External Factors and Size 135 



Graded concentrations. Cutler and Crump ('27) 

 noted that Hartmanella divided at smaller sizes as the 

 counted concentration of bacterial food decreased. 

 They found that both Hartmanella and Colpidium at- 

 tained unusually large sizes just after copiously re- 

 feeding. 



Graded concentrations of food flagellates were 

 tested upon Colpoda (Adolph, '29). The results, 

 shown in figure 41, indicate that all graduations of size 

 could be obtained in response to various conditions 

 ranging from starvation to an adequate supply of the 

 food organisms. Whereas at concentration 1, the low- 

 est, nearly all the Colpodae refused to grow and formed 

 protective cysts, at concentration 4 most of them tried 

 to grow and died in the attempt. But those that did 

 grow could not attain the size characteristic of well- 

 fed individuals, though they took longer times in which 

 to gather what food they could. 



As was the case with temperature, food appears to 

 influence body size primarily through fission rate. In 

 some species of unicellular organisms reproduction 

 keeps on for some generations after the food supply is 

 inadequate. In other species reproduction ceases as 

 soon as assimilation becomes inadequate for growth. 

 In the latter, size will decrease only in proportion as 

 bodily materials are used up in metabolic processes. 



3. Age of Culture Medium 



The study of reproduction in unicellular organisms 

 has in reality been for the most part a comparison of 

 methods and conditions of culture. The chief impedi- 

 ment to progress has been that most of the cultural 

 conditions were such as could not be described nor ex- 

 actly reproduced. Most of the studies of size in pro- 

 tista have been made under such unknown circum- 



