Morphological and ultrastructural variability of planktonic cyanophyceae in relation to seasonal periodicity.

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Abstract
Vegetative cells, heterocysts and akinetes in Anabaena solitaria are described under natural conditions by following wild populations of the alga over two successive field seasons. Ultrastructure of the vegetative cells and heterocysts is similar to that elucidated for other species of Anabaena in culture. Akinete development resembles that previously described for other members of the Nostocaceae except that the envelope is formed in two asymmetrical parts. Unlike the remainder of the envelope, fibres at the junction are oriented in one plane only, rendering it responsive to mechanical stress during germination. The need for a comparative study of natural and cultured material is emphasized by a demonstration of the morphological variability of akinete formation in Anabaena solitaria maintained in culture for ten years.