Azollaceae Wettst., Handb. Syst. Bot. 2: 77 (1903).
Free floating aquatic plants with copiously branched, delicate, horizontal, protostelic stems. Roots simple, on the ventral side of the axis, chlorophyllous, solitary or in clusters, extending a short distance downward in the water, deciduous. Leaves sessile, alternate, in two rows in the dorsal side of the stem; each leaf divided into two lobes, a (usually somewhat smaller) upper, aerial, and a lower floating lobe. The upper, aerial lobe photosynthetic, with epidermis and mesophyll, bearing on the lower surface a large cavity with mucilage and usually living colonies of the blue-green alga Anabae-na; the lower lobe floating with the dorsal side only immersed, unistratose except at the base, mostly without chlorophyll. Sporocarps borne on the first leaf of a lateral branch, the lower lobe of this fertile leaf reduced to two, sometimes up to four sporocarps, the upper lobe forming an involucre covering the sporocarps. Sporocarps containing either micro- or mega-sporangia; megasporocarps smaller. Sporangia without dehiscence mechanism. Microsporocarps containing numerous microsporangia; microsporangia forming 64 [according to Svenson (1944) 32 or 64] trilete spores; clusters of spores embedded in 3-10 massulae (massula: hardened tapetal mucilage containing groups of spores); surface of the massulae mostly with anchor-shaped outgrowths, called glochidia, except in A. nilotica. Megasporocarp containing only one mega-sporangium; Megasporangium with 8 spore-mother cells; after meiosis only one spore developing, the others aborting. Mature megaspore on the proximal pole with a columella, hair-like filaments, and with apical massulae (called floats). Megaspores plain, perforate, with sparse excrescences, or granulate, with dense excrescences, or densely filamentose, with fine or coarse perforations, or compact filaments forming a reticulum over coarse perforations, or rugulate, with mostly obscure perforations; microspores rugulose, germinating within the massula; microprothallium reduced, with one antheridium. Megaprothallium developing within megaspore, the growing cell cushion then forcing the megaspore open at the laesura, forming a (in transversal section) round to triradiate prothallium with several archegonia on the upper surface. C. 6 species, in the New World from Argentina to Alaska, in the Old World in Africa, E. Asia and Australia; introduced into Europe.
Anatomy and Morphology. The most important contributions are by Bonnet (1957), Demalsy (1958),
Eames (1936), Konar and Kapoor (1972, 1974), Strasburger (1873), and Warmbrodt and Evert (1978). Warmbrodt and Evert (1978) described the detailed structure of the leaf. The lower lobe is achlorophyllous except for several layers of photosynthetic cells surrounding the single vascular bundle. The aerial lobe consists of upper and lower epidermis with stomata, mesophyll tissue differentiated into spongy and palisade parenchyma, and a single vascular bundle. The upper epidermis is covered by numerous water-repellent trichomes (Pieterse et al. 1977). The palisade parenchyma consists of a single layer of cells, the spongy parenchyma of one or more layers. Within the spongy mesophyll a single large cavity is differentiated, which is connected to the outside through a small pore in the lower surface of the lobe. This cavity contains a muci-
laginous substance, colonies of the blue-green alga Anabaena azollae, and numerous multicellular hairs. The Anabaena-Azolla symbiosis is well studied (Duclc-ett et al. 1975; Peters et al. 1978; Hoist and Yopp 1979).
Single roots, or in A. nilotica groups of roots, develop at the base of the branches; the root is simple, 3-4 cm long, chlorophyllous, with or without root hairs. The sporocarps originate on the lower leaf lobe normally in pairs (sometimes in fours); two of a pair may be of the same or the opposite sex. According to Duncan (1940), the sporocarp initial forms a columella on whose base in the indusium initials soon differentiate. The indusium develops into a two-layered sporocarp wall. The mature sporocarp is then surrounded by a hood-like structure called involucrum, formed by the upper lobe of the fertile leaf (Fig. 20). The apical cell of the columella always becomes the apical cell of the developing megasporangium, so never more than one megasporangium is formed. While meiosis is taking place in the megasporangium, the columella elongates and some of its epidermal cells become micro-sporangia initials. If one megaspore in the megasporangium persists, the further development of the mi-crosporangia is suppressed and a megasporocarp develops. If all megaspores disintegrate, the megasporangium collapses, and numerous microsporangia develop, and a microsporocarp is formed. The sporangia of both types develop in the leptosporangiate manner but no annulus is found. Micro- and megasporogenesis were studied in detail by Duncan (1940) and Bonnet (1957). The massula, and certain characters of the megaspore, are highly distinctive structures peculiar to Azolla. The occurrence and the shape of glochidia (Svenson 1944), the number of floats, and the morphology (sporoderm architecture) of the megaspore (Fowler and Stennet-Wilson 1978) are important characters for separating taxa.
Ecology and Distribution. Species of Azolla are free-floating and occur on the surface of ponds or in sloughs of rivers, in warm-temperate or tropical climates. They propagate very successfully by vegetative means. The genus is of worldwide distribution. It was introduced to some areas, for example Europe, by man. Two main distribution centres can be distinguished: one in East Asia and Africa, the other in North and South America. The Old World species were originally lacking in Europe and W. Asia.
