[Induction of cell division in synchronous cultures of Chlorella]

In synchronous cultures of Chlorella a strong elevation of the average number of autospores occurs during the second half of the 16 hour light period. When the illumination was terminated during the course of this induction phase, the final spore number could be reduced significantly by lowering the temperature from 30 to 18°C for a period of 4 hours, provided the reduction in temperature ensued within the first 2-3 hours of the darkness. Accordingly a strongly reduced rete of DNA synthesis compared to the rate of protein synthesis was observed at the lower temperature. If, on the other hand, an additional light period of a few hours was given, starting within the same time, the final spore number was raised remarkably. Later on the spore number appeared to be irreversibly fixed, i. e. it was not changed by a similar cold treatment or illumination.The obvious correspondence existing between the time of fixation of the spore number and that at which the initiation of new DNA duplications ceases is discussed with regard to a previously postulated control mechanism which is involved in the induction of cell division.