IV.] 



PREDA TORY PLANTS. 



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 ^V&th, and 

 of hair weighing 7 J^th 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 Butter-wort (Pinguicula 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 



FIG. 74. 



