no NOT ALTOGETHER ABOUT PLANTS 



osmosis, more molecules of solutes move into these inner 

 cells of the cortex and replace those which have been 

 carried away. Thus we see cause for constant osmotic 

 movement of solutes inward toward the wood cells and 

 also cause for their continued entrance into the root-hairs. 



The other thing which helps in allowing solutes to con- 

 tinue to enter is a thing which is important wherever 

 osmosis occurs. Many of the molecules which enter the 

 protoplasts of the cortex are there changed to other substances. 

 So far as osmosis is concerned, this is about the same 

 as if they had disappeared entirely. Fresh molecules come 

 in to take their place, for they have ceased to be the 

 molecules which they were when they entered the plant; 

 they have become molecules of something else. This 

 change of the molecules permits osmosis to continue and 

 thus permits the plant to keep on getting the materials it 

 needs. This second cause of the continued entrance of 

 solutes probably does not have so much effect as the first, 

 but it is a thing which is important in keeping up osmotic 

 movements in other parts of the plant as well as in the roots. 



What is it that thus changes the nature of these mole- 

 cules? Think of the protoplasm in all the living cells. 

 You remember that it is the living substance. You re- 

 member that it is constantly changing itself and con- 

 stantly changing other substances which enter it. (See 

 page 72.) Along comes a molecule of one of the solutes 

 moving through the sap. If it happens to be a kind which 

 the protoplasm uses, it very quickly ceases to be a mole- 

 cule of that particular kind of solute. It becomes some- 

 thing entirely different. It has been lost ; lost by trans- 

 formation. It has made place for another molecule of 

 the kind which it used to be. 



