480 J. MYERS 



and by Sorokin in our lalioratory for a high- temperature strain of 

 Chlorella. 



In the life cycle of Chlorella a small cell increases in size by assimila- 

 tion to a large cell which divides into n small cells (autospores) . In 

 C. ellipsoidea, C. pyrenoidosa, and the high-temperature Chlorella 

 Tx71105, the number n lies somewhere between 4 and 8, although it 

 has a statistical rather than a fixed value. 



To depart from the main theme of this discussion, there should be 

 noted the exceptional case of the Emerson strain of C. vulgaris which 

 grows very poorly in the dark on glucose medium. Since its cells then 

 become large and packed with starch there appears to be no diffi- 

 culty in glucose assimilation in the dark. A light dosage so small as to 

 give no significant growth in the absence of glucose will greatly in- 

 crease the rate of growth in the presence of glucose and will prevent 

 the formation of unusually large cells. We interpret the phenomenon 

 as a light requirement or stimulation of the division phase in C. 

 vulgaris. However, the phenomenon is not shown by other Chlorella 

 and Scenedesmus strains examined. 



For cells growing photosynthetically our experience confirms the 

 conclusion of Tamiya: only the assimilation phase is light-dependent. 

 By a judicious regimen of light and dark periods it is possible to ob- 

 tain suspensions of cells which are nearly homogeneous or syn- 

 chronized with respect to division cycle. 



In our laboratory Sorokin has resolved the time course of changes 

 in photosynthetic activity occurring when a synchronized suspension 

 of small cells of his Tx71105 are illuminated at 30°C. and light satu- 

 ration over an 8-hour period. Light-saturated rate of photosynthesis 

 per dry weight of cells rises rapidly to a maximum at about 3 hours 

 and then falls continuously. After about 8 hours, the cell number be- 

 gins to rise, signifying the beginning of divisions. There are accom- 

 panying changes in chlorophyll concentration, respiratory rates, 

 and photosynthetic quotient. (Note added in proof: Following the 

 Gatlinberg conference there became available a report from Tamiya's 

 laboratory (4) detailing changes in the life cycle of C. ellipsoidea.) 



Variation accompanying the life cycle is not peculiar to the algae 

 but occurs also in other microorganisms. The problem may be 

 likened to a wave phenomenon, in this case a plot of volume of a cell 

 vs. time. In a bacterium such as E. coli, dividing rapidly by binary 

 fission, the waveform has an amplitude of 100% of the minimum and 



