4 THE BIOLOGY OF FLOWERING PLANTS 



The resultant mass now begins to take on the characters of 

 soil. Its basal constituent, the refractory portion of the 

 original rock, has been broken down and is mixed with finer 

 material of the nature of clay, derived from other more 

 soluble rock constituents, and with humus, derived from 

 the plant covering ; the whole is permeated by water held 

 especially by the finer material. Perhaps about this stage 

 a further important factor enters in the shape of the earth- 

 worm, the nature and magnitude of whose operations should 

 be followed in Darwin's monograph on vegetable mould 

 (1881). It brings down large fragments of dead plants from 

 the surface into the soil. It subsists on vegetable debris 

 which passes through the digestive tract where it is triturated 

 with mineral particles and whence it is ejected more finely 

 divided and more intimately mixed with these. The worm- 

 casts of fine earth — thrown on the surface in such numbers 

 that the face of a green lawn may be almost blackened in 

 the course of a single night — continually turn over the soil 

 as with a slow, invisible, but efficient plough. Burrowing 

 in every direction, 50,000 individuals in the acre keep 

 the soil light and prevent its rapid compacting to a solid 

 medium unfit for plant growth. Only in suitable soil 

 can the earth-worm thrive, but, as in the case of the 

 plant itself, the worm is largely the moulder of its proper 

 medium. 



Bacteria, too, are important soil organisms ; along with 

 fungi they are responsible for the breaking down of dead 

 organic matter to the humus stage, and also for the disap- 

 pearance of the humus, for in good soil the humus supply 

 is constantly renewed and yet does not increase. Certain 

 species have an effect of the first importance in controlling 

 the supply of nitrogenous compounds in the soil, by assimi- 

 lating atmospheric nitrogen or by various conversions of 

 nitrogenous compounds. 



Not all soils are formed in this fashion in situ. The 

 greater part of the British Islands is covered with soil 

 transported from a distance. The particles of sand and 

 silt and clay and, it may be, humus too, have been carried 



