LEAVES AND THEIR USES 251 



native of South America. A leaf of the sensitive plant has 

 four leaflets, each of which bears several pairs of secondary 

 leaflets. If the tip of a secondary leaflet is very gently 

 touched, that leaflet and the opposite one fold together ; the 

 bending takes place at the base of each leaflet. If instead of 

 merely touching the leaflet we stroke it slightly, not only the 

 one pair but all the pairs of secondary leaflets belonging to 

 the same primary leaflet fold together. But they do not 

 fold at the same time ; first the leaflet touched and its mate 

 fold, then the next pair, then the next, and so on, and finally a 

 bending occurs at the base of the primary leaflet so that it 

 droops. That is, the stimulus is conducted through the cen- 

 tral axis of the primary leaflet. This conduction through 

 the cells of the leaf is similar to the conduction of a stimulus 

 by means of nerves from one part of the human body to 

 another part. But the conduction is much slower in the case 

 of the plant. It seems to be through the cells of the vascular 

 bundles that the stimulus is chiefly conducted, and these 

 cells cannot be compared with nerves because their chief 

 business is not, like that of nerves, the conduction of stimuli. 

 If the leaflet is struck more violently, not only the parts 

 of one primary leaflet, but all the parts of the leaf, will be 

 affected, and we can follow the conduction of the stimulus to 

 the base of the first primary leaflet, and then from the bases 

 to the tips of the other primary leaflets. In this case, each 

 primary leaflet droops when the stimulus reaches its base, 

 and finally the whole leaf droops because of a bending at 

 the base of its stalk. The stimulus of a still more violent 

 stroke may be carried still farther, so as to affect other 

 leaves and even perhaps all the leaves of the plant. These 

 movements of the leaflets and of the whole leaf take place, 

 not only in response to the stimulus of a touch or a blow ; 

 they occur in the same way if a leaflet is stimulated by a 

 weak electric current, by a slight cut or other wound, or by 

 an irritating chemical substance such as hydrochloric acid. 



