ABSOKPTION OF FOOD MATEKIALS 137 



The other occasional constituents of the ash, which 

 have not so general a distribution as those already 

 mentioned, include a number of metals which play no part 

 in the nutritive processes. They are usually present in 

 very small amount, and appear to be of accidental occur- 

 rence, being absorbed by reason of the solubility of their 

 salts and their power of entering the root-hairs by the 

 ordinary process of osmosis. They are taken up in very 

 various combinations. Their presence is not generally 

 constant, and appears to depend entirely on the composi- 

 tion of the soil. 



The water which the plants take up is the chief source 

 of the hydrogen and oxygen which enter into the compo- 

 sition of the substance of the plant. A little of both these 

 elements is taken in in the several combinations of the 

 metals ; phosphates contain both, nitrates and carbonates 

 contain oxygen. The amount of them absorbed in these 

 forms is, however, relatively small. As we shall see 

 later, the water plays a very prominent part in the con- 

 structive changes which take place in the plant. 



The gases present in the water of the soil also make 

 their way into the root-hairs with the stream, but the 

 quantity is very slight compared with what is absorbed by 

 the sub-aerial parts. The gas carbon dioxide, which we 

 have seen to be present in the earth in considerable 

 quantity, is, however, not made use of in the constructive 

 processes. All of this particular food material is taken in 

 from the air. A little carbon is absorbed in the form of 

 carbonates. Many complex organic compounds of carbon 

 are taken in by those roots with which fungi, such as the 

 mycorhiza of certain trees, are living symbiotically, but 

 this is exceptional. The root-hairs are capable of absorb- 

 ing such organic compounds as sugar, but these materials 

 are rarely presented to them. 



The absorption of gases from the air takes place in the 

 leaves and other green parts. They enter freely through 

 the stomata so long as these are open, and find their way 



