MOVEMENTS OP SENSITIVE PLANT. 269 



bend down upon the stem ; and, if the plant he in a very irri- 

 table condition (from its functions being in a state of great acti- 

 vity), the other leaves are sometimes affected in a similar man- 

 ner. The explanation of this very curious phenomenon requires, 

 that the structure of the parts concerned in it should be explained. 

 It is evident that the cause of the movement must be in some way 

 propagated from the part touched, to the parts where the change 

 actually takes place, namely the points where the leaflets join 

 the veins, the veins come from the midrib, and the midrib from 

 the stem. At every one of these points, there is a little swelling 

 or intumescence, formed of very spongy cellular tissue, and con- 

 taining 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 vesicles, 

 being very irritable, contract and force out the fluid they contain; 

 and this necessarily pulls down or depresses the leaf-stalk and 

 all that it carries. If. on the other hand, anything 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 degree proportional to its 

 size ; and they are all connected together, by the vessels and 

 woody tubes of the midrib and veins. Now, when the tissue of 

 any of the leaflets be touched, it appears to contract in the same 

 manner as does that of the Wild Lettuce ; but instead of squeez- 

 ing 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 carried to the upper side of those 

 at a little distance ; and thus the neighbouring leaflets also are 

 depressed. The depression of the veins upon the midrib, and of 

 the midrib or footstalk itself upon the stem, will follow in like 

 manner ; the extent to which the movement is propagated, being 

 dependent on the amount of fluid expelled from the lower side 

 of the intumescence, in the parts where it has already taken 

 place. 



422. Various other stimulants, besides the touch of a hard 



