Chap. XII. CONCLUDING EEMAEKS. 5G3 



gyrating movements of the little lateral leaflets, seem 

 to be due proximately to the pulvinus, or organ ot 

 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 



stems, 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 ol 



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 



chano-ing 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 



circumnntating movements of pulvini. 



