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Title: The systematics of the aquatic fern genus Azolla Lam., with particular reference to section Rhizosperma (Mey.) Mett.
Author: Saunders, Richard M. K.
Awarding Body: Portsmouth Polytechnic
Current Institution: University of Portsmouth
Date of Award: 1990
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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 , evolutionary relationships 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 discussed. 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 descriptions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Botany