268 THE BIOLOGY OF STENTOR 



been that of Hammerling (1946) and Schulze (1951) who began 

 their work together but published separately. Their findings will 

 be reviewed together, noting points of difference in interpretation 

 or observation. In the cultures of polymorphus, the stentors 

 collected appropriately at the lighted sides of aquaria, yet too 

 intense an illumination was detrimental. Hammerling remarked 

 that the stentors divided only at night or in the dark. This observa- 

 tion may be important in providing a means for obtaining 

 simultaneous fission of animals in well-fed cultures. 



Stentors were not easily divested of their symbionts. When 

 grown in the dark, chlorellae decreased greatly in abundance but 

 a few algae were always retained, mostly toward the posterior pole. 

 Persisting symbionts in these pale ciliates might therefore have 

 been removed by cutting and culturing a number of anterior 

 fragments. Instead, the method employed by Pringsheim (I.e.) for 

 Paramecium hursaria was used: pale stentors previously grown in 

 the dark were cultured with abundant food (free-living algae) at 

 high temperature of 3o°C. Under these conditions for rapid 

 division, some stentors would outpace the chlorellae and emerge 

 entirely white. Three classes of animals from the same stock could 

 therefore be compared : green forms with abundant chlorellae, pale 

 stentors grown in the dark but always retaining some algae, and 

 white animals completely devoid of symbionts. 



The presence of actively metabolizing chlorellae promoted the 

 survival of starving stentors. This was demonstrated by " feeding " 

 the symbionts while starving their hosts. Light and the mineral 

 nutrients in soil extract or Benecke's solution provided conditions 

 for metabolism and growth of the chlorellae, as proved by the fact 

 that symbionts did increase and pale animals became green when 

 only these factors were supplied. Conditions for starvation of the 

 stentors were established by withholding the free-living algae which 

 they had been eating and digesting, and by repeated transfers to 

 remove bacteria. Controls were afforded by comparing white with 

 green stentors and survival in darkness as well as in the light. 



When kept in the light, green stentors survived twice as long 

 as white animals without symbionts, and pale stentors with few 

 but increasing chlorellae were in between. The capacity of the 

 stentors to undergo occasional fissions following starvation was in 

 the same order, green ones dividing the most. In the dark these 



