96 BRITISH PLANTS 
actually increase the amount of available nitrogen present 
in it (see Symbiosis, p. 132). 
3. Earthworms.—Charles Darwin, in his classic work 
on Earthworms, has revealed to us the role played by these 
humble creatures in Nature. They inhabit most soils 
containing a mild alkaline humus, which they pass through 
their bodies, depositing the “casts”? upon the surface. 
They are thus always transplanting soil, bringing it from 
below and exposing it to the air—ploughing, as it were, 
the land. Darwin estimated that several inches of soil 
are thus displaced in a century, and he attributed the 
eradual burial of stones and the sinking of old masonry 
to their agency. Worms also honeycomb the soil, pushing 
their tunnels several feet, or even yards, below the surface. 
This permits the free circulation of air in the deeper layers 
of the soil, keeping them sweet for the long exploring 
roots of the deeper-rooted trees. 
4. Fungi—Inm all moist soils rich in humus fungi 
abound. Not being green, these plants can make no 
carbohydrate. They live on rotting vegetation, much as 
bacteria do ; but whereas bacteria are concerned chiefly 
with the nitrogenous compounds, fungi attack the carbo- 
hydrates, which they remove from the soil. 
Manures. 
1. Natural Manure.—This consists of plant or animal 
remains, or animal excrement, which are added to the 
soil to replenish its fertility. It is usually exposed on 
or near the surface for a time, to allow it to rot or ferment ; 
afterwards it is dug into the ground, so that the soil- 
bacteria may get to work on it. To prevent the volatile 
compounds formed during the decomposition from escaping 
into the air, and to prevent the formation of free acids 
which would sour the ground, lime is added to the manure. 
It is important to remember that manure in itself is 
not plant-food ; it is only a source of plant-food. It must 
become rotten, and then be allowed to remain in the 
ground till the soil-bacteria have converted its organic 
material into mineral salts before plants can be said really 
to feed on it. In market-gardens, manure is employed 
not only as a source of food for plants, but also as a source 
of heat for raising early vegetables. For the latter pur- 
