Azollaceae

The family includes a single genus of small delicate heterosporous ferns that float on the water surface.

In appearance these tiny plants more closely resemble representatives of the leafy Jungermannialean liverworts than typical ferns. As a result of adaptation to a floating lifestyle, azollaceous plants have unique structural features, and the phylogenetic relationships of this family with other ferns are largely obscured. Undoubtedly, azollaceae are closely related to Salviniaceae and possibly, together with them, originated from ancestors common with the Hymenophyllales, although Azolla is more specialized and shares fewer traits with the Hymenophyllales than do the Salviniaceae.

The genus Azolla (Azolla) comprises six species. Fossil remains of these plants are found in Tertiary and Quaternary deposits of Eurasia and North America. Of the six species, only Azolla nilotica is restricted to a specific geographic region, occurring exclusively on the Nile River. The remaining species are widely distributed in tropical and temperate regions worldwide. Like the Salviniaceae, they inhabit standing or slow-flowing waters, forming extensive mats in places. The sporophyte of Azolla is a branched floating rhizome up to 25 cm long. On its upper side, tiny leaves (0.5–1 mm) are arranged in two rows and, like tiles or fish scales, closely cover the branches. From some nodes rather long adventitious roots hang into the water. The conductive system of the stem is a reduced siphonostele, which often assumes the form of a protostele. Most of the stem is occupied by cortex without intercellular spaces. The structure of the Azolla leaf indicates a high degree of specialization. Each leaf consists of two lobes, or segments. The upper segment, projecting above the water, is green, several cell layers thick, with stomata on both sides. The lower segment is submerged and is believed to function in water absorption. On some lower segments sori develop. A remarkable feature of Azolla is its symbiosis with the blue-green alga Anabaena azollae from the family Nostocaceae. The alga occupies a cavity located on the ventral side of the upper, aerial segment near its base. The cavity is surrounded by outgrowths of epidermal cells that gradually enclose it, leaving only a tiny central opening communicating with the external environment. It is entirely lined with epidermal cells from which hairs project, and is filled with mucilage (possibly a secretion product of these hairs). It has been suggested that the cavity originally served as a water-storage organ. How the algae enter the leaf cavity remains a mystery to this day. It does not seem to occur by chance penetration of the alga from the water, since filaments of Anabaena are almost always found in Azolla species. Interestingly, the blue-green algae are found among the hairs at the tips of young leaves. In older parts of the sporophyte, where hairs are absent, there are no traces of the alga. Some researchers believe that Anabaena enters the cavity from the leaf surface during early stages of its development. Others think that the alga enters an already formed cavity through the remaining small opening. This occurs while there is still mucilage between the young leaflets, along which the algal filaments migrate inward. There are reports, though very contradictory, that the alga accompanies Azolla at various stages of sexual reproduction. Akinetes of Anabaena are constantly found in the megasporangia, and their germination proceeds in parallel with the development of Azolla spores. As is known, both partners in a symbiosis should benefit from the association. The alga gains physical and chemical protection and a constant mineral supply. As for the fern, there is no doubt that Azolla containing the alga is able to fix atmospheric nitrogen, with Anabaena acting as the nitrogen-fixing agent. It has been suggested (not confirmed by precise data) that a more complex symbiotic consortium exists, with bacteria (Pseudomonas and Azotobacter) as a third partner, which are regularly found on Azolla leaves. Azolla reproduces vegetatively very frequently. Lateral branches easily break off from the main stem and are carried by currents to other water bodies. Sometimes water birds, animals and humans act as dispersal agents. The life cycle of Azolla is similar to that described for the Salviniaceae. On the very first leaf of each lateral branch, on the submerged segment, two, rarely four, sori develop. Usually there is a pair of either micro- or megasori on one leaf, but sori of both types can occur together. Unlike the Salviniaceae, Azolla sori are unequal in size. Microsporangia are large, containing from 7 to 100 microsporangia. Microsporangia are on slender stalks.

In representatives of the section Azolla of the genus Azolla, which include Azolla filiculoides, Azolla caroliniana, the small-leaved Azolla mexicana, hooked outgrowths or glochidia develop on the surface of the massulae. In Azolla pirmata and Azolla nilotica, which constitute the section Rhisosperma, glochidia are not formed. In the considerably smaller megasori there is only a single megasporangium, in which only one megaspore develops. The megaspore, enclosed in a large massula, lies in the lower part of the sporangium. The remaining three massulae are located in the upper part as pear-shaped appendages attached to it, which function as floats. Development of the male gametophyte occurs in close proximity to the female. Prior to gametophyte development, megasporangia detach from the maternal plant and float on the water surface. Massulae with microspores are brought to the megaspores by water movement. Microspores remain inside the massulae throughout the development of the male gametophyte, which in Azolla is highly reduced (consisting of an antheridium whose wall has 5 cells and two sterile cells). Eight sperm are formed in the antheridium. Development of the megasorus begins after development of the microspore, which ensures cross-fertilization. Germination of the megaspore and development of the female gametophyte begin with the division of the spore contents into two cells, of which the lower (larger) no longer divides and serves as a reserve of nutrients, while the upper cell divides to give rise to the female gametophyte. The developing gametophyte breaks the sporangial wall and emerges outside. One archegonium develops on its upper side, but if fertilization does not occur, several additional archegonia may appear. The fertilized egg gives rise to a young sporophyte, which floats on the water surface thanks to air enclosed in the air chambers of the first leaf. The practical importance of Azolla is mainly connected with its use in agriculture as a green manure enriching soil with nitrogen. In tropical Asia and some other countries, Azolla is cultivated for this purpose in rice paddies. However, as an integral part of rice cultivation, Azolla is used mainly in Indochina. Farmers of Thai Binh province in northern Vietnam have used Azolla on rice fields since time immemorial. According to tradition, this useful custom was begun by a poor Vietnamese peasant woman from the village of La Van in Thai Binh province. Farmers long kept this method of increasing fertility secret from foreigners. Many years have passed since then. Many researchers have devoted their work to studying Azolla as an agronomic crop. It has been found that in its ability to accumulate nitrogen Azolla is comparable to legumes, which are grown for the same purpose in temperate zones. In some countries, conversely, Azolla is regarded as a weed. Due to rapid vegetative reproduction this fern can cover the water surface within a short time, hindering boat movement. An interesting use of Azolla is as a plant that suppresses the growth of other weeds in rice fields and ponds. Azolla is also commonly found among aquarium plant enthusiasts. This is usually Azolla caroliniana.