ROOTS 



263 



the root-hairs, does the water enter freely. The root-hairs 

 push out among the soil particles and come into very close con- 

 tact with them, the particles sometimes being embedded in the 

 wall of the hair (Fig. 227). In this way the films of water 

 adhering to each soil particle are closely applied to the hair, 

 and water passes from them through the wall of the hair into 

 its cavity, and so into the plant. As 

 water enters from the films they be- 

 come thinner, and this loss is supplied 

 from neighboring films. In this way 

 a flow from regions of the soil deeper 

 and more distant than those to which 

 the root reaches is set up toward the 

 films losing water. The water supply 

 may not be able to make good such 

 loss indefinitely ; and if so, the films 

 gradually become thinner, until a 

 point is reached when the root-hair 

 can obtain no more water, the thin 

 film holding tenaciously to its par- 

 ticle of soil. After the roots have 

 obtained all the water they can 

 from the soil, and it seems per- 

 fectly dry, it still contains two to 

 twelve per cent of water in the 

 form of films. 



The water thus obtained by the 

 root-hairs passes inward through 

 the cortex ^nd enters the wood of the vascular cylinder, 

 and then is free to ascend to the wood of the stem, and 

 so to the leaves. 



145. Entrance of salts. In addition to water, the soil 

 supplies chemical substances in the form of salts, from which 

 the plant obtains certain elements that it needs in the manu- 

 facture of proteins, as nitrogen, sulphur, and phosphorus. 



FIG. 227. Root-hair of wheat, 

 which is shown to be an out- 

 growth of an epidermal cell in 

 close contact with the soil par- 

 ticles. 



