272 THE BIOLOGY OF STENTOR 



From these experiments it is clear that symbiosis with chlorellae 

 is a precise relationship involving a delicate equilibrium between 

 specific host and specific symbiont. Morphological differences 

 noted by Schulze certainly indicate that the chlorellae normally 

 found in polymorphus, igneus, and P. bursaria are distinct types. 

 Successful symbiosis was not related to the readiness with which 

 the ciliates took up chlorellae, for in some cases colorless animals 

 rapidly became green but not symbiotic, and vice versa. A first 

 requirement is that the chlorellae be not digested, though taken into 

 food vacuoles, and later infiltrated intimately into the cytoplasm. 

 It is not known how digestion is prevented, since the hosts seemed 

 to be able partially to digest symbionts when starved, and they 

 could also fully digest free-living chlorellae. The second require- 

 ment is that a harmonious equilibrium be established between 

 multiplication of host and symbiont such that the one does not 

 outpace the other in reproduction. 



According to Hammerling (1946), Ohler (1922) and Pringsheim 

 (1928) have found that P. bursaria can take up different types of 

 algae, but free-living forms are not capable of substituting for the 

 natural symbionts. Apparently this is also the case with S. poly- 

 morphus, for I have found animals containing both Chlorella and 



Fig. 75. Facultative multiple symbiosis, a : Stentor polymorphus 

 with both Chlorella and a needle-like algae free in endoplasm. 

 The needle forms were same as those growing in the culture. 

 b: On isolating onto slides, the needle algae were shed — 

 apparently alive — but not the Chlorella. 



