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. 
