One group, the water ferns (_Rhizocarpeæ_), produce two kinds of spores, large and small. The former produce male, the latter female prothallia. In both cases the prothallium is small, and often scarcely protrudes beyond the spore, and may be reduced to a single archegonium or antheridium (Fig. 71, _B_, _C_) with only one or two cells representing the vegetative cells of the prothallium (_v_). The water ferns are all aquatic or semi-aquatic plants, few in number and scarce or local in their distribution. The commonest are those of the genus _Marsilia_ (Fig. 71, _A_), looking like a four-leaved clover. Others (_Salvinia_, _Azolla_) are floating forms (Fig. 71, _D_). [Illustration: FIG. 71.--_A_, _Marsilia_, one of the _Rhizocarpeæ_ (after Underwood). _sp._ the "fruits" containing the sporangia. _B_, a small spore of _Pilularia_, with the ripe antheridium protruding, × 180. _C_, male prothallium removed from the spore, × 180. _D_, _Azolla_ (after Sprague), × 1.] Of the true ferns there are a number of families distinguished mainly by the position of the sporangia, as well as by some differences in their structure. Of our common ferns, those differing most widely from the types are the flowering ferns (_Osmunda_), shown in Figure 70, _C_, _D_. In these the sporangia are large and the ring (_r_) rudimentary. The leaflets bearing the sporangia are more or less contracted and covered completely with the sporangia, sometimes all the leaflets of the spore-bearing leaf being thus changed, sometimes only a few of them, as in the species figured. Our other common ferns have the sporangia in groups (_sori_, sing. _sorus_) on the backs of the leaves. These sori are of different shape in different genera, and are usually protected by a delicate membranous covering (indusium). Illustrations of some of the commonest genera are shown in Figure 70, _E_, _J_. CLASS II.--HORSE-TAILS (_Equisetinæ_). The second class of the pteridophytes includes the horse-tails (_Equisetinæ_) of which all living forms belong to a single genus (_Equisetum_). Formerly they were much more numerous than at present, remains of many different forms being especially abundant in the coal formations. [Illustration: FIG. 72.--_A_, spore-bearing stem of the field horse-tail (_Equisetum_), × 1. _x_, the spore-bearing cone. _B_, sterile stem of the same, × ½. _C_, underground stem, with tubers (_o_), × ½. _D_, cross-section of an aerial stem, × 5. _f.b._ fibro-vascular bundle. _E_, a single fibro-vascular bundle, × 150. _tr._ vessels. _F_, a single leaf from the cone, × 5. _G_, the same cut lengthwise, through a spore sac (_sp._), × 5. _H_, a spore, × 50. _I_, the same, moistened so that the elaters are coiled up, × 150. _J_, a male prothallium, × 50. _an._ an antheridium. _K_, spermatozoids, × 300.]