642 PLANT RESPONSE 



in this particular position, are enabled to absorb the largest 

 amount of sunlight. Such arguments, however, do not throw 

 any light on the mechanism by which the movement is 

 brought about. It is even somewhat difficult to understand 

 how this generalisation, that leaves place themselves at right 

 angles to light, has come to be accepted as a universal fact ; 

 for it is only necessary to make a visit to the open forest in 

 order to see that, so far from this being the case, many leaves 

 place themselves vertically upwards, others downwards, and 

 the rest in all possible intermediate angles between the two. 

 All that can be claimed on behalf of the dia-heliotropic 

 position is that no deviation from it is greater than plus or 

 minus 90''. 



Theory of De Vries. — De Vries, however, is in disagree- 

 ment with this theory. He establishes the importance of the 

 unequal rates of growth in the upper and lower halves of the 

 leaves — i.e. epinasty or hyponasty — as a factor in bringing 

 about their ultimate attitude. He next assumes, in accordance 

 with the generally accepted view, the existence of two opposite 

 responsive reactions, positive and negative, in regard to light 

 and gravity. He then proceeds to show that there can be no 

 necessity for the further assumption of a third or dia-heliotropic 

 tendency in the leaves, since epinasty and hyponasty, their 

 different combinations with either of the opposite actions 

 of positive and negative heliotropism and geotropism, and 

 considerations of the weight and balance of parts, are all 

 factors which take a share in determining the position ulti- 

 mately to be assumed by the dorsi- ventral organ with regard 

 to light. 



I shall now attempt to demonstrate the fact that the 

 responses of ordinary leaves are in every way similar to 

 those of the pulvinated, the mechanics of whose movements 

 have already been fully described. It will further be shown 

 how and under what circumstances the normal positive passes 

 into negative response, through certain intermediate phases. 



Subsidiary factors.— But before undertaking either of 

 these inquiries, it will be well to enumerate briefly the 



