96 GROWTH AND WORK OF PLANTS 



155. Response of insectivorous plants to touch. Remark- 

 able movements are shown by the leaves of some insectivorous 

 plant*. In the Venus's flytrap (fig. 81) the terminal part of the 

 leaf is shaped like a steel trap. The blade is broad and the mar- 

 gin rounded and beset with numerous hairs or spines resembling 

 the teeth of a steel trap. Upon the upper face of each side of the 

 leaf are three prominent hairs. When these hairs are touched 

 by pressure of sufficient force to compress certain cells at their 

 bases the leaf suddenly closes like a steel trap. As shown by 

 experiment the leaf closes also when other cells are compressed. 

 Likewise electrical and heat stimuli cause closure. The in- 

 tensity of the stimulus sufficient to produce closure may result 

 from one stimulus sufficiently prolonged, or from two or more 

 stimuli in succession. Flies which alight on the leaf are thus 

 caught and pressed between the folded leaf, and the leaf excretes, 

 through special glands, juices which digest portions of the insect, 

 which are then absorbed by the leaf and used for food. The 

 sundew has a rounded or elliptical leaf blade covered with long 

 glandular hairs which excrete a sticky substance. When an in- 

 sect alights on the leaf the sticky substance holds it, and the 

 hairs and leaf slowly fold inward around the insect and it is 

 digested by juices from special glands (fig. 82). 



