores are all of one kind or form, borne in 1-celled sporangia that open on the margin into two valves. The sporangia are borne in some species (Fig. [301]) as small yellow bodies in the axils of the ordinary leaves near the tip of the shoot; in other species (Fig. [302]) they are borne in the axils of small scales that form a catkin-like spike. The spores are very numerous, and they contain an oil that makes them inflammable. About 100 species of lycopodium are known. The plants grown by florists under the name of lycopodium are of the genus Selaginella, more closely allied to isoëtes, bearing two kinds of spores (microspores and macrospores).
Fig. 301.—A Lycopodium with Sporangia in the Axils of the Foliage Leaves. (Lycopodium lucidulum.)
