264 ELEMENTARY STUDIES IN BOTANY 



The most important salts of the soil, therefore, are nitrates, 

 sulphates, and phosphates. In order to enter the root, these 

 salts must be in solution, so that they pass in dissolved in 

 the water that enters from the films about the soil particles. 

 These films, adherent to the soil particles, naturally dissolve 

 any soil material that is soluble in water. It is very impor- 

 tant to know, however, that these dissolved salts are not 

 simply swept in by the moving water, for water and salts 

 move independently. 



The rate at which water enters the plant depends upon 

 the rate at which the plant is losing water ; and so the rate 

 at which a soil salt enters a plant depends upon the rate at 

 which the plant is using it. For example, if a soil salt, calico! 



A, is being used up constantly in the plant by being put into 

 new compounds, A will continue to enter from the soil. 

 If, on the other hand, a soil-salt, called B, is not being used 

 by the plant, it accumulates in the plant until the solution of 

 it in the plant is as concentrated as the solution of it in the. 

 soil films, and then no more of it can enter, even if it does 

 present itself to the root in solution. This is what was once 

 called the " selective power " of the root, by which was im- 

 plied that the root has some mysterious power of selecting 

 from the soil just what it needs. In our illustration, the root 

 would seem to have the power of selecting A and rejecting, 



B, but it is obvious that it is explained by a well-known law 

 of physics (osmosis). 1 It follows that when any soil-salt is 

 observed to accumulate in a plant, it is an indication that the 



1 Osmosis may be defined briefly by the following illustration : If 

 a membrane (like a cell-wall) forms a partition between two ma*sses 

 of water, and sugar is dissolved in the water on one side, it will pass* 

 through the membrane until the solutitJiion both sides'is of the same 

 concentration. Therefore, whenever one cell contains .a s,salt in 

 greater concentration than a neighboring cell, there will be a move- 

 ment of the salt from the former cell into the latter ; but if the Con- 

 centrations in the two cells are the same, there will be no movement. 



