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.
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, b2-b3); the third, the “water-leaf” (b1) 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]).
Fig. 219.—Marsilia salvatrix (natural size): K terminal bud; b leaves; f sporocarps; x point of branching of petiole.
The two genera (with 57 species, Temperate, Tropics) are land-and marsh-plants, whose dorsiventral, creeping stem bears roots on the under surface, and the leaves in two rows on the upper side (Figs. [219], [221]). The leaves of Marsilia are compound, and divided into four small leaflets springing from the apex of the petiole (Fig. [219]), and resemble the leaves of Oxalis. In the bud the leaves are circinate (Fig. [219] b), and at night they exhibit the well-known sleep-movements. The sporocarps are borne on the petioles of the fertile leaves, near their bases (Fig. [219] f); they are oblong and resemble small beans, the outer cells being hard and sclerenchymatous, while the inner ones are divided into a number of loculi arranged in two rows. On germination, water is absorbed, the two sides separate slightly, as valves (Fig. [220] A), and a long vermiform mass of gelatinous, parenchymatous cells (Fig. [220]), swollen by the water, emerges, bearing a large number of sori arranged pinnately. Each sorus (sr) is covered by a thin indusium. (The thin covering may be considered an indusium physiologically, though not morphologically).