RELATION OF AIR TO FOOD MANUFACTURE 69 



it. After several hours' exposure to sunshine, however, starch 

 will be abundant in the leaf, and the quantity will increase 

 with longer exposure to the sun's rays. The starch was made 

 in the leaf from carbon dioxide and water. This manufacture 

 may go on in any green part of the plant under the influence 

 of sunshine. Since the leaves are the principal green parts 

 of plants, it is in the leaves that most of the starch-making 

 is carried on. Starch and other starch-like substances, as the 

 sugars, may be used as food by the leaf or may be transferred 

 to other parts of the plant and then used or stored. 



74. Some problems. We shall now want to know how the 

 carbon dioxide gets into the leaf, how the water gets there, 

 what chemical changes take place, and what the light does in 

 this process. As an aid to the solution of these problems we 

 shall have to learn certain things about the structure of the 

 leaf. We shall therefore study next this organ of the plant, 

 which seems to be the food factory of the world. 



75. Structure of the leaves. The most conspicuous part of 

 the ordinary leaf is broad, thin, and green, so that it is com- 

 mon to call anything which is thin and flat leaf-like, and 

 many persons consider this part the whole leaf (fig. 38). This 

 is not the whole leaf, however, but is -the Hade. Besides the 

 blade there is usually a stalk, called the petiole, and sometimes 

 also, at the base of the petiole, two objects more or less like 

 little blades, which are called stipules. Stipules, petiole, and 

 blade make up the leaf. These are the parts of a complete leaf, 

 but the stipules are often very small or altogether missing, and 

 the petiole is sometimes absent. 



The stipules are not important parts of the leaf and will 

 not be studied here. The petiole is rather strong and woody 

 and serves to hold the leaf blade in the proper position. 



The blade of a leaf contains many veins, which serve as a 

 framework to support it, as well as for other purposes. The 

 arrangement of the veins differs in various kinds of leaves. In 

 many leaves there is a main vein extending from the petiole 



