The maximal frequency of heterocysts in the culture of Anabaena cylindrica grown in a nitrogen-free medium was found after 6 days of growth (6%); the number of heterocysts decreased to 1.8% with aging. The quantity of heterocysts in the culture decreased (they could even disappear completely) in the presence of ammonium salts as the source of nitrogen. The index of the shape (the length-to-width ratio of cells, %) decreased under these conditions though it increased in the other variants of the experiment. The incidence of heterocysts in the culture of A. cylindrica increased under the conditions of incubation at low temperature (+4 degrees C) to 8%. The effect was most pronounced when 1% glucose was added to the medium; then, the distance between heterocysts was 5-7 cells. The percentage of heterocysts in filaments increased to 8.3% when vegetative cells were decomposed at elevated temperature (+30 degrees C) in the light. Under the same conditions in the dark, heterocysts degraded and their number decreased to 0.8%. Spore formation in the culture was stimulated by calcium excess and phosphorus deficiency in the medium.
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