ACTIVE: PLANT FOOD, ETC. 197 



in contact with a solution of a substance which can penetrate the 

 membrane, diffusion will take place until the number of ions 

 of the same kind entering the cell walls is equal to the num- 

 ber leaving it. If the cell life appropriates any of the ions, and 

 holds them, so that they become incapable of diffusion, the ions 

 continue to enter the cell until the cell life becomes satisfied, and 

 the number entering and leaving become equal. The same 

 phenomenon occurs with an aggregate of cells or the entire plant. 

 It is then possible for plants to extract elements from very dilute 

 solutions, and accumulate them in their tissues, and also for plants 

 to live in comparatively strong solutions of a salt without taking 

 up large quantities of the substance. 



Diffusion into Cells. According to Pfeffer, plant cells are 

 composed of three parts ; an outer cell wall, of cellulose or some 

 other membrane ; a layer of protoplasm or living material adherent 

 to the cell wall ; and the cell sap. The cell wall is in general more 

 permeable to dissolved substances than the protoplasm, and hence 

 many substances pass through the cell walls but not through the 

 living plasma within. "It is indeed possible that the water and 

 salts absorbed by the roots pass mainly if not entirely through the 

 walls of living cells or the walls and cavities of dead wood fibers, 

 so that only on reaching the leaves of a tree do they penetrate the 

 living protoplasts there." The character of the cell wall and of 

 the protoplasm membranes determine whether a given substance 

 will penetrate to the interior of a cell, and any such substance 

 will continue to be absorbed until a condition of equilibrium is 

 reached, when all further absorption ceases. If this condition of 

 equilibrium is disturbed, absorption may continue, and relatively 

 large quantities of a particular substance may be absorbed from 

 a very dilute solution. 



Every substance which can pass through the different cellular 

 membranes penetrates the protoplasm independently of whether it 

 is useful, unnecessary, or even injurious. 



Cells may convert bodies which diffuse into them into non- 

 diffusing compounds, and the substance will continue to enter as 

 long as this takes place. The non-diffusing compound may be 



