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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