SENSITIVE PLANTS 



When man or any heavy animal is approaching certain 

 Indian plants, their leaves suddenly drop, and the leaflets close 

 together. The mere shaking of the ground or of the air pro- 

 duces these extraordinary movements in the sensitive Wood- 

 sorrel (Oxalis sensitiva), in two Leguminous plants (Smithia 

 sensitiva and Aeschynomene indica), and in several Mimosas. 



When one leaf-tip of Mimosa pudica^ the Sensitive Plant 

 (par excellence)^ is touched or injured, a series of changes 

 begin. All the little leaflets shut up one after the other ; then 

 the secondary stalks drop ; after this the main stalk of the 

 leaf suddenly droops downwards. After a short interval, the 

 next leaf above goes through identically the same movements. 

 If the shaking or injury is severe, every leaf from below 

 upwards moves in the same way. 



One probable advantage of these movements can be un- 

 derstood from the behaviour of flies, which alight upon the 

 leaves and make them drop. The flies are startled and go 

 away. Grazing animals will consider such behaviour in a 

 vegetable as very uncanny, and will probably go to some 

 other less ingeniously protected plant. 



Of course such extraordinary behaviour has been a chal- 

 lenge to the botanical world, and there is an overwhelming 

 mass of speculation, and observations about the Sensitive 

 Plant. 



It has been proved that the movements are caused by the 

 thickened part at the base of the main stalk of the leaf. 

 This is swollen, and full of water, and much thicker than 

 the stalk itself. It is by this thickened portion that the leaf 

 is kept at its proper angle. When the tip of the leaf is 

 shaken or injured, the cells on the under side of this swollen 

 part allow their water to exude into the spaces between 

 them, and in consequence down comes the leaf-stalk. 



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