IRRITABILITY. 



439 



the side most exposed to light (the concave side) ; in B, those of the 

 shaded side. The time of growth was in every case a period of 5 hours. 



We will now endeavour to arrive at some explanation of 

 the phenomena of heliotropism. The fact which has to be 

 explained is the change in the relative length of the two sides 

 of the organ. Various explanations have been offered, but it 

 would not be any advantage to discuss them all. We will 

 confine our attention to that of de Candolle, for this one is 

 assumed, implicitly at any rate, in much of the current 

 botanical literature. De Candolle was of opinion that the 

 curvature of a positively heliotropic organ is due to a difference 

 in the intensity of the light to which the two opposite sides of 

 the organ are exposed, the result being the more rapid 

 elongation of the side which is exposed to the less intense 

 light : in fact, he regards heliotropic curvature as a phe- 

 nomenon of etiolation, the shaded side of the organ becoming 

 to a certain extent etiolated. The reasoning upon which this 

 theory is based is this. It is known that light retards growth 

 in length ; hence when the two sides of a growing organ are 

 exposed to light of unequal intensity, the growth of that side 

 will be the more retarded which receives light of the greater 

 intensity. The assumption is that it is the shaded side of the 

 organ which is active in producing the curvature. 



The inadequacy of this explanation is at once apparent 

 when negative heliotropism is considered. Here we have 

 precisely the opposite effect produced under the same external 

 conditions. It was thought that negatively heliotropic organs 



