THE ROOT AND ITS FUNCTIONS 51 



of whatever kind, which is in solution. A solution of sugar, one 

 of salt, one of a mixture of the two, or one containing half a dozen 

 substances, may all have exactly the same osmotic concentration. 

 Third, the diffusion of water through a membrane and the diffu- 

 sion of salts through the same membrane occur quite independently 

 of one another. Water will move through a membrane from a 

 solution of lesser to a solution of greater total concentration, but 

 a dissolved substance, following the general law of diffusion, will 

 pass from a point where that substance is abundant to one where 

 it is scarce, always providing that the membrane is permeable 

 to it. Given the proper conditions, it is quite possible for a 

 dissolved substance to pass through a membrane osmotically 

 with no movement of water taking place at all, or for water to 

 move without a movement of dissolved substances, or even for 

 water to pass in one direction and dissolved substances in the 

 other. Fourth, if there is more than one substance in solution, 

 each will tend to diffuse quite independently of all others. 

 Differences in the concentration of each substance, considered by 

 itself, are what determine the rate and direction of diffusion of 

 that substance. 



Diffusion and Osmosis in the Plant Cell. — It is upon the 

 principles of diffusion and osmosis that the plant depends, not 

 only for the absorption of water and mineral substances from 

 the soil, but for most of the circulation of materials which goes 

 on within the plant body. We have already outlined briefly the 

 structure of a typical plant cell and may now consider the 

 osmotic interchanges which go on therein. 



The cell wall in plants is ordinarily composed of cellulose. 

 Like most organic materials, cellulose has the capacity of 

 absorbing water vigorously by imbibition and will therefore 

 swell considerably if placed, when dry, in contact with water. 

 This expansive ability of the cell wall is of some value in certain 

 of the plant's activities, as in the germination of the seed, but 

 the wall of an ordinary living cell is moist and has imbibed 

 water to the limit of its capacity. Water, and practically all 

 substances in solution, pass through this cellulose wall with great 

 readiness, and since it thus offers practically no resistance to 

 diffusion, its osmotic effect is slight. 



We have noted that, in the mature plant cell, the cytoplasm 

 is dispersed in a thin layer closel}^ pressed against the inner surface 

 of the cell-wall, and that it completely surrounds a large central 



