268 CONTRACTILITY OP VEGETABLE TISSUES. 



elasticity of the walls, therefore, occasions their violent con- 

 traction, when an aperture is formed in any way, by which the 

 distension is relieved. 



420. Such explanations, however, will by no means account 

 for all the evident movements of Plants ; and it is necessary to 

 suppose their living tissues to be endowed with a property 

 termed contractility, by which they are enabled to contract upon 

 the application of a stimulus, just as do the muscular fibres of 

 animals. The Vegetable kingdom affords many examples of 

 this kind of contraction. Thus, if the leaves of the common 

 Wild Lettuce be touched, when the plant is in flower, the part 

 will be covered with milky juice, which is forced out through 

 the stomata, by the contraction of the cells or vessels beneath. 

 Again, in the flower of the Berberry, if the base of the stamen 

 be touched with the point of a pin, the filament or stalk will 

 bend over, so as to strike its top against the style or central 

 pillar of the flower. This movement will hereafter be seen 

 (. 437) to be connected with the process of fertilisation ; and it 

 must be frequently caused by the contact of insects, which thus 

 assist in that function. There is a curious New Holland plant, 

 named Stylidium, sometimes cultivated in green-houses in this 

 country, which has a tall column rising from the centre of its 

 flower, and consisting of the stamens and style united ; this 

 usually hangs down over one side of the flower ; but if it be 

 touched ever so lightly, it starts up with a jerk, and rapidly 

 swings over to the opposite side. 



421. One of the most interesting of all the Vegetable move- 

 ments, however, is that displayed by the Sensitive plant (Mimosa 

 pudica). This is a Leguminous plant of the Acacia kind, which 

 has its leaves very much subdivided into leaflets. When spread 

 out in sunshine, they present no peculiarity of appearance ; but at 

 night they fold together as in sleep, more completely perhaps than 

 the leaves of any other plants. If, when expanded, one of the 

 leaflets be slightly touched, it will close towards its fellow; the 

 neighbouring leaflets will presently do the same ; the vein upon 

 which these are set will bend downwards, and meet the one on 

 the opposite side of the midrib ; the midrib itself will afterwards 



