INFLUENCE OF THE SUBSTRATUM. 499 



overcome by saprophytes which find a suitable habitat there and nourish in 

 abundance. The decayed portions of Saprophytes contain relatively little inorganic 

 material. No trace of lime (in particular) is to be found in their ash. In this way 

 a superficial layer of humus is formed which actually excludes a large number of 

 plants. The next deeper layer may contain a considerable quantity of inorganic 

 salts, but they are valueless to plants rooted in the upper (humus) layer, as they 

 cannot penetrate it. It has been shown by experiment that pure humus possesses 

 the power of holding back materials which are soluble in water. It possesses this 

 property to such an extent that if salt solutions are filtered through a layer of 

 humus the water which escapes below is almost pure. It is therefore impossible 

 for inorganic substances from the deeper layers of the soil, much less from the 

 underlying rock, to reach the surface layer of humus in solution by diffusion; and 

 if some mineral ingredients are not introduced by irrigation or flooding, the upper 

 layer of soil consists of pure humus on which only saprophytic plants can flourish. 



The formation of such layers of humus occurs much more easily and quickly on 

 slate mountains than on limestone, because in the former the rock and the products 

 of its decomposition retain water much better, and a uniform saturation promotes 

 the development of humus, and also because on slaty soil the second stage of the 

 development of the plant-covering consists of plants which require very few inor- 

 ganic food -substances, and accordingly very few inorganic materials are yielded 

 by the humus, which originates at the cost of the decaying portions of these plants. 

 But a thick stratum of pure humus may also arise in course of time on lime- 

 stone mountains. Only the soil must be uniformly moist in that spot, and 

 neither sand nor mud must be deposited on it. If these conditions are fulfilled a 

 deep humus will gradually spread itself over limestone rocks and de'bris in the 

 third stage of development, the superficial layer of which will contain no trace 

 of lime, but will afford an excellent soil for silica-loving plants (i.e. for those to 

 which lime is injurious). The isolated occurrence of so-called siliceous or slate- 

 plants on limestone mountains, even in the middle of a patch of plants which are 

 characteristic of a limestone soil, may be naturally explained in this fashion. 



The water which moistens the rock and soaks the soil has, apart from its 

 mechanical action, the important function of opening up mineral substances and 

 of forming solutions from which the absorbent plant-cells may take their choice. 

 The atmospheric water which penetrates into the earth from above is especially 

 valuable as a solvent on account of the carbonic acid gas it contains. It is 

 immeasurably more valuable to every part of the soil which is riddled by the 

 roots of living plants than the soil- water, so poor in carbonic acid, which collects on 

 impervious strata of the soil and soaks upwards through the superficial layers. 



The power of the soil to retain water depends mainly on the extent of breaking 

 up undergone by the rock whose disintegration has formed the soil and upon the 

 amount of clay which has arisen from this disintegration. But the amount of 

 humus which in course of time has mixed with the disintegration and the decom- 

 position products of the underlying rock is also an important factor, and thus very 



