Movements of Plants. 309 



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

 to the opposite side. 



One of the most interesting and best known of all vegetable 

 niovements, is that displayed by the Sensitive plant {Mimosa 

 pudica.) ^ When spread out in sunshine, the leaflets present no 

 peculiarity of appearance ; but at night they fold together as in 

 sleep. If, when expanded, one of the leaflets be slightly 

 touched, it will close towards its fellow ; the neighboring leaf- 

 lets 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 mid-rib ; the mid-rib will itself bend down upon 

 the stem ; and if the plant be in a very irritable state, the 

 other leaves are sometimes affected in a similar manner. The 

 explanation of this very curious phenomenon requires a know- 

 ledge of the structure of the parts concerned in it. It is evi- 

 dent that the movement must be in some way propagated from 

 the parts touched, to those where the change actually takes 

 place— that is, the points where the leaflets join the veins, the 

 veins come from the mid-rib, and the mid-rib from the stem. 

 At every one of these points there is a small swelling or intu- 

 mescence, formed of very spongy cellular tissue, and contain- 

 ing a great deal of fluid in its cells. If the under side -of the 

 intumescence at the foot of the leaf-stalk be touched, its vesi- 

 cles, being very irritable, contract and force out the fluid 

 which they contain, and this necessarily pulls down or depresses 

 the leaf-stalk and all that it carries. If, on the other hand, any 

 thing distend the cells on the upper side of the intumescence, 

 the leaf-stalk is pushed down, as it were, in a similar manner. 

 The intumescence at the origin of each vein, and at the base 

 of each leaflet, seems to possess the same properties in a de- 

 gree proportioned to its size, and they are all connected toge- 

 ther by the vessels and woody tubes of the mid-rib and veins. 

 Now, when the tissue of any of the leaflets is touched, it appears 

 to contract in the same manner as that of the Wild Lettuce ; 

 but instead of squeezing out its fluid upon the surface, it forces 

 it through the vessels into the upper side of the intumescences 

 at the base of itself and its fellow, and these leaflets are thus 

 caused to fold down and meet each other. The fluid forced 

 out from the under side of their intumescences is probably car- 



