142 VEGETABLE PHYSIOLOGY 



prominent in the grasses and the horsetails. It is taken 

 up from the soil in the form of soluble silicates, and possibly 

 to some extent in that of soluble silicic acid. 



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 occurrence, 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 composition of 

 the soil. 



The water which the plants take up is the chief source 

 of the hydrogen and oxygen which enter into the com- 

 position 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 constructive 

 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 subaerial 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 absorbing such organic compounds 

 as sugar, but these materials are rarely presented to them. 



The absorption of gases from the air takes place in the 



