METABOLISM 269 



differences disappeared and maximum survival times were the 

 same. Hence photosynthesizing chlorellae did confer an advantage 

 in survival of their starving hosts. Schulze further noted, hov^ever, 

 that in the dark the first individual to die was always a white one, 

 for minimal survival times were in the order : white, pale and green. 

 This result seems enigmatic since one would expect the catabolism 

 of stentor and symbiont to be additive. But according to 

 Hammerling, stentors receive some advantage from their 

 symbionts even in the dark. 



What is the nature of the aid to survival provided by Chlorella ? 

 First, this succor was not complete, because stentors with thriving 

 chlorellae did eventually die. Therefore the ciHates did not become 

 autotrophic, or capable of living indefinitely through their 

 symbionts on light and mineral nutrients alone, as is possible in 

 the more complete symbiosis found in Paramecium hursaria 

 (Pringsheim, 1928). Schulze showed this deficiency to be the same 

 in igneiis as in polymorphus. At first Hammerling said that starving 

 stentors simply digest their symbionts, so that animals with 

 abundant chlorellae should live longer on these food reserves ; but 

 he later softened this conclusion. And Schulze found that the 

 hosts only partially and never completely digest their symbionts. 

 Moreover, any minor aid from partial digestion should be com- 

 pensated by multiplication of the chlorellae at the expense of their 

 hosts. Digestion of the algae was therefore probably not the basis 

 of longer survival. Hammerling observed that pale animals with 

 very few chlorellae nevertheless lived much longer in the dark than 

 white animals, though of course not as long as green ones, indicat- 

 ing that the algae wxre supplying some minor factor important to 

 survival. The possibility that polymorphus has lost the capacity for 

 the synthesis of one or more vitamins was suggested by Schulze's 

 finding that his white stentors could not live indefinitely unless 

 fed on green, free-living algae. This species of Stentor may 

 therefore have become dependent on plants for certain vitamins. 



As stated by Hammerling the general conclusion from the tests 

 was that the presence of chlorellae enables stentors better to endure 

 a period of starvation. That such is the only advantage conferred 

 by the symbionts was further indicated by studies of the division 

 rate of well-fed stentors. He found that the fission rate was only 

 slightly higher if chlorellae were present and Schulze said that the 



