160 ROOTS 



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- 



