106 THE FACTORS [Part I 



all abound ; A. moschata and A. Millefolium on slate, A. atrata and A. Millefolium on 

 lime. Wherever the slate passes into limestone, A. moschata at once stops and 

 A. atrata begins. Both species are here therefore strictly confined to one soil ; and 

 this I have also observed in several places in Biindten, where the two species occur. 

 If one of the species be absent, the other becomes indifferent as regards its choice 

 of soil. A. atrata then inhabits both lime and slate indifferently ; the same fact also 

 applies to A. moschata : although, as it appears, this species does not thrive so well on 

 limestone as the other does on slate, yet, besides occurring on the primary mountains, 

 it is also found on pronounced lime formations, along with the usual characteristic 

 vegetation. In Bernina-Heuthal, I found in the midst of the slate that was stocked 

 with A. moschata a large fallen block of limestone with a layer of soil on it hardly 

 an inch thick. A colony of A. moschata had settled on it, as there all competition with 

 A. atrata was excluded.' 



8. HUMUS, 

 i. THE CHEMISTRY AND PHYSICS OF HUMUS\ 



Only a few natural soils consist exclusively of mineral matter, only 

 those in fact that are quite devoid of vegetation. As soon as plants have 

 settled on a mineral substratum, even if they be only bacteria, unicellular 

 Algae, or slowly growing lichens, they produce by their death and decomposi- 

 tion a finely grained organic substance, which, by the action of the rain and 

 of underground animals, becomes gradually and closely intermingled with 

 the mineral matter, so as to form dark earth, or ordinary soil, which is at 

 once distinguishable by its nearly black colour from purely mineral detritus. 



The organic products of decomposition of animals and plants are called 

 humus. During the process of humification dead animal and vegetable 

 remains produce by oxidation carbon dioxide and water, the latter however 

 in much greater quantity than the former, so that the residue is much richer 

 in carbon than were the living organisms. If sufficient air be admitted, 

 a formation of ammonia and nitric acid takes place at the expense of the 

 proteids ; nevertheless the greatest part of the nitrogen remains combined in 

 organic compounds that are decomposed with difficulty. What follows 

 here is chiefly concerned with the universal and important vegetable humus 

 only; animal humus is of mere local importance in determining the 

 distribution of natural vegetation, and will be discussed separately. 



As appears from the foregoing, humus is rich in two of the most im- 

 portant constituent elements of plants, carbon and nitrogen, which occur in 

 an inorganic nutrient medium only in a condition of extreme dilution. 

 These nutritive substances, in the form in which they are present in humus, 

 however, cannot be utilized by green plants, nor indeed by any of the 

 higher plants. Only certain bacteria and fungi can assimilate them to 

 a greater or less extent, or resolve them into simpler compounds. Many 



1 See especially Ad. Mayer and P. Miiller. 



