244 PLANT SOCIOLOGY 



is mostly the product of bacteria. The quantity of carbonic acid 

 formed serves to indicate the rate of decay. 



The aerobic bacteria involved in decay work with greater energy 

 than the anaerobic putrefactive bacteria. In the decay of cellulose B. 

 ferrugineus, Mijkogene 'puccinoides, Botrytis vulgaris, and others are 

 active. Streptothrix cromogena produces dark, humus-like material. 

 Vast quantities of insoluble carbohydrate compounds are reduced by 

 cellulose fermenters, and the carbonic acid released is returned to the 

 atmosphere. 



Decay is the usual method of forming humus, a substance of 

 extraordinary ecological importance. 



Humus and Humus Formation. — The course of humification 

 depends mainly upon the activity of minute living organisms in the 

 decaying layer. High temperature, moderate moisture, neutral or 

 basic soil reaction favor development of soil organisms and thereby 

 the decomposition of dead organic matter. 



Venema, in investigations at the International Geobotanic Station, 

 Montpellier, demonstrated that the layer of htter rich in mycelia, in 

 which decomposition occurs, is always more acid than the layers 

 immediately above and below. This is true even when it is only a 

 few millimeters in thickness. This acidification appears to be due 

 to the acids hberated by the microorganisms of decay. Even in acid 

 media and at low temperatures the process of oxidation goes slowly 

 forward. 



In addition to external factors, the nature of the decaying matter 

 also affects humification. The nature of the humus is determined also 

 by the destructibility, pH value, and buffering of decaying plant mat- 

 ter. The observations of Melin (1930) show that there is considerable 

 variation of the rate of decay within a single species but that on the 

 whole there is a distinct parallelism between the total nitrogen content 

 and the rate of decomposition. In leaves of different species there is no 

 correlation between nitrogen content and the rate of the primary 

 decomposition. Nevertheless, the process of humus formation seems 

 to be chemically the same in every case. According to the views of 

 Michaelis and of Page (cited in Hesselman, 1927), perhaps the whole 

 formation of humus, in so far as it is unaffected by the mineral matter, 

 consists in a substitution of the metallic cations of acid compounds by 

 H anions. The cations released are easily leached out in a moist 

 climate and conducted into deeper soil layers, where they accumulate. 



Local differences in humification, according to this view, are mostly 

 a matter of rate and quantity of humus formation. But these rela- 

 tions are directly dependent upon chmate. Thus the accumulation of 



