66 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
on in the body, and giving out carbon dioxide as a product of 
waste. It is quite a mistake to suppose that plants ‘‘ breathe in 
carbon dioxide and breathe out oxygen”, as often supposed. If 
there were not some compensating arrangement, it is clear that 
the amount of oxygen in air would rapidly diminish and the 
quantity of carbon dioxide rapidly increase. But it appears that 
this is not so, for the composition of air remains practically the 
same, at least for very long periods of time, though probably in 
earlier periods of the earth’s history its constituent gases were in 
different proportions from what is now the case. This constancy 
of composition at the present day is intelligible when we re- 
member how green plants feed, and of what their food consists. 
Such plants, in fact, bridge over in a fashion the gulf between 
non-living and living matter. For their food consists of carbon 
dioxide (obtained from the air), and water in which are dissolved 
certain substances of simple kind, foremost among these being 
nitrogen-containing compounds known as nitrates, of which salt- 
petre is a well-known example. In any sort of tree, shrub, or 
herb the carbon dioxide is taken in by the leaves (and the green 
part of the stem), while the roots absorb from the soil a watery 
solution of nitrates, &c., so dilute as to be comparable to the 
ordinary drinking water of most districts. These simple con- 
stituents of the food are converted step by step into the living 
substance of the plant by the agency of that substance itself. 
The first step in this series of up-building processes takes place in 
the leaves (and the green parts of the stem), and consists in a 
reaction between carbon dioxide and water, giving rise to a sub- 
stance which is more complex in nature than either of them. 
This can only go on in daylight and in the presence of chloro- 
phyll, which in some way not clearly understood enables the living 
substance associated with it to press the energy of sunlight into its 
service for the purpose of building up a comparatively complex 
substance from simple ones. And this first step in the manufac- 
ture of living matter is accompanied by the liberation of free 
oxygen into the surrounding air as a by-product. For the 
carbon dioxide and water, which are the raw materials in this 
work, contain more oxygen than is required for the purpose, and 
the surplus passes away to the exterior. It therefore follows that 
green plants in the course of their feeding (1) take carbon dioxide 
From the air, and (2) give out free oxygen Zo the air. And these 
