58 THE NATURE AND WORK OF PLANTS 



The water of the air is rarely used by the plant, 

 yet its presence prevents the plant from drying out, 

 and it is of great importance in this way. The other 

 substances are more or less useful or harmful, accord- 

 ing to the quantities present. 



64. Functions of the leaf. — To the leaf is in- 

 trusted the work of taking in the carbon dioxide 

 of the air, splitting it up, and combining it with 

 water sent up from the roots, in such manner as 

 to form sugars. The stream of water also brings 

 up mineral substances from the roots which are 

 needed in the leaf. The amount of water which 

 thus reaches the leaf is much greater than can be 

 used in the tissues, and most of it must be thrown 

 off. This is also done by the leaf. 



65. The colors of leaves. — The leaves of most 

 species are colored green by the presence of a sub- 

 stance the botanist terms chlorophyl, and this pig- 

 ment is also found in the outer tissues of some stems, 

 branches, or even roots. It is formed in special 

 masses of protoplasm, and the depth of the green 

 color of the leaf depends on the number of these 

 masses in the cells. Then again the living matter 



