430 LIFE MOVEMENTS IN PLANTS 



laid the scape horizontal, first with cooled side below, the 

 record showed that this did not affect the geotropic move- 

 ment. But on cooling the upper side, the geotropic niove- 



Fm. 158.— Effect of alternate application of cold on the upper and lower sides of 

 the organ. Application of cold on upper side (down-pointing arrow) induces arrest of 

 geotropic movement. Application below (up-pointing arrow) causes no arrest. 



ment became arrested, and it was not till the plant had 

 assumed the temperature of the surroundings that the 

 geotropic movement became renewed. Figure 158 shows 

 the effect of alternate application of cold, on the upper 

 and lower sides of the organ.* In the shoot, therefore, it 

 is the upper side of the organ that becomes effectively 

 stimulated. Before proceeding further I shall make brief 

 reference to the highly suggestive statolithic theory of 

 gravi-perception. 



THEORY OF STATOLITHS. 



With regard to the perception of geotropic stimulus 

 there can be no doubt that this must be due to the 



* " Plant Response " — p. 50o. 



