ABSORPTION OF FOOD MATERIALS 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 
