The systematics of the aquatic fern genus Azolla Lam., with particular reference to section Rhizosperma (Mey.) Mett.
The small aquatic fern Azolla Lam. (Azollaceae Wettst. ) maintains a
symbiotic association with the nitrogen-fixing cyanobacterium Anabeena
azollae Stras., and is consequently of great potential as an organic
fertiliser for tropical lowland rice production in developing countries.
Recent-agricultural research has been severely hampered by the lack of a
sound taxonomic framework, however, and an extensive revision of the
genus was therefore conducted using both herbarium and living specimens.
A numerical taxonomic analysis of sect. Rhizosperma (Mey. ) Mett. was
initially conducted using morphological, ultrastructural and anatomical
characters. Several other sources of taxonomic data were also employed,
including chromosome number and morphology, and the electrophoretic isozyme
banding patterns of all Azolla species. - The
between the extant taxa was elucidated using cladistic methodology,
enabling a discussion of the evolutionary trends in the genus and
a phylogenetic interpretation of the classification. The historical
biogeography of the genus and extant sections and species is also
Sect. Azolla is shown to be a coherent and evolutionarily natural
taxon; the two species that comprise sect. Rhizosperma (A. pinnata R.
Br. and A. nilotica Decne. ex Mett. ) are morphologically, cytologically
and isozymically very distinct, however. Sect. Rhizosperma is also
shown to be paraphyletic since A. pinnata possesses numerous synapomorphies
with sect. Azolla. A new tripartite supraspecific classification
of the genus is therefore proposed, viz.: (i) sect. Azolla as
currently delimited; (ii) sect. Rhizosperma sensu stricto, containing
only A. pinnata; and (iii) sect. Tetrasporocarpia Saunders et Fowler,
sect. nov., for the inclusion of A. nilotica. Both A. nilotica and A.
pinnata are shown to be valid taxa. The latter species is shown to
contain considerable geographically related intraspecific variation,
however, and this has been taxonomically recognised as: (1) subsp.
pinnata (occurring in northern and eastern Australia and New Caledonia);
(ii) subsp. asiatica Saunders et Fowler, subsp. nov. (occurring in
India, southern China, South East Asia and southern Japan); and (iii)
subsp. africans (Desv. ) Saunders et Fowler, scat. et comb. nov.
(occurring in tropical Africa and Madagascar). A sequential dichotomous
key is provided for the identification of taxa, with new taxonomic