Division: Pteridophyta (Filicophyta)
Class: Pteridopsida (Filicopsida)
Azolla Lamarck, Encycl. Meth. Bot. 1:343 (1783).
Azolla is probably the most unfernlike of all fern genera, but numerous studies have clearly shown it to be a true fern. Studies have also demonstrated a relationship between Azolla, Salvinia and the Marsileoid ferns, although they are morphologically different.
Like Salvinia and the Marsileoid ferns, Azolla is heterosporous. Heterospory in these groups is clearly an adaptation to an aquatic environment.
Azolla plants are very small clusters floating on water surfaces. In most species, each cluster consists of only a few small branches. Only the clothing and highly reduced leaves may be seen from above. When a cluster branches to a certain point, the older portions die and the newer portions break off to form new clusters.
In some areas such as the Amazon basin, Azolla colonies may densely cover the surface of a pond or lake, presenting a difficult environmental problem. On the other hand, a thick covering of Azolla may prevent mosquitos from breeding. In southeast Asia, Azolla is grown on rice paddies, and becomes fertilizer when the paddies are drained.
Known chromosome counts for the genus are centered around x=22, with many variations. This probably indicates that a tetraploid n=22 was the original count, deriving from n=11, while a reduction to n=10 preceded the differentiation of Salvinia and the Marsileoids.
The treatment of Azolla here follows that of Saunders and Fowler, in Plant Systematics and Evolution 184: 175-193 (1993).
Species of Azolla:
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This page was last revised on 11-21-1997.