448 



PLANT RESPONSE 



tion per minute at that point. The rise of temperature in 

 one minute, then, under the experimental conditions described, 

 is through "4° C. The growth-elongation of the specimen, 

 therefore, while the temperature is rising from 33*8° C. to 

 34'2° C, gives us the rate of growth for the mean temperature 

 of 34 C. This is found in the magnified record to be 

 10 mm. The absolute value of the rate of growth is thus 

 •01 mm. per minute. In this way we can determine from the 

 curve the rate of growth corresponding to any temperature. 

 It will thus be seen how in the course of an experiment 

 lasting for thirty-five minutes only, we are able to obtain 

 data which give us the various rates of growth through a 

 wide range of temperatures. 



This operation can, moreover, be made entirely automatic. 

 The breadth of the circular electrolytic trough may be appro- 

 priately varied at different parts of the circle, so that turning 

 the handle through equal arcs raises the temperature of the 

 plant chamber by equal degrees. The handle of the rheostat 

 may then be rotated by the recording drum itself. Hence 

 in the record, equal lengths of the abscissa will represent not 

 only equal times, but also equal rises of temperature. And 

 finally by taking the record photographically, the whole pro- 

 cess becomes automatic. From the data furnished by fig. 184 

 we obtain the following table : 



Table showing Rates of Growth at Different Temperatures in 

 Flower of Crinum Lily. 



The curve shown in fig. 184 exhibits the relation between 

 these various temperatures and their corresponding rates of 

 growth. It is here seen that the rate gradually rises till we 



