SENSITIVE PLANTS 



them to take measures for their safety, serves a 

 useful purpose in their struggle for existence. 

 Instances of this kind are so famihar in animal 

 life that we often fail to realise how very valuable 

 to an animal is this delicate sense of touch. Indeed, 

 this familiarity with the movements of animals 

 is apt to make us oblivious even to their sensi- 

 tiveness to external stimuli, and much more 

 oblivious to their dependence on the possession of 

 such sensibility for self-protection. 



Turning now to plants, we should hardly 

 expect to find developed in them the sense of 

 touch, because we regard them as organisms with- 

 out feeling. Also, we have to recognise that plants 

 possess no nerves and brain centres such as we are 

 familiar with in most animals. But, nevertheless, 

 there are numerous plants which are just as sensi- 

 tive as the caterpillar or hedgehog, or even more 

 so. There are plants so sensitive that if, when 

 standing by them, 3^ou should suddenly spread 

 your umbrella or sunshade, it would be quite suffi- 

 cient to cause them instantly to close together 

 their leaflets and turn down their leaf stalks, just 

 as if they were startled and alarmed by the 

 movement. On a sunny day, when the tempera- 

 ture is sufficiently high, you need not make even 

 so decided a movement ; merely your shadow 

 falling on their leaves will often cause them to 

 droop slightly. 



In Fig. 45 (Plate 28) is shown one of the most 

 celebrated of these sensitive plants — Mimosa 

 ptidica, a native of Brazil — as it appears when 



59 