Relationships. There are similarities to Salvinia, but since these are not very close, it does not seem indicated to unite the two genera into one family but to distinguish a family Azollaceae, and to include Salvinia-ceae and Azollaceae in one order, the Salviniales. The phylogenetic derivation of Azolla remains unknown. Fossil remains were found in the Cretaceous (Hall 1969; Sweet and Hills 1976). Some affinity to the Hy-menophyllaceae has been considered (Eames 1936; Takhtajan 1953), but most of the anatomical and morphological features are very different from those of that family.
Karyology and Hybridization. Only the chromosome numbers of A.pinnata, n= 22 (Loyal 1958), 2n=44 (Loyal et al. 1982) and A. caroliniana, 2n =48 (Tschermak-Woess and Dolezal-Janisch 1959) are known. So far no hybrids have been found.
A single genus:
Azolla Lamarck Fig. 20
Azolla Lamarck, Encycl. M6th. Bot. 1: 343 (1783); Reed (1953,1965).
Characters as for family.
Bonnet, A L. M. 1957. Contribution à l'étude des Hydroptéri-dêes. III. Recherches sur Azolla filiculoides Lamk. Rev. Cy-tol. Biol. Vég. 18:1-88.
Campbell, D. H. 1893. On the development of Azolla filiculoides Lam. Ann. Bot. (London) 7: 155-187.
Demalsy, P. 1958. Nouvelles recherches sur le sporophyte d'Azolla. Cellule 59: 235-268.
Duckett, J. G, Toth, R., Soni, S. L. 1975. An ultrastructural study of the Azolla-Anabaena azollae relationship. New Phytol. 75:111-118.
Duncan, R. F. 1940. The cytology of sporangium development in Azolla filiculoides. Bull. Torrey Bot. Club 67: 391-412.
Eames, A J 1936. Morphology of vascular plants, lower groups. London: McGraw-Hill.
Follieri, M. 1977. Classification and phylogeny of living and fossil water ferns of the genus "Azolla". Webbia31: 97-104.
Fowler, K. 1975. An escape mechanism for spermatozoids in Azolla massulae. Amer. Fem J. 65: 7-10.
Fowler, K., Stennet-Willson, J. 1978. Sporoderm architecture in modern Azolla. Fern Gaz. 11:405-412.
Hall, J. W. 1969. Studies on fossil Azolla: primitive types of megaspores and massulae from the Cretaceous. Amer. J. Bot. 56: 1173-1180.
Holst, R.W., Yopp, J. H. 1979. Studies of the Azolla-Anabaena symbiosis using Azolla mexicana. I. Growth in nature and laboratory. Amer. Fem J. 69:17-25.
Konar, R. N., Kapoor, R. K. 1972. Anatomical studies on Azollapinnata. Phytomorphology 22: 211-223.
Konar, R. N., Kapoor, R. K. 1974. Embryology of Azolla pinnata. Phytomorphology 24: 228-261.
Loyal, D. S. 1958. Cytology of two species of Salviniaceae. Curr. Sei. 27: 357-358.
Loyal, D.S., Gollen, A.K., Ratra, R. 1982. Morphological and cytotaxonomic observations on Azolla pinnata. Fem Gaz. 12: 230-232.
Lucas, R.C., Duckett, J.G. 1980. A cytological study of the male and female sporocarps of the heterosporous fem Azolla filiculoides Lam. New Phytol. 85:409-418.
Peters, G.A., Toia, R. F., Raveed, D., Levine, N.J. 1978. The Azolla - Anabaena relationship. VI. Morphological aspects of the association. New Phytol. 80: 583-593.
Pieteise, A H., de Lange, L., van Vliet, J. P. 1977. A comparative study of Azolla in the Netherlands. Acta Bot. Neerl.26: 433-449.
Rao, H. S. 1935. The structure and life history of Azolla pinnata R. Brown with remarks on the fossil history of the Hy-dropterideae. Proc. Indian Acad. Sei. 2B: 175-200.
Reed, C.F. 1954. Index Marsileata et Salviniata. Bol. Soc. Brot. II, 28: 5-61.
Reed, C.F. 1965. Index Maisileata et Salviniata. Suppl. Bol. Soc. Brot. II, 39: 259-302.
Strasburger, E. 1873. Über Azolla. Jena: Gustav Fischer.
Svenson, H. K. 1944. The New World species of Azolla. Amer. Fern J. 34: 69-84.
Sweet, A. R., Hills, L. V. 1976. Early tertiary species of Azolla subg. Azolla sect. Krematospora from western and arctic Canada. Can. J. Bot. 54: 334-351.
Takhtajan, A. L. 1953. Phylogenetic principles of the system of higher plants. Bot. Rev. 19:1-45.
Tschermak-Woess, E., Dolezal-Janisch, R 1959. Über die karyologische Anatomie einiger Pteridophyten sowie auffallende Unterschiede im Kernvolumen bei Cyrtomium fal-catum. Österr. Bot. Z. 106: 315-324.
Warmbrodt, R. D., Evert, R. F. 1978. Comparative leaf structure of six species of heterosporous ferns. Bot Gaz. 139: 393-429.
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