176 PROTOZOA 



showed a similar temperature tolerance, although the latter has 

 been found only in Florida whereas the former never has been 

 found in warm climates and is most abundant in cold climates. 

 However, this particular strain of variety 3 came from near the 

 southern limit of its range, and there may be strain differences 

 in temperature tolerance within a variety. Indeed, some strains 

 of variety 4 grow well at 35° (Margolin, 1956b), while others 

 die at that temperature. Preer (unpublished) has even found 

 temperature mutants within a strain of variety 2. The mutant 

 cannot survive at 31° while the wild type can. Of the tolerance 

 for low temperatures even less is known. We have some indica- 

 tions that variety 4 strains become abnormal if long grown at 

 10°. There is much need for a systematic study of temperature 

 tolerance. It may prove to be diagnostic for varieties with limited 

 natural range of climates, but more variable in varieties with 

 wide natural temperature distributions. 



The relation of temperature to fission rate may be studied in 

 several ways: comparing different varieties at a standard tem- 

 perature, comparing the maximal fission rate at whatever tem- 

 perature yields it, comparing the temperatures which yield the 

 maximal fission rate, and comparing the entire curve relating tem- 

 perature to fission rate over the whole viable range of tempera- 

 tures. The latter method is the most revealing. Sonneborn and 

 Dippell (unpublished data of 1942) did this for the representa- 

 tives of varieties 1, 2, 3, 4, and 7 mentioned above. Up to about 

 23°, they found only minor differences in fission rate among these 

 strains; but at higher temperatures the differences became 

 strongly accentuated and each variety gave a distinctive curve. 

 The question of the existence of strain differences within a variety 

 was not satisfactorily analyzed in their work. For the strains 

 examined, the maximal fission rates attained were in part related 

 to the maximum temperature tolerated, for fission rate increases 

 with temperature almost up to that point, but even at lower tem- 

 peratures differences appeared. Thus both at 28.5° and 30.5°, 

 the order of the varieties in fission rates, from high to low, was 

 4, 1, 7, 3, 2, covering a range (at 28.5°) from 5.8 down to 4.2 

 fissions per day. Varietal differences in fission rate thus unques- 



