ONLY on rare occasions have heterocysts of blue-green algae been seen to germinate. The observations of a number of investigators have supported various suggestions as to non-reproductive functions of these cells. However, no definitive elucidation of their function has yet been presented1. Indicative of the apparent infrequency of heterocyst germination under natural conditions is Fritsch's2 comment that in a six-month survey of algal material from all over the world, he had observed no single case of a germinating heterocyst. If conditions could be established which would invariably result in germination of these cells, these conditions could then suggest natural environments in which heterocyst germination might be successfully sought.