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The changes in the heterocyst density (a,b) and heterocyst frequency (c, d) of Anabaena variabilis under various nitrate (a,c) and ammonium (b,d) treatments over 14 d of incubation. N-free treatment was set as the control. Measurements on day 0 were made 3 h after inoculation. The experiments were carried out in triplicates and the average of all the values was obtained. The error bars represent the standard deviation among the replicates.
