Azollaceae

Azollaceae Wettst., Handb. Syst. Bot. 2 (1903) 77; C. Chr. in Verdoorn, Man. Pterid. (1938) 550; Wettst., Trat. Bot. Sist. (1944) 416; Reed, Bol. Soc. Brot. II, 28 (1954) 15. Small aquatic plants with flabellate branched stems bearing roots and leaves. Leaves sessile, alternate, imbricate, bilobed. Dorsal leaf lobe fleshy and chlorophyllous, held above water surface; with uni- or bicellular trichomes, and anomocytic stomata; with cavity containing mucilage and filaments of the cyanobacterium Ventral leaf lobe Anabaena azollae. generally unistratose and translucent (except at base), resting on water surface. Roots either solitary or in fascicles, growing from stem branching points; with numerous root hairs and two semi-persistent root caps and a basal root sheath. Sporocarps borne in pairs or fours at base of branches, initially covered by involucre of dorsal leaf lobe. Plants monoecious, with separate mega- and microsporocarps. Megasporocarp containing a solitary indehiscent megasporangium, which contains a single megaspore attached by placenta. Megaspore with trilete mark, and 3 or 9 proximally positioned alveolate ‘floats’ (up to 24 in fossil taxa) attached by filosum of megasporoderm. Megasporoderm sculpturing and stratification variable and often highly complex. Megagametophyte endosporic, forcing megaspore open at laesura, bearing several archegonia. Microsporocarp containing numerous indehiscent microsporangia that develop successively from apex to base; each microsporangium with c. 64 trilete microspores. Microspores aggregated together in alveolate structures (‘massulae’), analogous to floats. Massulae partially or fully covered with simple or glochidiate trichomes. Microspores germinating within massula; microgametophyte reduced, with one antheridium; antherozoids multiflagellate, released through flask-shaped cavities in massula. One genus only.