LEAVES 



381 



FIGURE 120 



the trap closes rapidly upon it and stays closed until 

 as much as possible of the insect is digested, when the 

 trap again opens. Carnivorous plants of this kind usually 

 grow in places where it is difficult to get nitrog- 

 enous foods. As nitrogen is absolutely neces-. 

 sary for the growth of protoplasm (page 371) 

 these plants may have had to adopt this way 

 to supply the need. 



Some leaves extend themselves into spiny 

 points, like those of the thistle (Figure 120), 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 (Figure 121). Sometimes these veins extend 



parallel to one another 

 through the leaf, as in 

 the corn and palm. This 

 is generally characteris- 

 tic of monocotyledonous 

 leaves. In other leaves, 

 the veins form a network, 

 as in the maple and apple. 

 This is characteristic of 

 dicotyledonous plants. 



Experiment 119. Place 

 the freshly cut stem of a 



white rose, white carnation, variegated geranium leaf, or any thrifty 

 leaf which is somewhat 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. 



FIGURE 121 



