FACTORS THAT HINDER ABSORPTION 159 
of the substances in solution in the cell sap. Water is drawn 
into the root hairs only when the density of the cell sap is 
sufficient to exert an osmotic force that overcomes the osmotic 
force of the solution without and the forces by which the soil 
holds on to the water. On the other hand, when the forces 
without are stronger than the osmotic force of the cell sap, 
then water will be drawn out of the root. This latter condition, 
which is likely to be disastrous to the plant, occurs when there 
is an excessive amount of mineral salts in the soil solution, or 
when the soil becomes so dry that the forces with which the 
soil holds on to the water become so great as to overcome the 
osmotic force of the cell sap. By watering plants with nutrient 
solutions which are too strong, the soil solution may become so 
concentrated as to injure the plants. 
The entrance of the dissolved mineral salts into the root hairs 
depends chiefly upon two things: First, the cell membrane must 
be permeable to them. Second, the membrane being permeable 
to them, they pass into the root hairs by the laws of diffu- 
sion. Thus, if a salt is more concentrated without than within 
the root hairs, it passes into them until it is as plentiful within 
as without. Also substances may diffuse out of the root hairs 
when more concentrated within than without. Although the 
movement of the salts may be influenced in rate by the move- 
ment of the water, experiments show that the amount of min- 
eral salts which enter the plant is quite independent of the 
amount of water absorbed. However, in being alive, the cell 
membrane presents some features not found in connection with 
dead membranes. One peculiar feature is that it can alter 
its permeability from time to time, and another is that, al- 
though it allows many substances to pass in, it allows very 
few to pass out. In being permeable to some substances and 
not to others, roots are thereby able to exercise selective ab- 
sorption, which in general favors the entrance of the more useful 
substances, although roots by no means keep out all harmful 
substances. 
From the epidermal cells, the water and mineral salts pass 
through the cortex to the xylem vessels through which they reach 
the shoot. (Fig. 141.) 
Factors that Hinder Absorption. — The forces concerned in 
capillarity and surface films increase as the water of the soil de- 
