Salvinia is a free-floating hydrophyte and inhabits freshwater pools, tanks and lakes gregariously. It is mainly an African genus, though a few species are very common in India.
The genus comprises of 10 species. The common Indian species are S. natans and S. cucullata.
The Sporophyte of Salvinia:
The stem is a branched rhizome attaining a length up to 10 cm. and the entire body is densely clothed with short-stalked or sessile leaves, arranged in whorls of three (two lateral and one submerged). The plant is rootless but the submerged leaf gets highly dissected and covered with multicellular hairs and thus resembles the root-system. The two lateral leaves are covered with stiff hairs and papilose projections and are highly modified as floating organs.
Internally, the rhizome has a siphonostele with greatly reduced vascular tissues and a broken cylinder of xylem.
The sporocarps are oval, nut-like bodies which appear to arise in clusters near the base of the much dissected submerged leaf. Actually each segment of the leaf terminates into a sporocarp. The sporangia in each sporocarp arise on a raised Placenta, invested by an indusium. Thus each sporocarp is nothing but a sorus containing clustered sporangia.
The sporocarps are differentiated into microsporocarps bearing microsporangia only and megasporocarps bearing mega-sporangia only (Fig. 400).
Each sporangium is provided with a slender stalk and distinctly leptosporangiate in origin and it is surrounded by a wall with a tapetal layer. In mega-sporangium 32 megaspores are formed in tetrads, as a result of reduction division of mother cells but all excepting only one degenerate (Fig. 401).
In micro-sporangia, 64 microspores are produced by the reduction division of the microspore mother cells. In the micro-sporangium, the tapetal cells disintegrate and the cytoplasm forms a hardened mass in the sporangial cavity forming massula which pushes the microspores towards the periphery.
In mega-sporangium, the hardened matrix formed from the tapetal cells and degenerated megaspores later become triangular, vacuolated and lobed at the tip forming perespore or epispore, with the formation of spores, gametophytic germination begins.
The Gametophytes and The New Sporophyte of Salvinia:
1. Male Gametophyte (Fig. 402):
The microspores germinate within the massula of the micro-sporangium and develops into the male gametophyte. It cuts off a small prothalial cell and two antheridia. There is a large basal cell below the antheridia by which the antheridia are pushed out of the micro-sporangial wall. Each antheridium consists of four sperm mother cells within each of which a spirally coiled multiflagellate sperm is formed.
2. Female Gametophyte (Fig. 403):
The single megaspore germinates within the perispore of the mega-sporangium and develops into the female gametophyte. A mature female gametophyte consists of a large basal, multinucleated nutritive cell and an archegonium which projects out of the tri-radiate ridge of the megaspore and gradually turns green.
Each archegonium consists of a neck made of binucleate neck canal and venter of a ventral canal cell and an egg.
The New Sporophyte of Salvinia:
After fertilization the egg cell is converted into an oospore. The first division of the oospore is longitudinal or nearly so, giving rise to two unequal parts. By subsequent divisions an embryo, with a foot, the first leaf and a stem apex, is formed and ultimately produces a new sporophyte.