METABOLISM 267 



according to Stolte (1922), a condition which could be corrected 

 by supplying oxygen through algae added to the medium. His 

 interpretation was that oxygen is necessary for the elaboration 

 and activity of digestive enzymes. 



5. Symbiosis with green algae 



As in many other ciliates and in simpler metazoa, certain stentors 

 may bear spheroid, grass-green cells of Chlorella living within 

 them. These species include polymorphtis, igneiis (Balbiani, 1893), 

 amethystinuSy and niger (Maier, 1903). The algae reside in the 

 endoplasm (Johnson, 1893), where they are scattered at random. 

 That the relationship is symbiotic is shown by the demonstrations 

 that the stentors in question can live without the algae, that the 

 algae do not disintegrate on death of the stentor (Balbiani, 1893) 

 and may even continue life as free-living cells, and that both 

 stentors and algae receive advantages from their association. 



The first experiments on symbiosis in Stentor were made by 

 Prowazek (1904). He reported that chlorellae can multiply within 

 dead and crushed polymorphus, indicating that the algae in stentors 

 can probably be grown in '' tissue culture " like those of Para- 

 mecium bursaria (Loefer, 1936). Having obtained artificial sym- 

 biosis by infecting Stylonychia and Euplotes with free-living 

 Chlorella, Prowazek tried unsuccessfully to obtain the same with 

 Stentor coeruleus. Failure occurred in spite of the fact that the 

 coeruleus digested the chlorellae only partially and ejected the 

 remainder. Even the enucleated stentors would not accept chlorellae 

 intimately into their cytoplasm, ingesting the algae but retaining 

 them, undigested, within food vacuoles. Prowazek concluded that 

 the cytoplasm of coeruleus is unfavorable for Chlorella and he there- 

 fore doubted Kessler's (1882) report that symbiosis can be 

 established between this Stentor and the chlorellae from a fresh- 

 water sponge. Certain experiments of mine were confirmatory 

 (Tartar, 1953). In interspecific grafts (a convenient method for 

 introducing symbionts) any substantial admixture of coeruleus 

 cytoplasm with polymorphus resulted in the ejection of symbionts 

 natural to the latter species. In Prowazek's observations neither 

 enucleated polymorphus nor enucleated coeruleus with ingested 

 chlorellae survived longer than controls without algae. 



The most comprehensive study of symbiosis in polymorphus has 



