A morphological taxonomic revision of Azolla Lam. section Rhizosperma (Mey.) Mett. (Azollaceae)

Abstract

SAUNDERS, R. M. K. & FOWLER, K., 1992. A morphological taxonomic revision of Azolla Lam. section Rhizosperma (Mey.) Mett. (Azollaceae). Recent research on the agronomically important aquatic fern Azolla has been hampered by the lack of a sound taxonomic framework. The classification of section Rhizosperma is revised using multivariate statistical analyses of ultrastructural, gross morphological and anatomical characters. The disparate morphologies and current delimitations of A. pinnata and A. nilotica are corroborated. Three distinct geographically related subspecies of A. pinnata are identified and formally named and described as: 1. subsp. pinnata, occurring in northern and eastern Australia and New Caledonia; 2. subsp. asiatica subsp. nov., occurring in India, southern China, south‐east Asia and southern Japan; and 3. subsp. africana stat. et comb, nov., occurring in tropical Africa and Madagascar. A sequential dichotomous key is provided for the identification of taxa, with new taxonomic descriptions.

Paleoazolla, a new heterosporous fern from the Upper Cretaceous of Argentina

“…As a result, a number of evolutionary trends have been hypothesized for the reproductive morphology of the family, and especially for Azolla (e.g., Fowler, 1975;Martin, 1976;Collinson, 1980;Saunders and Fowler, 1993). It has generally been accepted, and recently restated, that one of the trends in hydropteridinean megaspores has been the reduction in float number (e.g., Jain and Hall, 1969;Hall and Bergad, 1971;Jain, 1971;Hall, 1974;Saunders and Fowler, 1993). This idea is based on the fact that modern representatives of Azolla have few floats, while many fossil genera such as Glomerisporites, Azollopsis, and several extinct sections of Azolla have numerous floats.…”

“…The microspores of this family (except Ariadnaesporites) are variable in number and embedded in a pseudocellular matrix termed the massula (Fowler, 1975;Lucas and Duckett, 1980;Saunders and Fowler, 1992). Generally, massulae have external projections termed glochidia, which serve to attach them to megaspores by entanglement with the filosum (Hoffman and Stockey, 1994).…”

“…Azolla is divided into six sections on the basis of several vegetative and reproductive characters (Saunders and Fowler, 1993): Rhizosperma, Azolla, Simplicispora, Kremastospora, Filifera, and Antiqua. The first two are known from both fossil and extant records, whereas the last four are known exclusively from fossil material.…”

“…Paleoazolla patagonica and extant species of Azolla both have three floats per megaspore; however, other characters of the modern species clearly differentiate them from Paleoazolla. These include the presence of single-barbed glochidia on the massulae, a large megaspore collar, and three-zoned floats in the modern species (Saunders and Fowler, 1993) as compared to mutibarbed glochidia, no collar, and two-zoned floats in Paleoazolla. The megaspore wall structure is also significantly different.…”

“…Fossil Azolla species with three floats have been described from the Upper Cretaceous to the Lower Paleocene by several authors. However, most of these reports have not been verified as the materials are poorly preserved (Collinson, 1980(Collinson, , 1991(Collinson, , 1992Saunders and Fowler, 1993). The earliest well-known species with three floats are A. indica (Trivedi and Verma, 1971) and A. intertrappea (Sahni, 1941;Sahni and Rao, 1943) from the Cretaceous/Tertiary boundary of India (Courtillot et al, 1988;Duncan and Pyle, 1988).…”