LIFE HISTORIES OF FAMILIAR PLANTS 



Seeing that the Mimosas and the Wood-sorrel 

 tribe are distinct groups of plants, their families 

 being in no way related, it is, I think, reasonable 

 to contend that the sensitive characteristic of their 

 leaves was evolved from the sleeping habit. How- 

 ever, it still remains to explain how the plants 

 acquired their habit of shrinking at the approach 

 and the touch of animals. 



The explanation is very simple. A plant that 

 has developed the sensitive or sleep movement to 

 a high degree is necessarily affected when light 

 decreases. Thus, at the approach of a storm, 

 when the sky becomes cloudy and dark, its leaflets 

 quickly close together. The rain pelts down with 

 the characteristic force of storms in the tropical 

 countries where these extremely sensitive plants 

 are found ; the leaflets, however, huddle still more 

 closely together, for in that position they most 

 readily throw off the water ; and the probability 

 of their getting damaged is, of course, compara- 

 tively small. So, in process of time, the leaflets 

 would acquire the habit of drooping at the first 

 spots of rain that touch them, and this quite 

 independently of the influence of light — simply 

 because the leaves benefit by the habit. This, I 

 think, was how sensibility to touch was first 

 acquired and manifested by these leaves. Later, 

 when the habit had become fixed, they quite 

 naturally drooped their leaves also when animals 

 touched them ; for they had learned that such 

 was the proper and wise course to pursue when- 

 ever anything external came in contact with them. 



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