T. M. SONNEBORN 189 



much longer than an ordinary interfission interval. Also pro- 

 longed, but not so much so, is the time from the first to the sec- 

 ond fission. This is about 10 hours in variety 4, about 16 to 17 

 hours in varieties 13 and 15. These are probably the extreme dif- 

 ferences among the varieties. Similar but smaller differences may 

 well exist among certain other varieties. Information is at present 

 lacking on that point. In general the timing of autogamy seems 

 to be about the same as for the comparable nuclear stages of con- 

 jugation in varieties in which both processes occur. 



(c) Environmental conditions for conjugation. The tempera- 

 ture optima and ranges for the occurrence of conjugation differ 

 for certain varieties (Sonneborn, 1938, 1939, 1941, 1956a, unpub- 

 lished; Sonneborn and Dippell, 1943, 1946). Varieties 2, 3, 11, 

 and possibly a few other less studied varieties, have relatively 

 low optimal temperatures for mating and conjugate with reduced 

 frequency or not at all at high temperatures within the tolerated 

 range. These varieties mate avidly at 19° or less, poorly or not at 

 all at 27° or more. Other varieties, such as 1, 4, 8, and 9 conju- 

 gate avidly at 27°, or even higher temperatures, as well as at 

 19°. Sonneborn and Dippell (1946) compared the sexual reactiv- 

 ity of representatives of several varieties over a wide range of 

 temperatures and found for each one a unique pattern of re- 

 sponses to temperature. Possibly extension of such a study to all 

 varieties would reveal unique features for each. Nothing is yet 

 known as to the possibility of strain differences in these respects 

 in a variety — like 1, 2, 15, or 16 — which occurs in nature over a 

 wide range of temperatures. 



Visible light inhibits sexual reactivity in variety 3 (Sonneborn, 

 1938, 1939). Under the usual alternation of day and night, this 

 variety is sexually reactive only between about 1 a.m. and 1 p.m. 

 Diurnal periodicities also exist in some other varieties; the period 

 of reactivity is during the night in variety 2; from about 4 p.m. to 

 10 a.m. in variety 11; and from about 8 p.m. to 10 a.m. in va- 

 riety 15, peak reactivity after 11 p.m. (Sonneborn, 1956a; unpub- 

 lished ) . 3 There are also indications of periodicities in varieties 12 

 and 13, but the details are not yet known. Some of these varieties 

 can become sexually reactive at any hour after they have been 



