The gametophyte (sexual generation) is represented by prothalli which, in the homosporous genera, may live underground as saprophytes, or the upper portion may develop chlorophyll and project above the surface of the ground as an irregularly lobed green structure (_e.g._ _Lycopodium cernuum_)[85]. In the heterosporous forms the prothalli are much reduced and do not lead an independent existence outside the spore by the membrane of which they are always more or less enclosed. The sexual organs are represented by antheridia and archegonia; the male cells are provided with two cilia except in _Isoetes_ which has multiciliate antherozoids like those of the ferns. The existing Lycopods, though widely distributed, never grow in sufficiently dense masses to the exclusion of other plants to form a conspicuous feature in the vegetation of a country. The inconspicuous rôle which they play among the plant-associations of the present era affords a striking contrast to the abundance of the arborescent species in the Palaeozoic forests of the northern hemisphere. • • • • • =Lycopodiaceae.= _Lycopodium_, represented by nearly 100 species, forms a constituent of most floras: epiphytic species predominate in tropical regions, while others flourish on the mountains and moorlands of Britain and in other extra-tropical countries. For the most part _Lycopodium_ exhibits a preference for a moist climate and appears to be well adapted to habitats where the amount of sunlight is relatively small and the conditions of life unfavourable for dense vegetation. Mountains and islands constantly recur as situations from which species have been recorded. Some species are essentially swamp-plants, _e.g._ _Lycopodium inundatum_, a British species, and _L. cruentum_ from the marshes of Sierra Nevada. A variety of the American species, _L. alopecuroides_ (var. _aquaticum_) affords an instance of a submerged form, which has been collected from an altitude of 12–14,000 ft. on the Andes and Himalayas. It is noteworthy that a considerable variety of habitats is represented within the limits of the genus and that many species are sufficiently hardy to exist in circumstances which would be intolerable to the majority of flowering plants[86]. The British species frequently spoken of as Club Mosses, include _Lycopodium Selago_, _L. annotinum_, _L. clavatum_, _L. alpinum_, and _L. inundatum_. • • • • • =Selaginellaceae.= The species of _Selaginella_, over 300 in number, are widely spread in tropical and subtropical forests, growing on the ground with trailing, suberect or erect stems climbing over taller and stouter plants or as pendulous epiphytes on forest trees.