The internal anatomy of the Lycopodiaceæ is somewhat complex, but their external organization is simple. A club moss consists of a cylindrical stem covered with overlapping leaves, spirally arranged, of small size relatively to the stem, and always simple or undivided. The stem branches in a peculiar forked manner, which gives the plant its characteristic candelabra-like form. Existing lycopods are creeping plants, seldom exceeding two feet in height, but many extinct species attained the dimensions of large trees. On the ends of certain branches the leaves are crowded together, giving the terminal portion of each shoot some resemblance to a pine-cone. The crowded leaves on this portion bear, on their upper surfaces, little sacs called sporangia. Certain of these sacs contain very numerous small, rounded bodies, the microspores; others have fewer spores of larger size, distinguished as macrospores. Sacs containing the small male spores are termed microsporangia; those having the large female spores, macrosporangia. When ripe, a sporangium bursts and discharges its spores, which are scattered by the wind. Should a spore alight on a favorable spot, it germinates after a time and gives rise to a structure called a prothallus, which is really an independent plant. This stage in the life-history of a cryptogam is, however, much better seen in ferns, where the prothallus is entirely expelled from the spore and attains a higher degree of independent development. The prothallus throws out root-hairs, nourishes itself and grows, but the leaf-like form it assumes bears not the remotest resemblance to the parent fern from which it sprang. This phenomenon, characteristic of the higher cryptogams, is known as the “alternation of generations,” or “alternate generations.” Similar phases are observed in certain animals, the medusæ or jelly fishes, for example. In the course of its development, a fern passes through two distinct phases; first, the spore-bearing stage or sporophyte, represented by the fern frond; second, the egg-bearing stage, the oöphyte or prothallus. As we ascend in the scale of vegetable life, the egg-bearing or sexual generation diminishes in importance, while the sporophyte preponderates more and more. In club mosses, the prothallus has all but lost its independence; in the case of the selaginella it is formed almost entirely within the spore, only a small part being extruded when the spore ruptures. Some of the lycopods are inosporous--that is, they have, like the ferns, but one kind of spore. Where this is the case, the prothallus developed from the spore bears two sets of sexual organs; the prothallus of one of the heterosporous cryptogams, on the other hand, produces sexual organs of one kind only. Antheridia appear on the prothallus developed from a small spore; archegonia on that from a large one. The former are the male organs, and from them are emitted numerous antherozoids, minute ciliated bodies, which swarm over damp surfaces