316 PHYSIOLOGY 



soil can be improved by mechanical and chemical treatment designed 

 to remove or destroy the harmful compounds. The rotation of crops 

 may find partial explanation herein; the excretions and decomposition 

 products of a given crop may be injurious to the same plants, but 

 less so or not at all to others. Even manuring may prove to have its 

 value less in the compounds put into the soil than in the improvement of 

 soil texture and the destruction of the deleterious compounds in it. 



Entry of water. The cells bearing root hairs and the adjacent ones 

 are so constructed as to facilitate the immigration of water and various 

 solutes. The cell walls are thin and the protoplast apparently forms 

 only a thin sheet over the inner surface, the greater part of the cell 

 being occupied by a huge sap cavity. The cell sap is usually a more 

 concentrated solution than the water outside; the internal pressure of 

 the water is consequently less (p. 308), and water enters, distending the 

 cell until the elastic recoil of the stretched wall is sufficient to balance 

 the osmotic pressure of the solutes, or to exude as much water as enters. 



Entry of solutes. At the same time, if any solutes to which the pro- 

 toplast is permeable exist in the soil water, but either not at all or in less 

 amount in the cell sap, they will diffuse into the cell. But their move- 

 ment is as independent of the movement of the water as are the condi- 

 tions of such movement ; water and solutes move independently. If any 

 solute which enters thus is not changed or stored in the plant, i.e. if 

 it is not removed as such from solution, it may attain equilibrium inside 

 and outside the plant, so that no more enters ; but if it is removed by 

 being chemically changed or by being stored, more constantly enters. 



Entry and exit via roots. The root therefore possesses permeable 

 surface cells always in contact with soil water, through which water 

 and a variety of solutes, chiefly oxygen and mineral salts, make their 

 way, under the conditions already set forth regarding osmosis. At the 

 same time, the root permits through these same surfaces the outgo of any 

 solute formed in the cells, to which the cytoplasm is permeable, that 

 does not exist at equal or greater pressure in the soil water. It is even 

 conceivable that water would pass out thus, were it possible for the soil 

 to become sufficiently dry. Artificially this can be demonstrated; it 

 has not been shown that it occurs in nature. When the roots are exposed 

 to air, as in transplanting, especially if the plants are to be transported 

 far, it is necessary to guard against excessive loss of water by evaporation 

 from the roots; and the quick drying of exposed roots is a most obvious 

 danger in transplanting. 



