=The sexual generation.= The prothallium is green and leaf-like, as in the majority of Ferns, but irregularly branched and curled. It is often unisexual. The male prothallia bear antheridia only, and are smaller and less branched (Fig. 222 _A_) than the female; the latter may attain a diameter of ½ an inch, and bear archegonia only (Fig. 222 _B_). The antheridia and the archegonia resemble those of the Ferns, but the spermatozoids (Fig. 223) are larger and less twisted. On the last curve is situated a more or less elongated appendage of cytoplasm (Fig. 223 _c_). =The asexual generation.= The embryo is similar to that of the Ferns. The fully developed _Equisetum_ is a perennial herb, with widely creeping (in some species tuberous) rhizome, from which extend erect, aerial, most frequently annual shoots. [Illustration: FIG. 224.--_Equisetum arvense_: _a_ fertile branch with cone; _b_ vegetative shoot; _c_ cone; _d_ sporophylls.] The vegetative aerial STEMS are divided into a number of internodes by the whorls of leaves (Fig. 224). The internodes are hollow, the cavities being separated from each other by the transverse partitions of the solid nodes. The lower portion of the internode, which is encased by the leaves, has much thinner and softer cell-walls, so that the stem is easily separated into segments just above the nodes. Each internode has a large number of ridges and furrows, and bears at its apex a whorl of leaves whose number and position correspond to the ridges of the internode. As in the case of other verticillate plants, the whorls are placed alternately, one above the other; the same arrangement is also found in the ridges on two successive internodes. In addition to the large air-cavity in the centre of each internode (the central cavity), a whorl of tubular air-passages is found in the cortex of the stems, opposite the furrows (vallecular canals). There is also a similar air-passage (carinal canals) in each of the vascular bundles, which are placed in a ring, one opposite each ridge, and therefore alternating with the vallecular canals. The vascular bundles are _collateral_ as in the majority of Flowering-plants, but poorly developed. The xylem of each bundle consists of two groups of annular or spiral vessels, close to the outer border of the carinal canal, and two groups of scalariform tracheides, each placed on a radius passing through a group of spiral vessels. The phloëm is placed between these four groups, each of which has only a few vessels. The stiffness of the stems is mainly due to the large amount of silica in the cell-walls of the epidermis, and to the sclerenchymatous cells of the ridges.