130 Regulation of Size in Unicellular Organisms 



nutritional energy entirely from light and their chemi- 

 cal materials entirely from the same inorganic me- 

 dium. 



Duration of photosynthesis. The energy which 

 green flagellates receive from light is equivalent to as- 

 similated food. Absence of light is therefore exactly 

 equivalent to starvation. Hartmann ('21) found an in- 

 teresting effect of prolonged supply of light upon the 

 colonial species Eudorina elegans. Standard cultures 

 were maintained at maximum rates of growth in inor- 

 ganic salt solutions which were radiated by strong ar- 

 tificial light for twelve hours daily. When light was 

 furnished continuously, then reproduction occurred 

 faster. Whereas the colony of thirty-two individuals 

 ordinarily produced thirty-two new colonies at one 

 time every fifth day, or every sixty hours of radiation, 

 the colonies now reproduced at first after sixty hours, 

 then faster and faster. Insufficient time was taken for 

 growth, and so the individuals and the colonies became 

 smaller with each generation until finally they died. 

 This is an effect quite similar to that of a high tem- 

 perature; but the effects of temperature are usually 

 adjusted or compensated within the organisms. 



Kinds of organic compounds. In the green alga 

 Scenedesmus a great variety of foods can modify the 

 size of the individuals, according to Roach ('26). Some 

 of the results are shown in table 11, though it is not 

 certain that this investigator would wish those cul- 

 tures which were made at different times compared 

 very closely with one another. Not only the mean 

 volumes of individuals were calculated, but also the 

 total bulk which resulted after various numbers of 

 days. Our comparison at the ninth day is shortly be- 

 fore the end of "logarithmic" multiplication in all the 

 cultures. 



The hexose sugars and sucrose hastened growth 



