ANALOGY WITH THERMONASTY 245 



the second half the record became true ; on the third day 

 the inverted plant gave a record (c) which from an external 

 point of view was similar to that given by the plant in the 

 first or normal position. 



There is another aspect of the subject which may be 

 of interest, namely, whether geotropic irritability itself 

 undergoes variation under high temperatures in the tropics. 

 The results of various investigations have shown (as 

 will be described in a later chapter) that geotropic 

 stimulation is effected by the fall of starch-grains in the 

 cells of the statolithic apparatus. Is the efficiency of this 

 apparatus modified by high temperature ? This problem 

 I investigated by the method of geo-electric response, in 

 which the excitatory reaction under geotropic stimulation 

 was detected by the concomitant electric response. I found, 

 for example, that while the intensity of geo-electric response 

 was, generally speaking, very marked in Calcutta during the 

 month of February with an average temperature of 20° C, it 

 disappeared by the middle of April, when the average tem- 

 perature had risen to about 30° C. With Tropaeolum majus 

 I could get no response even in March. On repeating the 

 experiments with the same plant three months later at my 

 Mayapuri Research Station, Darjeeling, I was considerably 

 surprised to find that the geo-electric response of Tropaeolum 

 was fully vigorous at the hill station, where the temperature 

 was lower than 20° C. This would appear to show that 

 geotropic irritability is accentuated within limits by a fall of 

 temperature and depressed by a rise. Though some plants 

 exhibit this change in a marked degree, yet it cannot be 

 asserted that all plants exhibit it. The particular periodic 

 movements under consideration can, however, be explained 

 by the effect of variation of temperature on an organ 

 rendered anisotropic by geotropic stimulation. 



Analogy with Thermonasty 



It has been shown that in an anisotropic growing organ, 

 rise of temperature, acting on it as a whole, induces a 



