444 PLANT RESPONSE 



temperature thus attained. It is also necessary, for reasons 

 to be fully explained in a subsequent chapter, that the 

 specimen should not receive thermal radiation from the 

 heating coil, as such radiation, I find, retards growth. A 

 shield of mica, opaque to thermal radiation, is interposed 

 between the heating coil and the plant, which is thus subjected 

 only to the action of changes of temperature. For many of 

 my experiments I selected specimens of the Crinum Lily, on 

 account of its extreme regularity of growth, which is so 

 uniform that on adjusting the record under balance, the 

 external conditions being constant, the line of record remained 

 horizontal for a period of certainly over an hour. 



The rate of growth at the temperature of the room, say 

 30 C, is first taken on the recording drum, which is covered 

 with paper divided into millimetres. The horizontal distance 

 or abscissa represents time, which, with the particular speed 

 of drum which I used was 6 mm. per minute. The ordinate 

 represents growth-elongation, and as the growth-recorder, or 

 crescograph, produces a magnification of 1,000 times, 1 mm. 

 distance of the ordinate is equal to an actual growth of 

 •001 mm. The ordinate corresponding to an abscissa of 

 6 mm. would thus be equivalent to a growth in thousandths 

 of a millimetre per minute. 



The rheostat handle is now turned to the index-number 

 corresponding to the raising of the temperature of the chamber 

 by 1 ° C. There is first a variable period of rise of temperature, 

 after which a permanent degree is attained. During this 

 preliminary stage, variation of temperature acts as a stimulus, 

 giving rise to responsive contraction or retardation of growth. 

 But after this transient disturbance, the growth attains a 

 constant rate, characteristic of the given temperature. These 

 peculiarities will be better understood on following the record 

 given in fig. 182 — reduced here to half the original size — 

 which was taken with a specimen of Crinum Lily, during ten 

 minutes. The record during the first five minutes is for the 

 temperature of 34 C. It will be seen that in two minutes 

 the growth-elongation is fifteen divisions, and as each division 



