248 THE CULTURE OF FARM CROPS. 



the root througlj ihe stem is exposed to the action of the 

 oxygen of the air which is absorbed, and is elaborated into 

 nutritive matter which is deposited in the cells of the leaves 

 and of the stem, as the current descends, as woody fiber; 

 starch; and albuminous matter. This process is analogous 

 to the circulation of nutriment through the digestive and 

 assimilative organs of animals, into and through the lungs 

 and through the arteries and capillary vessels which are 

 scattered in the finest network all through the tissue, and 

 from which the solid substance of the animal is deposited. 

 Thus the stem increases in growth by deposit of new mat- 

 ter; but in the manner above described as the character of 

 the plant differs. This deposit is made on the inner part 

 of the stems of most of the plants which are grown as farm 

 crops, but on the outside of the wood and under the bark 

 of trees, forming what is called the cambium layer, between 

 the wood and the bark. 



But by far the larger part of the water absorbed by the 

 roots and passing upwards through the stem is evaporated 

 by the leaves. A small sunflower plant no more than 3 

 feet high, draws up from the roots through the stem to the 

 leaves and exhales from these, about 30 ounces of water in 

 24 hours; and the enormous quantity of water which passes 

 through the stems of the trees upon one acre of forest land 

 is estimated at several tons per day. 



