Lycopodiaceæ form a class of isosporous vascular Cryptogams, containing two orders with four genera. Order I.: Lycopodiæ includes the genus Lycopodium, with about 100 species which are universally distributed; and the genus Phylloglossum with only one species (P. Drummondii), found in Australia and New Zealand. Order II.: Psilotæ includes the genus Psilotum, with two species which are found in the tropics of both hemispheres, and the genus Tmesipteris, with only one known species, which is epiphytic on tree-ferns in the southern hemisphere.
Of the four genera the Lycopodium is best known under the name of 'Club-moss' or 'Stag's-horn moss,' but there is no more than a superficial resemblance between it and the true mosses (Muscinæ). The stem may be creeping as in
L. clavatum, the common club-moss of the British Isles, erect as in L. Selago, which is also a British species, or shrubby and stout as in some tropical species.
The Lycopodiaceæ have mostly a dichotomous form of branching. The stems, except in the shrubby forms, are slender, and never reach more than a few feet in length. The leaves are small and undivided, usually overlapping and completely protecting the stem. Special branches are spore-bearing, and one sporangium is borne in the axil of each leaf. Only in some fossil forms are there two kinds of spores (heterosporous). The spore develops a green prothallus, which sends root-hairs into the soil, or a colourless, tubercular subterranean prothallus; but both forms produce on the same individual antheridia and archegonia; the former produce spermatozoids, and the latter produce oospheres. Fertilisation occurs as in the Fern (q.v.). The sporophyte plant which results from the fertilisation of an oosphere by a spermatozoid is the conspicuous generation, while the oophyte or prothallus is the inconspicuous generation. The roots are simple, and may arise at any point of the stem near the ground. A number of flattened vascular bundles unite at intervals longitudinally in the centre of the stem to form a single axile cylinder, which is surrounded by a sheath, while the rest of the stem around this is made up of thick or thin walled cells. The elements of the bundle resemble those of ferns. A simple strand passes from the axile cylinder to the midrib of each leaf. Alternation of generations is very strongly marked in Lycopodium, but vegetative reproduction of the sporophyte may occur by means of bulbils in the axils of the leaves or on the roots, or by means of adventitious buds on the stems.
The Lycopodiaceæ are very closely related to the higher forms, the Selaginellaceæ, which are heterosporous plants, and to the lower forms, the Ferns, which have simpler sporangia.
The spores of Lycopodium are used for coating pills, and the hands rubbed over with the spores may be dipped in water without being wet; they are also used for flash lights in pyrotechnic displays. Many species are medicinal. L. clavatum is emetic, and L. Selago, cathartic. The spores of many form a powder which is beneficial in ulcerations, &c., and L. alpinum is used as a dye. The Lycopodiaceæ may be regarded as the degraded survivors of tree-like forms that were very plentiful in the forests of the Carboniferous period.