36 PLANT PHYSIOLOGY 



and of the processes which result in their formation, lags 

 behind our knowledge of the organisms themselves, the 

 treatment of the germ-diseases must continue to be largely 

 a groping in the dark. 



In addition to those fermentations or decompositions ac- 

 complished in the living body or in lifeless substances by 

 distinct species of organisms, there are other equally pro- 

 found chemical disturbances which are due to the combined 

 activities of organisms of more than one species. In the 

 comparatively simple case of the so-called nitrogen bacteria 

 one species decomposes and converts ammonia salts into 

 those of nitrous acid, and another these nitrous salts into 

 nitric. Many other decompositions taking place in nature 

 are the result of the co-operation of several or many species 

 of organisms, some anaerobic, others aerobic, the former 

 preparing the way for the latter. The final products and all 

 the intermediate ones will vary according to the organisms 

 co-operating. The simplest, though not necessarily the 

 mildest, diseases are due to the activity of single species oi 

 organisms. The simplest fermentations are the same. Both 

 are decompositions, with or without oxygen. The more 

 complicated diseases are those due to mixed infections, 

 in which two or more species act together. The one species 

 makes life easier for the other, the products of the one fur- 

 nish food or the sources of energy for the other, the main 

 or the by-products of both supplement each other by weak- 

 ening the host. Typhoid-malaria, and even the mild pro- 

 tective disease which is the result of vaccination, are mixed 

 infections due to the co-operation of two or more ( probably 

 three or four ) species, no one of which alone is able to pro- 

 duce the disease. 



We have then in nature a series of chemical decompositions 

 accomplished by living organisms, animals and plants, for 

 the liberation of needed kinetic energy, and resulting in suc- 

 cessive simplifications from the most complex compounds to 

 the simplest carbon-dioxide, ammonia, and water. The 

 normal or aerobic respiration of all higher and most lower 

 organisms results in the direct destruction by oxidation of 

 the most complex compounds. Anaerobic or intramolecular 



