Culture Techniques 



145 



very long, will kill the cultures (Fig. 44). It is 

 because of this extreme sensitivity to high tem- 

 peratures that cultures, when maintained at 

 "room temperature," fluctuate so much more 

 widely in growth rate in summer than in winter. 

 Cultures will, on the other hand, endure low tem- 

 peratures — down to + 5° C. — for long periods 



o 



Q_ 

 Ld 



tr 



3 

 O 



a. 



I5h 



10 



o= Growth rates at the indicated temperatures. 



x = Growth rates at 22', of cultures previously 

 held for one week at the indicated 

 temperatures. 



x— x-x 



10 



20 30 



DEGREES C. 



Fig. 44. Average increment rates of isolated tomato roots main- 

 tained for one week at a series of controlled temperatures ranging 

 from 5° to 40° C. (open circles) and then all transferred for a sec- 

 ond week (crosses) to a temperature of about 22° C. All except 

 those maintained at 35° and 40° show an immediate reversion to the 

 growth rate typical for 22°. (From White, P. R. 1937. Plant 

 Physiol. 12 : 773, fig. 1, 266.) 



