Five species of _Lycopodium_ are found in Great Britain. _L. clavatum_ and _L. selago_ are common in mountainous districts. _L. annotinum_ is common in the Highlands of Scotland. The other genus of the order is _Phylloglossum_, with one species, _P. drummondi_ (Australia, Tasmania, and New Zealand), a small plant only a few centimetres high, with two tubers, and about eleven linear leaves at the base of the stem which is terminated by a cone of sporophylls.--FOSSIL Lycopodiaceæ in the Carboniferous period. OFFICINAL: “Lycopodium,” the spores of _L. clavatum_. Family 2. =Psilotaceæ=. The sporangia are placed on the apex of short, two-leaved stems, as 2–3, seldom four, small capsules. Small herbs, with angular stems; leaves small, simple, and one nerved. Only four species.--_Psilotum_ (Madagascar, Moluccas, Sandwich Islands, etc.) is destitute of roots, their place being supplied by special underground stems which bear a few modified leaves, very much reduced, especially when buried deeply in the soil. Three species.--_Tmesipteris_ (Australia), one species. Sub-Class 2. =Selaginelleæ= (HETEROSPOROUS Lycopodinæ). Micro-and macrospores. The prothallia are very much reduced, especially the male; the female does not leave the spore. The leaves are ligulate. [Illustration: FIG. 233.--Germination of the microspores of _Selaginella_: _A_ the spore rendered transparent, seen from above. In the interior is seen the prothallium (_f_), and the first divisions of the antheridium (_a_, _b_, _c_, _d_); in _B_ the spore-wall is removed and all the spermatozoid-mother-cells formed; in _C_, the microspore has opened and the spermatozoids and the mother-cells are escaping together.] =The sexual generation.= In the ~MICROSPORES~ are formed: (1) a very small “vegetative” cell, representing the vegetative part of the prothallium (_f_ in Fig. 233 _A, B_), and (2) a cell many times larger and which divides into a number (4–8) of primordial cells, each of which divides into four spermatozoid-mother-cells, though all of these may not develope spermatozoids. On germination, when the spore-wall is ruptured, the spermatozoids and spermatozoid-mother-cells are ejected into the water. The ~SPERMATOZOIDS~ in _Selaginella_ are elongated and club-shaped, with two cilia (Fig. 234); but in _Isoëtes lacustris_ they are spirally-twisted threads which differ from all other spermatozoids by having a bunch of cilia _at each end_; the other species of _Isoëtes_ have cilia only at the anterior end. The MACROSPORES. Shortly after the macrospores have been set free, or in _Selaginella_, while still enclosed in the sporangium of the mother-plant, they germinate and soon become filled with the cellular tissue of the prothallium, and even in _Selaginella_ the archegonium begins to be formed before the rupture of the spore-cell-wall has commenced (Fig. 235 _A_).