METABOLISM 271 



were first encapsulated in food vacuoles but not digested and later 

 freed into the endoplasm. By this means normal symbiosis could 

 be reestablished in either Stentor polymorphus or Paramecium 

 bursaria provided with their own type of chlorellae. The procedure 

 itself was therefore adequate. 



Exchanges of symbionts made by Schulze, in part confirmed by 

 Hammerling, are diagrammed in Fig. 74 and may be summarized 

 in the following propositions: 



(a) Colorless P. bursaria readily acquired Chlorella from 

 S. polymorphus but did not become autotrophic as with their own 

 chlorellae. Enduring symbiosis was not established, for the ciliates 

 later became white. This was confirmed by Hammerling. 



(b) In contrast, the paramecia established a true symbiosis and 

 became capable of autotrophic nutrition when chlorellae of -S". igneus 

 were substituted for their own. 



(c) However, if chlorellae from a stock of igneus had previously 

 resided for 7 months in polymorphus^ they then gave no satisfactory 

 symbiosis when introduced into white P. bursaria. The host can 

 therefore alter the symbiont, and in this case the pyrenoid of the 

 igneus chlorellae was lost. 



(d) As the preceding implies, colorless polymorphus could 

 establish an enduring symbiosis with chlorellae from igneus, in 

 spite of the fact that the algae were not readily taken up and the 

 hosts remained paler than polymorphus with their own chlorellae. 

 Using certain stocks of both species of Stentor, chlorellae from the 

 igneus were said to be rendered capable of free-living existence 

 after passage through the polymorphus, again indicating an eflrect 

 of host on partner. 



(e) These independent algae could then quickly establish a good 

 symbiosis when taken up by colorless polymorphus. 



(f) But when normally free-living Chlorella were offered to white 

 polymorphus no symbiosis developed. 



(g) Colorless polymorphus estabUshed with chlorellae from 

 P. bursaria what Schulze called a labile partnership. Only with 

 difficulty was a symbiosis estabhshed, and this relationship per- 

 sisted only if bright light and soil extract were provided, but 

 Hammerling succeeded in maintaining a green culture for one year. 

 Notable was the fact that the chlorellae did not render the stentors 

 autotrophic as they do the paramecium. 



