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 ig 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 
