[Illustration: FIG. 214.--_Salvinia natans_: _A_ microsporangium with germinating microspores and protruding prothallia (_s_); _B_ a prothallium with the bicellular antheridium (_s_) growing out of the microsporangium; _C_ the two cells of the antheridium have opened by transverse clefts; beneath is seen the microspores enclosed by the hardened mucilage; _D_ spermatozoids still enclosed in the mother-cells.] [Illustration: FIG. 215.--_Salvinia natans. A_, _B_ Female prothallia, _f-f_, protruding from the macrospore which is still enclosed in the macrosporangium; _œ_ archegonia. _C_ An embryo (× 16) still in connection with the spore (_s_): _a_ the scutiform leaf; _b-e_ the subsequent foliage-leaves, of which _b_ and _c_ stand singly, _d-e-v_ in a whorl; _v_ the submerged-leaf; _f-f_ wing-like lobes of the prothallium: _m_ the foot.] Sub-Class 2. =Hydropterideæ= (formerly Rhizocarpeæ), =Water Ferns=. The following further characteristics must be added to those given on page 205:-- =Sexual generation.= The MICROSPORES produce an extremely rudimentary prothallium, formed of only a single cell, and having also a very much reduced bicellular antheridium with a small number of spermatozoid mother-cells in each cell (in _Salvinia_ 4, in _Marsilia_ and _Pilularia_ 16). In _Salvinia_ the microspores remain embedded in a hard mucilaginous mass (at first frothy) which fills up the cavity of the sporangium. The prothallium must therefore grow out through this slime and also through the wall of the sporangium (Fig. 214), and it thus terminates in a relatively long cell. In _Marsilia_ the microspores are set free from the microsporangium, and the prothallia, with the antheridia, remain in them until the spermatozoids are liberated. The latter are spirally-twisted threads. The MACROSPORES, on germination, give rise to a very reduced prothallium, which in _Salvinia_ bears 3 archegonia; but, if these are not fertilised, the prothallium may continue to grow and become a fairly large, green body with several archegonia (Fig. 215 _A_, _B_). In _Marsilia_ the prothallium is still more reduced, it is enclosed in the macrospore, and only bears one archegonium. The archegonia are similar in structure to those of the Ferns, but are smaller, and sunk more deeply in the tissue of the prothallium. [Illustration: FIG. 216.--_Salvinia natans. A_ An archegonium, unripe, seen in longitudinal section: _h_ the neck-cells; _k_ the neck-canal-cells; _c_ the central cell. _B_ An open archegonium of which the neck-cells have separated off. _C_ An open, old archegonium seen from the top.] =The asexual generation= is developed from the fertilised egg-cell. It is a dorsiventral, horizontal shoot. In _Salvinia_ it bears at first a shield-like leaf, the scutiform leaf (Fig. 215 _C_, _a_), which is succeeded by the ordinary foliage-leaves. The young plants of _Marsilia_, likewise, have less perfect leaves in the very early stage.