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 problem of 
securing suitable exposure, and thus to food manufacture. 
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