HARMFUL EFFECTS OF CROWDING UPON GROWTH 107 



GROWTH INHIBITION IN ANIMAL CULTURES 



The work on animal cultures most closely connected with these 

 investigations on bacteria and on tissue culture is that dealing with 

 the growth in a protozoan infusion. In two studies (1911 and 1914) 

 Woodruff demonstrated that Paramecia excrete substances that are 

 toxic to themselves when present in their environment and that 

 probably play an appreciable role in determining the time of maxi- 

 mum number, rate of decline, and other characters. Similar conclu- 

 sions were reached as a result of work with the hypotrich as regards 

 their own excreta, but they are immune to the effects of Paramecium 



Fig. 4. — ^A record of the rate of division of Paramecium aurelia in a series of four 

 experiments (A, B, C, D) to determine the effect of different volumes of culture medium, 

 changed every 24 hours, on the rate of reproduction. The ordinates represent the 

 average daily rate of division of the four lines of organisms in the respective volumes 



of medium, averaged for 4-day periods. Rate of division in 2 drops, ; 5 drops, 



; 20 drops, ; 40 drops, -• • (From Woodruff, 191 1.) 



excreta. In a protozoan infusion the appearance of dominant Pro- 

 tozoa at the surface runs in this order: Monad, Colpoda, Hypo- 

 trichida, Paramecium, Vorticella, and Amoeba. This ecological se- 

 quence is due in part to accumulation of toxic material and in part 

 to the supply of available food. 



This problem is closely related to the consideration of the effect 

 of the size of the effective environment, whether lake, pool, or labo- 

 ratory container, upon the contained organisms, which in turn is 

 closely related to the whole problem of crowding. 



