CHAPTER XI 



LEAVES 



Characteristic Feature of Leaves 



Ordinary green leaves, known as foliage leaves, may be defined 

 as the food-making organs of the plant. The green cortex of 

 stems makes some food, but usually the greater part of it is made 

 in the leaves. Leaves are constructed especially for utilizing the 

 carbon dioxide of the air and the water brought up from the roots 

 in the manufacture of sugar. Although starch may sometimes 

 be the first food product formed by the leaves from carbon dioxide 

 and water, the evidence indicates that ordinarily the first product 

 is sugar from which starch is formed later. Sugar then may be 

 regarded as the fundamental plant food, since from it or from the 

 proteins of which it is the chief element, plants build by chemical 

 transformations all of their structures and organic materials of 

 whatever kind. Leaves transform sugar, when it is abundant, 

 into starch which serves as a storage form of sugar. Although 

 proteins can be made in any living part of the plant, it has been 

 demonstrated that leaves are very active in the formation of this 

 food. Although proteins may be regarded as a secondary food 

 since they depend upon sugar as their chief foundational ele- 

 ment, they are exceedingly important because from them the pro- 

 toplasm, the living substance of the cell, is formed. Proteins, 

 although of various kinds, are formed by combining chemically 

 the mineral elements of the soil, such as nitrogen, sulphur, and 

 phosphorus, with the elements of sugar. Since leaves manu- 

 facture sugar and are well supplied with the mineral elements, 

 they are well equipped for the manufacture of proteins. 



The efficiency of green tissue in making sugar depends upon 

 exposure to light and air, and the foliage leaf may be considered 

 a device for securing good exposure of green tissue. The elevat- 

 ing of leaves into light and air by the stem, and their arrangement, 

 position, form, and structure are related to the prol:)leni of 

 securing suitable exposure, and thus to food manufacture. 



233 



