Osmosis. 47 



molecules will be removed from solution and prevented 

 from exercising pressure, thus causing a reduction of the 

 osmotic water pressure in that cell ; this will permit more 

 water from the adjacent cells to be driven in to make good 

 the loss. 



So, too, if water is being lost by evaporation from the 

 leaves, this loss will result in a reduced osmotic water 

 pressure in the leaf cells which will permit the heavier 

 pressure in the cells extending backward toward and to 

 the root hair in the soil to force more water onward 

 toward the leaves and thus maintain the flow of water by 

 powerful osmotic pressure toward the leaves as long as 

 evaporation continues. 



When nourishment is being stored in the seed the sub- 

 stances in solution in the sap are being taken out and 

 laid down in solid form, thus tending to maintain at that 

 place a reduced osmotic pressure which permits the sub- 

 stance of that sort to be forced continually toward the 

 place where the formation is going on. In this way the 

 starch and other products are supposed to be brought from 

 the leaves and stems to the seeds or places in stems, like 

 the potato, where food products are being stored. 



In the gathering of nitrogen from the nitrates in the 

 Boil water, too, the process would be the same. Wherever 

 the nitrate is being transformed there its osmotic pres- 

 sure would be falling and this permits more to be forced 

 to the same point. 



The so-called selective power of plants, whereby they 

 obtain those substances dissolved in the soil water which 

 they need, is thus explained. It should be understood 

 that unless the mole-cules of a substance in solution are 

 too large to pass from cell to cell through the walls these 

 substances will do so until the solution inside the plant 

 has a strength equal to that outside, but if this substance 

 chances to be one which the plant does not use there will 

 be no further concentration of that substance in the plant 

 unless it be at places where evaporation is taking place. 



If a poisonous principle exists in the soil water os- 



