Azolla microphylla
Azolla microphylla Kaulf.
(Greek: mikros, micro, small or little, and phyllon, leaf, = small-leaved)
Local names: Mexican mosquito fern
Similar to A. caroliniana and distinguished as in the key above; frequently fertile (Lumpkin 1993); 2n = usually 44, rarely 66 (Stergianou & Fowler 1990). Quiet waters; Bowie, Cameron, El Paso, Tyler (BRIT, specimens annotated by J. Peck), Jeff Davis (W. Hess 3201, BRIT), Jim Wells (BRIT), Presidio (Warnock & L.C. Hinckley 46865, TEX-LL, annotated as A. Mexicana by T. Reeves), and Brewster (Turner et al. 2003) cos.; B.C. and widespread in c and w U.S. (occasionally introduced in the e U.S.—Lumpkin 1993); also Mexico to South America. Sporulating summer–fall. [Azolla Mexicana Schlect. & Cham. ex C. Presl] Recent DNA sequence data based on multiple genes support the biological reality of this species as separate from A. caroliniana but show that it is conspecific with individuals that had been going under the name A. Mexicana (a more recent name now placed in synonymy under A. microphylla) (Reid et al. 2006; Metzgar et al. 2008). Evrard and Van Hove (2004) raised the possibility that the type specimen of A. microphylla is actually of another species—if this turns out to be the case, a nomenclatural change will be necessary. Within the state this species (as A. Mexicana) has in the past usually been considered to be restricted to far w TX; for example, Turner et al. (2003) mapped it (as A. Mexicana) in TX only in the Trans-Pecos. However, there has long been confusion between it and the morphologically very similar A. caroliniana (J. Peck, pers. comm.) (see more discussion under that species). Peck and Taylor (1995) showed A. microphylla (as A. Mexicana) nearly throughout AR, including adjacent to the extreme ne border of TX, and Lumpkin (1993) mapped it in OK immediately adjacent to the ne border of TX. More recently, Peck (2011a) treated all material occurring in AR as A. microphylla. Because of the difficulty in distinguishing it from A. caroliniana, the species is probably much more widespread in TX than previously thought. Lumpkin (1993) also stressed that because of inconsistent characters used in the past to identify Azolla species, “Literature that attributes a particular species of Azolla to a particular state or province must be questioned. …” Further work thus needs to be done to clarify the distribution of both A. microphylla and A. caroliniana in TX. Azolla microphylla (as A. Mexicana) was erroneously reported to be “generally less cold tolerant” and to have “a narrower environmental range than A. caroliniana” (Lumpkin 1993). However, it occurs in the upper Mississippi River northward to Minneapolis-St. Paul, MN (Peck 1982) and also to the Canadian province of British Columbia. Van Cat et al. (1989) reported hybridizing this species with A. filiculoides. Jill Reid (pers. comm.) has suggested that A. microphylla may have arisen from a hybridization event between A. caroliniana and A. filiculoides. She came to that conclusion based mainly on the studies of the reproductive structures—the megasporocarp of A. caroliniana is distinctive and non-variable (likewise with A. filiculoides), but the megasporocarp of A. microphylla is variable, with overlapping morphology of both A. caroliniana and A. filiculoides. The species is occasionally cultivated (Lellinger 1985).