238 TEXTBOOK OF BOTANY 



is always a drop of thick liquid that glistens in the sun like 

 dew. If an insect alights on the upper surface of the leaf, 

 touching some of the hairs, it is caught in this sticky liquid. 

 The touch of the insect serves as a stimulus to the leaf, which 

 becomes curved, and the hairs bend over from all sides upon 

 the insect. The soft parts of the insect's body are then 

 digested by enzyms contained in the sticky secretion. 



" Venus' fly-trap " (Fig. 144) has a leaf -blade of two parts, 

 separated by the midrib ; the two parts fold together upon 



A 



FIG. 144. Venus' fly-trap (Dionaa muscipula). A, a plant, some of 

 whose leaves are open and some closed as the result of stimulation by an 

 insect ; B, a single leaf ; C, a cross section of the upper part of a leaf, closed. 

 After Kerner. 



an insect that has touched any of the sensitive hairs on the 

 upper surface of the leaf. 



253. Arrangement of Leaves. A plant like the lilac 

 or the horse-chestnut, whose leaves are borne in pairs at 

 the nodes of the stem or branch, is said to have opposite 

 leaves. In such plants, very commonly, though not always, 

 the leaves of one pair grow in a direction at right angles to 

 that of the next pair above or below ; this is the case in the 

 horse-chestnut. Some plants, for example some lilies 

 (Fig. 145), have their leaves in circles of three, four, or more 

 at each node, instead of in pairs. Very many plants, how- 

 ever, like the bean and the cucumber, have only one leaf at 

 each node ; their leaves are alternate. In an alternate- 



