iv Movement and Nervous Action in Plants 75 



theory of circumnutation Darwin acknowledged that it did 

 not explain all movements ; he did not propose to apply it 

 to the collapse of the Sensitive plant's leaves when they are 

 touched, or to the movement of the stimulated Sundew and 

 Venus Fly-Trap, or to the movement of the Barberry's 

 stamens when they are jostled by an insect's legs. 



In regard to the means by which the circumnutating 

 movements are brought about, Darwin expressed himself 

 in his Movements of Plants more definitely than he had 

 done in the previous volume. Following several German 

 botanists, he emphasised the importance of the turgescence 

 or state of water-tension of the cells of the plant, and he 

 also recognised the extensibility of the cell-walls. " On 

 the whole," he says, " we may at present conclude that 

 increased growth, first on one side and then on another, is 

 a secondary effect, and that the increased turgescence of 

 the cells, together with the extensibility of their walls, is 

 the primary cause of the movement of circumnutation." 



Having stated the general thesis of Darwin's book, 

 namely, that most of the movements of plants are modified 

 forms of circumnutation, and his general conception of the 

 internal processes involved, we may next briefly discuss the 

 movements of plants in their relation to gravitation, light, 

 and other external influences. In so doing we shall not in- 

 sist upon Darwin's generalisation, for perhaps no part of his 

 work has met with more adverse criticism, as notably on the 

 part of Sachs, than this. It is indeed the opinion of most 

 botanists that Darwin's theory of circumnutation was over- 

 strained. 



