PLANT LEAVES 



195 



a portion which is hinged at the middle and has on the 

 inside of each half three short hairs, while the outside 

 is fringed by stiff bristles. As soon as an insect touches 

 the hairs, the trap closes rapidly upon it and 

 stays closed until it is digested, when the trap 

 again opens. Carnivorous plants of this kind 

 usually grow in places where it is difficult to 

 get nitrogenous foods, and they have adopted 

 this way to supply the need. 



Some leaves extend themselves into spiny 

 points, like the thistle (Fig. 94), in order to 

 keep animals from destroying the plant, or they 

 may develop a sharp cutting edge, like some grasses, 

 or emit a bad odor, or have a repugnant bitter taste. 



The veins or little 

 ridges extending 

 through the leaf from 

 the leaf stem vary (Fig. 

 95.) Sometimes these 

 veins extend parallel to 

 each other through the 

 leaf, as in the corn arid 

 palm. This is generally 

 characteristic of mono- 

 cotyledonous leaves. In 

 other leaves, the veins 

 form a network, as in the maple and apple. This is charac- 

 teristic of dicotyledonous plants. 



Experiment 99. Place the freshly cut stem of a white rose, white 

 carnation, variegated geranium leaf, or any thrifty leaf which is some- 

 what transparent, in a beaker containing slightly warmed water 

 strongly colored with eosin. Allow it to remain for some time. The 

 coloring matter can be seen to have passed up the stem and spread 

 through the leaf or flower. 



Fig. 95. 



