THE ROLE OF GREEN PLANTS 245 



just under the wall. Diffusion takes place following the laws of 

 osmosis, according to which water passes through a selectively per- 

 meable membrane from a point of its greater to a point of its lesser 

 concentration. This means that water passes from the soil into the 

 cell sap, which has a higher concentration of solutes than does the 

 water. Since the cell sap within the root hair has received a greater 

 quantity of water, it in turn becomes a point of higher concentration 

 of water than the cells lying next to it interiorly, and consequently, 

 the flow continues from these outer cells to the adjoining cells which 

 have a higher concentration of solutes. In this manner water is 

 passed through the cells of the root i)ito the woody cylinder inside 

 the cortex. Once having reached this region it passes up the tubes 

 into the stem and on into the leaves as will be shown later. 



The Stem, Structure and Functions 



In thinking of the tree as a li^'ing organism, we are not so much 

 concerned with the internal structure of the stem as with the way it 

 functions. For many centuries it has been known that water passes 

 up through the wood. If a tree is girdled — that is, if a narrow strip 

 of bark extending inward as far as the wood is removed — the tree will 

 keep its leaves for some time, indicating the upward passage of water 

 which keeps them from wilting. If, however, a strip of wood directly 

 under the bark is removed, enough of the bark being left intact to 

 allow for passage of fluids, the leaves will wilt within a very few mo- 

 ments. A cut branch of apple or willow placed in red ink after a few 

 hours shows by a red circle, visible in sections cut across the stem, 

 that the colored water has passed up through the outer layers of the 

 new wood. 



In order to understand better the pathways for the rise of sap in 

 the dicotyledon stem, one must study its growth. When seen in 

 cross section, the vascular tissues of such stems are arranged in a circle. 

 In some herbaceous stems, the woody bundles are separated by a 

 parenchyma, but in trees, shrubs, and a good many herbs, the bundles 

 are united to form a complete ring around the stem. These vascular 

 bundles are open at each side and grow more or less continuously 

 from a single row of meristem or embryonic cells which form a layer 

 around the stem. This layer is called the cambium, and the growth 

 of the wood and bark of our large trees is due to the activity of this 

 always youthful layer of cells which, like the cells of embryonic tissue, 

 continually divide and multiply to form internally the xylem or wood 



