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 

 gradual 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 r 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. In 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- 



