Azolla is a free-floating plant occurring in fresh water pools, tanks and back waters. The genus comprises of 6 species, of which A. filiculoides may be regarded as a cosmopolitan one.
The common Indian species is A. pinnata (= A. imbricata).
No fossil representative is known from earlier than the Lower Caeneozoic bed.
The Sporophyte of Azolla:
The sporophyte of Azolla looks more or less like the gametophyte of a leafy Jungermannia. It has a freely branched, delicate rhizome, which is horizontally floating, bearing solitary or clustered adventitious roots on the underside, and alternately arranged two-ranked leaves on the dorsal side.
Each leaf is dorsiventrally lobed, the lobes being approximately of equal size; the thick dorsal lobe is green and aerial, while the thin ventral one is almost hyaline and submerged. This ventral lobe is believed to be helpful in absorbing water.
On the lower surface of the dorsal lobe there is a conspicuous cavity harbouring the blue- green alga, Anabaena azollae. It has been demonstrated experimentally that this blue-green alga is capable of fixing free nitrogen for its host.
Internally, the rhizome has an apparently siphonostelic cylinder, but it is practically impossible to differentiate the central cylinder from the cortex. The vascular tissues are greatly reduced. The parenchyma cells in the cortical region are without intercellular spaces.
The sporocarps are small globular nut-like bodies and formed on the submerged lobe of the lowermost leaf of a branch. Each submerged lobe gives rise to two sporocarps. The sporangia in each sporocarp arise on a raised placenta and incrusted by an indusium.
Thus, each sporocarp is nothing but a sorus consisting of clustered sporangia. Early in the stage of development, a sporocarp contains both microsporangia and a single large terminal mega-sporangium. Later on the sporocarps are differentiated into microsporocarps and megasporocarps respectively, (Fig. 409).
Thus, of the two sporocarps in a submerged lobe, the larger one is the microsporocarp bearing microsporangia only with abortive mega-sporangium and the smaller one is the megasporocarps bearing a mega-sporangium with abortive microsporangia. The sporangia are shortly stalked and the wall contains a layer of tapetium and these are distinctly leptosporangiate in origin.
In the megasporangium, there is only one functioning megaspore matter cell which divides reductionally and form four megaspores of which three degenerate and only one persists. In the microsporangium 64 microspores are formed into tetrads as a result of reduction division of mother cells.
The tapetal cells in the microsporangium as usual degenerate and cytoplasm organises into two to sight masses known as massulae embedding the microspores. The massulae are invested on the surface by hair-like appendages with sagittate tips known as glochidia (Fig. 410). In mega-sporangium four massulae are formed of which basal layer one contains the functioning megaspore. With reduction division and formation of spores, the gametophytic generation begins.
The Gametophytes and The New Sporophyte of Azolla:
1. Male Gametophyte (Fig. 411):
The microspores germinate within the massulae and develop into male gametophytes. The microspore cuts off a small prothallial cell and a single antheridium. A large basal cell is also developed below the antheridium by which it is pushed out of the sporangial wall. The antheridium consists of eight sperm mother cells within each of which a spirally coiled multiflagellate sperm is formed (Fig. 412).
2. Female Gametophyte (Fig. 413):
The single megaspore germinates within the massula giving rise to the female gametophyte. A mature female gametophyte consists of a large basal multinucleate nutritive cell and a number of archegonia which project out of the megaspore wall but never turn green. Each archegonium consists of neck of one neck canal cell and venter of a ventral canal cell and an egg.
The New Sporophyte of Azolla:
As a result of fertilization the egg cell in the archegonium is converted into an oospore. This oospore becomes elongated vertically and undergoes a transverse division at first. By subsequent divisions the embryo forms a quadrant, from which develop the first leaf, the stem, the root and the foot. From this embryo ultimately a new adult sporophyte is established.