260 



THE BIOLOGY OF HYDRA : 1961 



FED DAILY 



GREEN 



ALBINO 



10 



TIME IN DAYS 



Fig. 4. Semi-log plot of growth rates of duplicate cultures of green and 

 albino C. yiridissima fed daily in the light. 



number of hydranths was counted and then each hydranth was fed 

 on a dense suspension of Artemia nauphi. One hour after feeding 

 and again, six hours later, the culture medium was renewed. This 

 routine was followed for 5-7 days. 



Figure 4 shows that green and albino C. viridissima, when fed 

 daily, have nearly identical logarithmic growth rates. These results 

 imply that the algae do not contribute anything to the host that 

 cannot be acquired from an exogenous food supply. Under optimal 

 conditions, nutritional benefit would not be expected to manifest 

 itself in terms of growth of the host because the maximum growth 

 rate (kmax), a property intrinsic to the species, cannot be exceeded, 

 regardless of the magnitude of the algal contribution. Therefore, we 

 conducted growth experiments in which the amount of food was 

 limited, reasoning that this would then allow benefit from the algae 

 to express itself. Figure 5 demonstrates that, when fed every second 



