122 University of California Publications in Zoology [Vol.22 



Reproduction as a factor influencing diurnal oscillations of num- 

 bers may not be important, but, for the reason just stated, it probably 

 is. If, in addition to this, it could be shown that any considerable 

 number of plankton forms accelerate both growth and fission pro- 

 cesses with diurnal rise in temperature and that fission may occur 

 two or more times in twenty-four hours, the argument in favor of 

 dominance of temperature through the reproductive factor would be 

 convincing. The writer has examined large numbers of publications 

 without finding definite discussion of this point. Apparently the 

 only cases in which the number of generations in a twenty-four hour 

 period has been accurately determined have shown what occurred 

 under special laboratory conditions only, and they do not tell what 

 occurs with a mixed population under natural conditions. Even so, 

 the published records of such studies deal almost entirely with animal 

 types. Since the increase of numbers due simply to acceleration of 

 vital processes is sufficient to account for any probable influence of the 

 reproductive factor in this series, it is doubtless best to say that the 

 addition of a distinct generation during the twelve-hour period is 

 improbable. 



It is also true that reproduction may be influenced by light. Some 

 organisms may be stimulated to greater reproductive capacny, some 

 to less. In the green organisms, with which we are now mainly con- 

 cerned, it may be seen that light, through acceleration of food manu- 

 facture, might cause increase of fission, due to increased availability 

 of energy producing and building materials. On the other hand, it 

 seems to be pretty well known that in higher algae and phanerogams 

 food manufacture is characteristic of day time, growth and repro- 

 duction of night, i.e., the plant does not carry on all its functions 

 equally well at the same time. 



The gaseous content of the water would be mainly effective, under 

 ordinary conditions, through its influence on the irritability of the 

 organisms and through its more or less direct connection with their 

 feeding and respiration. A very slight difference in dissolved gases 

 would surely change the responses of some organisms to light and 

 other stimuli, but it is hard to determine the definite connection of 

 that fact with the conditions now under consideration. As Birge and 

 Juday have clearly shown, the physiology of plankton organisms can- 

 not be fully determined by tests under artificial conditions such as 

 those of the laboratory, e.g., the capacity of various animal forms for 

 meeting anaerobic conditions of the environment is vastly greater in 



