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. [Illustration: FIG. 217.--_Salvinia natans_ (natural size): _A_ seen from above, floating on the water; _B_ a portion seen from the side in its natural position in the water.] [Illustration: FIG. 218.--Sori of _Salvinia_ in longitudinal section: _h_ microsporangia; _m_ macrosporangia. (× 10.)] Order 1. =Salviniaceæ.= This order more nearly approaches the true Ferns, especially so on account of the form of the indusium. Only one species is found in Europe, _Salvinia natans_ (Fig. 217). This is a small, floating, annual, aquatic plant, entirely _destitute of roots_. The dorsiventral, horizontal stem bears two kinds of leaves, which are arranged in whorls of three. Two of these which turn upwards are oval, entire, “_aerial foliage-leaves_” (Fig. 217 B, _b^2_-_b^3_); the third, the “_water-leaf_” (_b^1_) is submerged and divided into a number of hair-like segments, similar to the submerged leaves in many aquatic plants, for instance, Water-buttercup (see also Fig. 215 _C_). The whorls of leaves alternate with each other; there are thus 4 rows of dorsally-placed aerial leaves, and two rows of ventrally-placed submerged leaves. The sporangia are situated in sori, each sorus being borne on a small column (receptacle or placenta) and enveloped by a _cupular_, but _entirely closed indusium_ (Fig. 218). _The sori are situated on the submerged leaves_ (Fig. 217 _B_, _s-s_) _and are unisexual_, _i.e._ each sorus contains microsporangia only, or macrosporangia. _Azolla_ belongs to this order. It is a very small, floating, tropical water-plant (America and East India), with horizontal, root-bearing stem. The stem branches profusely by lateral buds, and bears the two rows of leaves on its dorsal side, the roots on the ventral side. Each leaf is bifid, and divided into an upper dorsal, and a lower ventral portion. The upper segments float on the surface of the water and are arranged like tiles on a roof, each one overlapping its neighbour. In each floating segment a large cavity is found, in which _Anabæna_ is always present. The lower segments are submerged. Order 2. =Marsiliaceæ.= The characteristic feature of this order, and one not possessed by other Fern-like plants, is that the sori (2–many) are enveloped _in leaf-segments_ which _close round them_ and form a “sporocarp,” just in the same manner as the carpels, in the Angiospermous Flowering-plants, close round the ovules and form ovaries. The sori contain both micro-and macrosporangia. When the spores are ripe, the sporocarp opens in order to disperse the spores (Fig. 220).