plants, approaching so nearly to a monocotyledon, exhibit a high organization, only inferior to that of the Lycopods. The group contains the genera Marsilea, which has leaves made up of cuneiform lobes, and resembling those of some leguminous plant; and Salvinia and Azolla, both of which consist of small floating plants, which mostly occur in tropical or subtropical countries. SECTION XI. LYCOPODIACEÆ, OR CLUB MOSSES. THE Club Mosses are mostly perennial plants, with slender creeping stems, often several feet or yards in length, occasionally erect, and clothed by small, sessile, closely set, often imbricated leaves without veins. They have in some instances a habit resembling that of Conifers. The stems consist of a mass of thick-walled, often dotted cells, enclosing one or many bunches of scalariform tissue, which sends off branches to every leaf and bud. The scalariform tissue is accompanied by fine, elongated, and sometimes rather coarser cells, which are occasionally reticulated. The stem approaches to that of ferns, but the bundles of vascular tissue are confined to the centre. The branches of the stems are often bifurcate, and terminate in one or a pair of cone-like spikes, which are either sessile or stalked. The sporangia are sessile in the axils of the imbricated leaves or bracts that cover the cones. Two kinds grow on the same plant, one of them bivalved, containing a powdery substance, whose particles a high magnifying power shows to be globular spermatozoids: these do not germinate; and the other three-valved, enclosing comparatively large, nearly spherical granules, marked with three prominent ridges, radiating from one extremity. These certainly germinate, forming by cell-division a prothallus of hexagonal cellular tissue, adherent to and confluent with the spores, as in Marsileaceæ, or penetrating their cavity, but without the protrusion of threads, as in ferns or mosses. On this prothallus, archegonia are soon produced, the embryo being formed from a cell at its base; this sends down roots on one side, and a minute stem with the two primary leaves of the young plant at the other. The plant thus bears a close resemblance to a young plant of the dicotyledons, and exhibits, with regard to fructification, the highest organization of which the Cryptogams are capable, although in their tissues the Lycopods are inferior to the Equisetaceæ and Marsileaceæ.