242 CHAP. XXII. DIURNAL MOVEMENTS OF PLANTS 



intensity of light increases, while it decreases in the early 

 afternoon as the intensity of light diminishes. Variation 

 of temperature within normal limits does not, according to 

 him, appear materially to affect the daily period. In other 

 words, it is the variation of light, and not of temperature, 

 that has a marked influence on the tension of the tissues. 

 In the case of diurnal movement of trees, however, the 

 effective factor is not light but variation of temperature. 



Even if variation of tension were due to variation of 

 temperature, it is by no means clear how this could cause 

 a definite up and down movement of the tree. Rise or fall 

 of temperature acting on the tree as a whole cannot produce 

 any unidirectioned movement unless one side is relatively 

 more active than the other, as is the case only in anisotropic 

 organs. 



In a young tree the geotropic effect is outwardly mani- 

 fested by the upward curvature induced in the growing 

 region. I have shown further that the cortex of an old 

 rigid stem, though incapable of outward movement, is still 

 sensitive to stimulus.^ The cortex of the tree throughout 

 its length may thus be rendered differentially excitable by 

 the prolonged action of geotropic stimulus which renders 

 it anisotropic. The fact that a condition of anisotropy is 

 essential for the production of diurnal movement under 

 diffuse stimulation will presently be demonstrated. 



Diurnal Variation of Tissue-Tension in Mimosa 



It has just been stated that the characteristic response 

 under variation of temperature is best exhibited by an 

 organ which is anisotropic. Millardet found a daily 

 periodicity of tension in Mimosa pudica, shown by the up 

 and down movements of the leaf ; the organ of response is 

 here the pronouncedly anisotropic pulvinus. Millardet was 

 thus able to correlate the periodic movements of the leaf 

 with variation of temperature and of tension. The maximum 



1 The Motor Mechanism of Plants (1928), p. 141. 



