Taxonomy of the American Azolla species (Azollaceae): a critical review.
The taxonomy of the New World species of Azolla has been the subject of much debate and remains unsatisfactory. Since 1944 most authors, mainly Americans, recognize four species: A. caroliniana, A. filiculoides, A. mexicana and A. microphylla. The present study is based on a comprehensive review of the literature and on original observations, by optical and scanning electron microscopy, of numerous samples cultivated in our laboratory as well as herbarium exsiccata. Observation of the type specimens confirms the opinion of some authors that A. caroliniana and A. microphylla are synonyms of the previously described A. filiculoides. The ferns named A. caroliniana and A. microphylla by most authors, including the American taxonomists in their recent works, are different from their type specimens. The study also shows that the Mettenius conception, proposed as early as 1867, has to be rehabilitated: two species only exist in America. According to the priority rule they must be named A. cristata and A. filiculoides.