654 



PLANT RESPONSE 



determined by the fact that it is always the more excited 

 side of the motile organ that undergoes contraction and 

 concavity. That various types of this effect arise is due only 

 to the unequal excitabilities of the two halves, and to the fact 



that the stimulus remains 

 in some cases localised, 

 while in others, owing to 

 the better transverse con- 

 ductivity of the tissue, it 

 becomes internally diffused. 

 The position ultimately 

 taken up by the leaves is 

 thus determined primarily 

 by the action of light, and 

 secondarily by that of 

 various subsidiary factors, 

 which are : (i) the natural 

 movement of the organ, 

 due to hyponasty or epi- 

 nasty ; (2) the differen- 

 tial excitability of the 

 two halves of the dorsi- 

 ventral organ under the 

 stimulus of gravity ; (3) the turgescent condition of the plant, 

 as determined by its suctional activity ; and (4) the limits of 

 flexibility of the organ, as determined by its anatomical 

 peculiarities. It is out of the innumerable possible combina- 

 tions of these factors that the variety of attitudes ultimately 

 assumed by the leaves directly arises. 



With regard to the nature of geotropic action on leaves, 

 it will be shown in the next chapter that here also, as in the 

 case of dia-heliotropism, the assumption of a dia-geotropic 

 property is unnecessary ; for the observed effects are ex- 

 plained by the fact, which I shall demonstrate, that a dorsi- 

 ventral organ possesses differential geotropic sensibility. 



Fig. 264. Negative Heliotropic Response 

 of Leaves of Arfocar^ns \mder Sunlight 

 acting from Above 



Tlie upper record exhibits the response of 

 a very young leaf, second in order from 

 the top of the shoot. The lower record 

 shows that of an older leaf, fourth in 

 order on the same shoot. 



