CHAP. XII. CONCLUDING REMARKS. 5(33 



gyrating movements of the little lateral leaflets, seem 

 to be due proxirnately to the pulvinus, or organ of 

 movement, not having been reduced nearly so much 

 as the blade, during the successive modifications 

 through which the species has passed. 



We now come to the highly important class of 

 movements due to the action of a lateral light. When 

 sterns, leaves, or other organs are placed, so that one 

 side is illuminated more brightly than the other, they 

 bend towards the light. This heliotropic movement 

 manifestly results from the modification of ordinary 

 circumnutation ; and every gradation between the two 

 movements could be followed. When the light was 

 dim, and only a very little brighter on one side than 

 on the other, the movement consisted of a succession 

 of ellipses, directed towards the light, each of which 

 approached nearer to its source than the previous one. 

 When the difference in the light on the two sides 

 was somewhat greater, the ellipses were drawn out 

 into a strongly-marked zigzag line, and when much 

 greater the course became rectilinear. We have 

 reason to believe that changes in the turgescence of 

 the cells is the proximate cause of the movement 

 of circumnutation ; and it appears that when a plant 

 is unequally illuminated on the two sides, the always 

 changing turgescence is augmented along one side, 

 and is weakened or quite arrested along the other 

 sides. Increased turgescence is commonly followed by 

 increased growth, so that a plant which has bent itself 

 towards the light during the day would be fixed in this 

 position were it not for apogeotropism acting during 

 the night. But parts provided with pulvini bend, as 

 Pfeffer has shown, towards the light ; and here growth 

 does not come into play any more than in the ordinary 

 circumnutating movements of pulvini. 



