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. 
