T axonomic re-evaluation of the Azolla genus in Portugal A.L. PEREIRA, G. TEIXEIRA, I. SEVINATE-PINTO, T . ANTUNES and F. CARRAPIÇO Azolla Lam. is an aquatic fern with deeply bilobed leaves. It has a very thin and hyaline ventral leaf lobe and an aerial and chlorophyllous dorsal lobe, which have an extracellular cavity . Populations of bacteria belonging to s everal genera, and a population of nitrogen-fixing cyanobac teria, Anabaen a azollae live in t his cavit y (CARRAPIÇO, 1991; GRILLI CAIOLA & FORNI, 1999). This association is very important as a natural organic fertilis- er for rice production in several countries ( LUMPKIN & P LUCKNETT , 1980). Research involving Azolla has often been hindered by confusion on the taxonomy of the genus. This genus is divided into two subgenera: Azolla, with two sections – Azolla ( A. filiculoides Lam., A. rubra R. Br., A. carolini - ana W illd., A. microphylla Kaulf., A. mexicana Presl.) and Rhizosperma ( A. pinnata R. Br .) - and subgenus T etrasporocarpia, which only includes A. nilotica Decne (SAUNDERS & FOWLER, 1993). The taxonomy of the genus Azolla is traditionally based on vegetative charac- ters such as plant dimensions, leaf characteristics (shape, papillae and hyaline border of dorsal leaves lobes), and on reproductive characters like the number of massulae and microsporangia (S VENSON, 1944). The number of glochidia septa in the microsporangium massulae has been used to distinguish between species. This is, how- ever , a variable character, just like the above-mentioned ABSTRACT - The Portuguese and the European Flora refer to the presence of two or three Azolla species in Portugal: A. filiculoides Lam., A. caroliniana W illd. and/or Azolla mexicana Presl., the latter included in the last edition of Flora Europaea. In the present work, the taxonomy of Azolla species is reviewed using the two most important characters that can distinguish between these two/three species: papillae in the dorsal leaf lobe and perine architecture of the megaspore apparatus. Other characteristics, such as the hyaline border cells of the dorsal leaf lobes and the number of glochidia septa in microsporangium massulae, are also used. All the Azolla specimens, collected from several locations in Portugal, were identified as Azolla fili culoides Lam. T h is identification disagrees with previous published reports on Azolla taxonomy in Portugal as well as with herbarium identification. KEY WORDS - Azolla filiculoides Lam., perine surface, perine zonation, papillae, hyaline border, taxonomy r eceived 13 December 2000; revised version accepted 4 June 2001 P LANT B IOSYSTEMS , 135 (3) 285-294, 2001