286 



PLANTS 



may be broken up into leaflets, as in the clover; it is then 

 called a compound leaf. 



Where the heat of the sun is not too severe, as in temperate climates, 

 leaves are arranged on the stem in such a way that they shall receive 

 as much light as possible. Sometimes leaves alternate upon the stem; 

 sometimes two leaves are opposite each other. Often many leaves 



FIG. 233. Leaves of the Maple, Birch, and Wild Lily-of- the- Valley. 



are opposite one another; they are then said to be whorled. Often all 

 the leaves are attached at almost the same place on the stem, but take 

 the form of a rosette, so that all shall get light. 



The leaf is made up of more or less spongy tissue, con- 

 sisting of layers of cells. The cells contain living, cell 

 material, or protoplasm (cf. 323), and a green material, 

 called chlorophyll (cf. 110). The outside of the leaf is 

 covered with a skin, or epidermis. This is usually color- 

 less. In the wall of the epidermis there are slits, which 

 can be varied in size by the plant; these are called sto- 

 mata (Fig. 234), meaning "mouths." The stomata are 

 the "breathing pores" of the leaf, and enable the plant 

 to take in air, and to give off the gases it does not 

 need. They also control the rate of evaporation of water 



