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.
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.
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.
The formation of the sporangium is the same as in the Leptosporangiate Ferns. (The 16 spore-mother-cells originate from one central, tetrahedric archesporium.)
The Hydropterideæ are divided into 2 orders, the chief differences between them being found in the asexual generation.