ROOTS 265 



plant is not using it ; and that the soil-salts the plant is using 

 are not apt to be found in it. This will explain why the 

 chemical analysis of a plant does not detect what it is using 

 from the soil, but only what it cannot use. 



A distinction must be made between the entrance of the 

 salts of the soil into the root, and their movement through the 

 vascular system (xylem) of the root, stem, and leaves. The 

 entrance is through living cells until the xylem is reached, 

 and then the movement is through dead cells. As has been 

 said, a salt enters a living cell only when the water of the cell 

 is poorer in the salt tHan the water outside (osmosis) ; but 

 in dead tissue (as the water-carrying xylem) osmosis does not 

 work, and the salts dissolved in the water are carried along 

 with it. When they reach their destination, as the mesophyll 

 cells of a leaf, they must pass from the xylem (of the veins) 

 into the working cells according to the law of osmosis. There- 

 fore, salts enter the plant by the selective power of osmosis, 

 they are carried through the xylem by the movement of water f 

 and they are delivered to the working cells by the selective 

 power of osmosis. 



146. Special forms of roots. Roots in soil serve the 

 double purpose of anchoring the plant and receiving water, 

 but certain roots hold other relations and need special men- 

 tion. 



(1) Prop-roots. In certain plants roots are sent out from 

 the stem or the branches, and finally reaching the ground 

 establish the usual soil relations. Since these roots resemble 

 braces or props, the name prop-roots has been applied to them 

 (Fig. 228). A very common illustration is that of the corn- 

 stalk, which sends out such roots from the lower nodes of the 

 stem. More striking illustrations, however, are furnished 

 by the banyan and the mangrove. The banyan sends down 

 from its wide-spreading branches prop-roots, which are 

 sometimes very numerous. When they enter the soil- they 

 often grow into large trunk-like supports, enabling the 



