56 THE LIFE OF THE PLANT 



place in the nutrition of a vegetable cell. It is the iron 

 salt which moves toward the tannin, not the tannin 

 towards the iron, because the iron salt is a crystalloid, 

 whereas tannin is a colloid. Going back to the nutrition 

 of the cell we meet, roughly speaking, the same pheno- 

 menon. What, in fact, are the substances a cell finds 

 in its environment ? Gases, water, and salts dissolved 

 in water ; that is, crystalloid substances — ^which means, 

 generally speaking, extremely mobile substances which 

 easily pass through the cell membrane. What sub- 

 stances does such a cell contain, into what does it 

 transform the substances absorbed from the outside ? 

 It transforms them chiefly into albuminoids, oils, gums, 

 starch, or cellulose — ^in other words, into colloids, 

 scarcely mobile substances which will not pass through 

 membranes or into other substances totally insoluble. 

 This may be easily grasped with the aid of the following 

 table : — 



MAIN SUBSTANCES 



Insoluble bodies and colloids. 



Gases and crystalloids. 



Throughout its life the cell is continually surrounded 

 by substances which easily diffuse into it. Thus, for 

 instance, the carbonic acid of the air rushes constantly 

 into every cell with which it comes into contact. But if 

 this carbonic acid remained in the cell unchanged, only 

 a very little of it would penetrate into the cell ; however, 

 as we shall see later on, once in the cell it is changed : 

 carbonic acid and water form a carbohydrate, and this 

 transformation leaves room for the entry of fresh 



