THE BEHAVIOUR OF PLANTS 27 
delicate spines, the slightest touch on any one of which causes 
the two halves of the specially modified leaf-end to fold inwards 
on the midrib as a hinge. ‘The transmission of impulse is 
more rapid, the trap closing in a few seconds ; and electric 
currents have been observed to accompany the change. Tooth- 
like spines at the edge of the trap interlock, and serve to 
prevent the escape of small insects, while short-stalked purple 
elands secrete an acid digestive juice. Division of labour has 
been carried further; and organic behaviour, not less pur- 
posive, is carried out in a manner even more effective. 
Fig. 7.—Venus’s Fly-trap (Dionxa), Leaf viewed laterally in its ex- 
panded state. (From Darwin’s “ Insectivorous Plants.”) 
In other plants adaptive movements are well known. 
“Few phenomena have such a peculiar appearance as the 
movements which occur in the sensitive Oxalis when rain comes 
on. Not only do the leaflets on which the finest rain-drops 
fall fold together in a downward direction, but all the neigh- 
bouring ones perform the same movement, although they have 
not themselves been shaken by the impact of the falling drops. 
The movement is continued to the common leaf-stalk bearing 
the numerous leaflets. This also bends down towards the 
ground. ‘The rain-drops now slide over the bent leaf-stalk and 
down over the depressed leaflets, and not a drop remains 
behind on their delicate surfaces.” * The waves of impulse 
are said to be transmitted along definite lines, and to cause 
* Kerner, “ Natural History of Plants,” vol. i., p. 536. 
