The Plant. 85 



of their high starch content. In these cases the stems serve as 

 storage places for reserves of plant food. The bulbs of the onion, 

 lily and other plants, are permanent buds, formed of fleshy, 

 closely packed scales. They are properly a part of the stem of 

 the plant, serving as reserve material for growth. The fleshy 

 portion of the crocus, gladiolus and some other plants is not a 

 bulb, but is an enlargement of the base of the stem. 



The stem also serves as a means of support for the leaves and 

 fruit, favoring the exposure of both to the air and sunlight, es- 

 sential to the chemical processes which promote growth. 



The leaf. The leaf is the seat of greatest constructive activity 

 in the plant. The important process of transpiration, or escape 

 of water from the plant, is controlled by minute openings upon 

 the plant's surface. These openings, known as stomata, occur 

 in small numbers upon the stems of plants, but they are most 

 abundant upon the leaves. They are especially numerous upon 

 the protected under surface of leaves, where, as in the case of the 

 cabbage or apple, their number may reach 200,000 per square 

 inch. The outlet of a stoma is lined by two peculiar cells which 

 face each other, forming a miniature mouth opening outward 

 from the surface of the leaf. These cells, called guard cells, are 

 the seat of control in the action of the stomata. When the water 

 supply is abundant and the plant cells are turgid, the guard cells 

 are elongated vertically to the leaf surface and contracted par- 

 allel to it, thus drawing apart and exposing an outlet for the 

 evaporation of water. On the other hand, when the water sup- 

 ply is limited and the plant cells wilt or shrink, the guard cells 

 flatten and become elongated parallel to the leaf surface, thus 

 automatically closing the stomata and checking evaporation from 

 the plant. This process partly controls the supplying of plant 

 food from the soil and is an important means of maintaining 

 optimum temperatures in the plant as a result of increased or 

 decreased evaporation of water. 



The leaf inhales air through the stomata. From this supply 

 of air it assimilates carbon dioxide for the construction of plant 



