Iv.] PREDATORY PLANTS. 63 
detain an insect, but to excite the leaf it is necessary 
that the victim should touch the knob itself. On 
finding itself detained, the insect 
struggles to free itself, and in so 
doing excites the glands. The 
irritation is communicated to all 
the filaments or “tentacles,” which 
thereupon bend towards the cause 
of excitement, and effectually im- 
prison it. Kicks and struggles are 
useless, the acid secretion is poured 
out, the insect killed, and finally 
digested. But the Sundew is not 
particular as to its food, provided 
it is of an animal nature. Small 
fragments of meat placed upon 
it will produce precisely similar 
effects. The size of the substances 
causing irritation is of little moment. Mr. Darwin 
found that a fragment of cotton weighing syooth, and 
of hair weighing zg2gath of a grain was sufficient to 
cause the tentacles with which they were in contact 
to bend. Such exquisite sensitiveness probably ex- 
ceeds that of the most sensitive of human nerves. 
Recently an addition has been made to our insec- 
tivorous plants by the discovery of certain habits 
of the Butterwort (Pimguicula vulgaris). It is, like 
Drosera, a stemless plant, with a tuft of spreading - 
leaves, incurved at the edges, and covered with a 
greasy matter, from which probably its name is 
derived. It sends up several leafless stalks, each 
bearing at the summit a solitary drooping purple 
