ENTRY OF SOLUTES 81 



considerable height in the tube. If the latter be connected with 

 a mercury manometer the pressure can be measured. 



116. Nutrients Other than Water. All other sub- 

 stances can enter the plant only in solution in water. 

 This is true of the gases as well as of mineral salts, for 

 although a gas may enter the air spaces of a leaf in the 

 gaseous state, it cannot penetrate the wet cell walls in this 

 state but must go into solution. It is then subject to the 

 same physical laws of diffusion as the other solutes. 



117. The wet cell wall presents no (at least marked) 

 obstacle to the diffusion of any solute. The plasma 

 membrane, however, is impermeable for some, difficultly 

 permeable for others, and easily permeable for still other 

 substances. Accordingly the molecules of the substances 

 in solution outside of a cell will penetrate into the cell 

 with different degrees of rapidity and independent of the 

 direction that the water is passing. The result will be 

 that the solution inside of the cell may have its compo- 

 nents in entirely different proportions from the solution 

 outside. 



118. The process by which solutes pass into the cell 

 and from cell to cell is diffusion. This is the molecular 

 passage of a solute from that part of a solution where the 

 concentration of that particular solute is greater to where 

 it is less. As long as the plasma membrane is easily 

 permeable for the particular solutes they have no osmotic 

 effect and may diffuse in the same direction with or 

 counter to the osmotic stream. Thus the dissolved salts 

 that enter a plant do so independently of osmosis and 

 diffuse toward those parts of the plant where these 

 particular salts are less abundant. They will not 

 become more concentrated anywhere in the plant than 

 outside of it as long as they retain their same composition 

 and the permeability of the plasma membrane remains 



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