ABSORPTION OF FOOD MATERIALS 127 
duced by the addition of certain compounds to the culture 
fluid, or how the absence of any particular salt affects their 
well-being. 
In carrying out experiments in this way, it is usual to sow 
some large seeds, such as those of the broad bean, in damp 
sawdust, and allow them to germinate. When the radicle 
of the seedling has elongated to the extent of about an 
inch, the seed is placed upon a perforated cork inserted into 
the neck of a bottle containing the liquid which is the 
subject of the investigation. It is so arranged that the 
radicle dips down through the cork into the liquid. As 
growth proceeds the radicle develops a root-system in the 
way appropriate to the particular plant used, which absorbs 
from the liquid the salts which are required by it, so far as 
these are present. At the same time the plumule grows 
upwards, and soon a shoot appears, which develops pari 
passu with the root. 
By this method various plants can be cultivated with 
different degrees of success ; in some cases not only leaves, 
but flowers and even fruit can be produced. The progress 
of the plant, and the readiness with which it will develop, 
will depend upon the salts which are supplied to it in the 
water, if it is maintained in normal conditions of light, 
temperature, and aeration. In preparing the solution, 
particular mixtures can be employed, and the most favour- 
able one ascertained, while subsequent analysis of the 
liquid will show to what extent the various constituents of 
the culture fluid have been abstracted from it. 
This method is, however, only of use in determining 
particular points, such as the effect of the presence of 
certain metals in particular combinations, or the influence 
of different concentrations of particular substances. It 
does not give an account of what is happening to a 
plant with its roots embedded in the soil, for the com- 
position of the latter cannot be compared with that of 
a solution definitely made up for purposes of experiment. 
The composition of the soil, as we have seen, is very far 
