450 PLANT RESPONSE 



fallen to "0045 mm. per minute, or almost the same as at 

 30 C. Thus, while 5-5° C. of rise of temperature before the 

 optimum enhanced the rate of growth by three times, a 

 further rise of only 15 C. beyond that point was sufficient to 

 bring it back to almost the same value as at 30 C. The 

 individual characteristics of each specimen are seen, not by 

 any perceptible variation of the optimum point, but rather by 

 differences in the steepness, during rise or fall, of the curve. 

 With some specimens, for example, the increase of rate of 

 growth during an equal rise of temperature from 30 C. to 

 35"5° C. is only half of that seen in the figure. The steepness 

 of fall, on the other hand, beyond the optimum may be 

 much greater ; that is to say, a rise of i° C. or less above 

 the optimum will sometimes reduce the rate of growth to its 

 value at 30 C. 



(3) The Method of Balance. — I shall now describe an 

 extremely delicate method of determining the rate of growth 

 at different temperatures, which is especially suited for the 

 exact determination of the optimum point. A balanced line 

 of record is first obtained by the turning of the balancing 

 wheel of the Crescograph (fig. 168). This regulates the 

 difference of level of the syphon tube, until the spot of light 

 is stationary at the temperature of the room. As the tem- 

 perature is now raised and the rate of growth increased, the 

 balancing wheel has to be rotated, say to the right, in order 

 to keep the spot of light stationary. The reading of the 

 circular scale at different temperatures thus gives the balanced 

 readings for the corresponding rates of growth at those 

 temperatures. A previous calibration of the value of the 

 circular scale enables us to determine the absolute growth- 

 movements at various temperatures. For the determina- 

 tion of the optimum point, however, this is not necessary. 

 All that has to be done in this case is to keep the spot, 

 which would otherwise drift to the right under a constantly 

 increasing rate of growth, on the point of balance, by the 

 right-handed rotation of the balancing wheel. This must be 

 done as long as the temperature and rate of growth are 



