CHAPTER TWENTY-TWO 



THE PROCESSES OF ROOTS 



The absorption of water and mineral salts is the process most 

 generally associated with roots. The three physical processes 

 involved in absorption were defined in Chapter XI, and are here 

 briefly summarized. 



Diffusion is the movement of molecules and atoms from places 

 of greater concentration to places of less concentration. When 

 the diffusion of water into a substance, or body, results in swell- 

 ing, the process is called imbibition. The diffusion of water 

 through a differentially permeable membrane that separates a 

 mass of water, or a dilute solution, from another is called osmosis. 

 In osmosis water moves from the place of its greater concentration 

 through the membrane to the place of its less concentration. If 

 the solution is inclosed by walls, the movement of water into 

 the solution produces a pressure known as osmotic pressure. 



Absorbing mechanism of roots. The epidermis is the primary 

 absorbing tissue of the root. We have seen that it consists of 

 delicate walled cells, some of them prolonged outwardly as root 

 hairs. The wall is composed inwardly of cellulose and outwardly 

 of pectic material, the latter having a powerful imbibing capacity 

 for water. The wall is permeable to both water and dissolved 

 salts. 



The cytoplasm of the epidermal cell is the differentially per- 

 meable membrane which separates the cell sap from the water in 

 the soil. It prevents the outward diffusion of sugar and other sol- 

 uble organic substances, and permits the inward diffusion of water 

 and mineral salts. The epidermal cell, then, has a wall with a 

 great capacity for imbibition of water ; it forms an efficient 

 osmotic cell for the taking up of water ; and it affords a permeable 

 medium for the inward diffusion of salts. Because of the presence 

 of sugars and other substances made by the plant, the concentra- 

 tion of soluble substances in the cell sap is necessarily greater 

 than in the soil solution in which a plant grows. 



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