All LEAVES are situated in _whorls_. The VEGETATIVE are simple, undivided, 1-nerved, and are united into toothed sheaths (Fig. 224 _a_, _b_). The branching of the stems in some species (_E. arvense_) is very abundant. The branches break through the base of the leaf-sheaths (Fig. 224 _b_), and generally _alternate with the teeth_ (leaves). The FERTILE LEAVES (_sporophylls_) are different from the barren ones. They are _free, shield-like_, each one having a short stalk bearing usually an hexagonal plate (Fig. 224 _d_), and closely compressed into an ear or cone (Fig. 224 _a_, _c_). The _Equisetums_ thus present an advance in development distinctly beyond that of the Ferns, which is further emphasized by the circumstance that a transition from the sheath-leaves to the fertile-leaves is found in the involucre or annulus, a “collar” of specially modified leaves situated at the base of the cone (Fig. 224 _a_ and _c_). The cone may be considered as a very rudimentary flower, and the annulus may be regarded as a very early stage in the formation of a flower (perianth). See page 235. The SPORANGIA are situated on the underside of the sporophylls, one at each angle; they are sac-like, and open inwardly by a longitudinal cleft (Fig. 224 _d_). An annulus is wanting; but in the wall of the sporangium, as in the pollen-sacs of the Flowering-plants, a layer of cells, with annular or spiral thickenings, is developed, which assists in the dehiscence of the sporangium. The SPORES are green; the walls composed of four distinct layers, of which the outer is gradually separated, except at one point, and becomes split into four long bands (_elaters_) (Fig. 225). The elaters are extremely hygroscopic, coiling round the spore when moistened, and expanding as soon as dry, presenting a most lively object under the microscope when breathed upon and allowed to dry. The second layer, when germination commences, becomes detached from the inner wall, which is formed of the exospore and endospore. The order has become much reduced, and at the present time includes only one genus, _Equisetum_, with about twenty-five species, which are distributed over the entire globe, particularly in damp situations. In SOME SPECIES the barren shoots are green and very much branched, but the fertile ones are unbranched, pale brown, and possess no chlorophyll (_E. arvense_, Field-Horsetail, Fig. 224, and _E. maximum_). IN OTHERS the fertile and barren shoots are alike green, and either both unbranched (_E. hiemale_), or branched (_E. palustre_, _E. limosum_, etc). The fertile shoots of _E. silvaticum_, up to maturity, resemble those without chylorophyll of _E. arvense_, but after that period they produce green branches, and thus resemble the barren ones. [Illustration: FIG. 225.--Spores of _Equisetum_: _A_ damp, with elaters (_e_) coiled round the spore; _B_ dry, with elaters expanded.]