ACTION OF PLANTS ON THE SOIL. 263 



Dead leaves, haulms, branches, and tree-trunks, when they rest upon damp 

 ground, as also lifeless roots, rhizomes, bulbs, and tubers, buried in moist earth, pass 

 into a state of putrefaction, provided that their temperature does not fall below 

 freezing-point, that is to say, they are resolved into carbonic acid, water, and 

 ammonia, the rapidity of the process varying directly as the supply of water and 

 the degree of temperature to which the dead matter is exposed, and inversely as the 

 quantity of compounds of humous acid present. If more dead fragments of plants 

 accumulate within a particular interval of time on one spot than decay, a formation 

 of vegetable mould takes place there; on the other hand, the ground remains 

 destitute of humus when the entire accretion of organic matter is quickly decom- 

 posed as soon as it is dead. The general fact turns out to be that the decomposition 

 of organic bodies is prevented, or at least limited, by a dry condition, and is 

 promoted by moisture, and that it can only be prevented in moist surroundings by 

 the presence of large quantities of humous acids, or by the temperature being low 

 enough to turn water into ice. 



This result directs attention to those inconceivably small animate beings, which, 

 as has been proved by experience, are arrested in their activity by scarcity of water 

 and are killed by the antiseptic substances referred to. That they are the cause of 

 the resolution of dead plants is corroborated by the facts that they are always 

 present where vegetable putrefaction is in progress, and that, on the other hand, 

 decomposition can be prevented by rendering the access of these minute organisms 

 impossible. First in importance in this respect of course are bacteria, the causal 

 connection of which with processes of dissolution, and especially with those decom- 

 positions, which are known by the name of putrefaction, is established. Of these 

 bacteria. Bacterium Termo, and several micrococci, bacilli, vibriones, and spirilla, 

 are the commonest. Their multiplication and the withdrawal for this purpose of 

 substances from dead plants cause a splitting up of the organic compounds in the 

 latter. The albuminoid compounds are first of all peptonized; next, tyrosin, leucin, 

 volatile fatty acids, ammonia, carbon-dioxide, sulphuretted hydrogen, and water 

 are formed, this stage of the process being accompanied by the evolution of an 

 offensive odour of decomposition, and later, nitrous and nitric acids are produced by 

 further oxidation. The carbohydrates, too, chiefly cellulose and starch, are split up, 

 and the products of this analysis, in so far as they are not used up by the bacteria 

 for their growth and reproduction, pass in a gaseous condition into the atmosphere, 

 or into the water surrounding the dead plants. Moreover, the bacteria themselves 

 do not remain at the spots where they have been battening on vegetable matter, but 

 swarm away through the water, or else come to rest in a short time, in which case 

 if the seat of their activity dries up they are blown away by currents of air, and so 

 conveyed to other dead plants. Similar decompositions can be induced by moulds 

 (Eurotium, Mucor, Botrytis cinerea, Penicillium glaucum) as well as by bacteria, 

 and, in addition, the disintegration of wood occasioned by the mycelium of Dry-rot 

 (Merulius lacrymans), the green-rot of trunks of oaks, and beeches, caused by 

 Peziza ceruginosa, the mouldering of wood induced by the mycelium of Polyporus 



