THE ROOT HAIRS 93 



but are unable to pass the plasma membrane may by travel- 

 ling in the wall alone, enter the plant body and become dis- 

 tributed throughout its length without once having entered the 

 living cells. 



The selective action of the plasma membranes has never been 

 satisfactorily accounted for on the basis of chemical and physical 

 processes alone, although, presumably, this could be done if all 

 the conditions were accurately understood. As soon as the pro- 

 toplast dies the membranes lose their power of selection and the 

 cell-sap readily escapes from the cell; and so it seems the mem- 

 branes are able to do their work because they are living. When 

 we are unable to give a chemical or physical explanation of a 

 physiological phenomenon of this sort we speak of it as due to a 

 vital power, by which we mean that its seat is in the living pro- 

 toplasm and its origin is shrouded in mystery. And the mystery 

 in this instance is the more profound because the selective action 

 varies in purposeful ways through the self-regulatory action of the 

 protoplast, as will be brought out in subsequent chapters. 



The root hairs excrete organic acids and carbon dioxide, and 

 these go into solution in the soil water and have a solvent effect on 

 some of the soil constituents. 



The water and solutes (substances in solution in the water) 

 absorbed by the root hairs pass into the adjoining cortex cells 

 and thence across the root to the tracheal tubes and tracheids of 

 the xylem where they begin their ascent into the stem. The 

 relative positions of the primary xylem and phloem strands (Fig. 

 42) which make it possible for the water and solutes (now after 

 their entrance into the plant called the crude sap) to reach the 

 xylem without traversing the phloem is evidently a device to keep 

 the crude sap and the elaborated sap (sap containing soluble food 

 such as sugar in solution) distinct and apart. As will be seen in 

 Chapter X the elaborated sap makes its way in the phloem long- 

 itudinally throughout the plant from the leaves to the roots. 



The root hairs are very short lived and the older ones die away 

 about as fast as the new appear. After the root hairs die the 

 walls of the outer cortex cells become more or less suberized and 



