AZOLLA PRIMAEVA AND ITS PHYLOGENETIC SIGNIFICANCE

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Abstract
The genus Azolla belongs to the family Salviniaceae of the hydrophytic ferns. It is represented by 25 fossil and 6 extant species.Azolla primaeva (middle Eocene) bears bisexual sporocarps. Microsporangia are almost invariably accompanied by megasporangia, whereas megasporangia, three per sporocarp, commonly occur alone. Microsporangia tend to be located near the main axis, whereas megasporangia occur out to the tips of lateral branches.The evolutionary tendencies within the genus are discussed briefly and it is concluded that section Azolla gave rise to section Rhizosperma. Three possible routes whereby the nine-floated condition of the latter could have been derived are presented: fusion of three megaspores and megaspore apparatuses and abortion of two megaspores; reduction in float number from an ancestral form similar to A. teschiana; the segmentation of the three-floated form into the nine-floated condition. The last hypothesis, with progressive simplification and ultimate elimination of glochidia, is favored. Changes in the glochidia were not synchronous with changes in the megaspore, but occurred later with remnants still present on A. pinnata.

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