FOLIAGE AND SCALE LEAVES. 73 



at night (fig. 29). Among insectivorous plants Venus' fly-trap is 

 a good example of motility. If an insect alights on the upper side 

 of the leaf and happens to touch one of the sensitive hairs on 

 the lamina, the two halves move rapidly upwards, and by the 

 interlocking of the marginal bristles a very efficient trap is 

 formed. 



Leaves show a high degree of irritability. They are trans- 

 versely geotropic and heliotropic, that is to say, under the 

 influence of gravity and light bifacial leaves at any rate tend 

 to place themselves horizontally. Sensitiveness to contact is 

 shown by leaf-tendrils as well as stem-tendrils, and other more 

 obvious cases are the sensitive plant and Venus' fly-trap. The 

 tentacles of sundew when a fly alights upon one of them all bend 

 towards the centre of the leaf and entangle it. All the insecti- 

 vorous plants pour out their digestive excretions as the result of 

 contact, and this often depends on the chemical nature of the 

 touching substance, so that we have sensitiveness to chemical 

 stimuli. 



Spontaneity is shown in the protoplasmic movements in 

 hairs, &c., and on a larger scale in an Indian form, the tele- 

 graph plant (Desmodium gyrans), which possesses ternate leaves. 

 The lateral leaflets of these are in a constant state of up and down 

 movement, quite rapid enough to be visible with the unaided 

 eye. 



