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 Anabaena; 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.
KeywordsChlorophyll Europe Germinat Cretaceous Perforation
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