136 BACTERIA AND THE CARBON DIOXIDE CYCLE 



This brief description of the phenomena of fermentation will serve to 

 indicate the scope and nature of the problems involved. 



There remain to be said a few words regarding the by-products of 

 fermentation, carbon dioxide, free hydrogen, and (in putrefactive processes) 

 ammonia and free nitrogen. Careful investigation shows that there is a 

 whole scale of products ranging from compounds whose heat of combus- 

 tion is high, such as alcohol, down to the gases just mentioned, in which it 

 is very low or nil. There seems every reason to believe that the combus- 

 tion of both the chief and the by-products is gradual, by stages. In this way 

 there must be going on a number of different processes simultaneously in 

 the cell, the various compounds being decomposed, and their energy tapped 

 step by step, a process which must necessarily result in the formation of 

 a large number of bodies of different composition ' by-products.' 



The description given in the last five chapters of the circulation of the 

 elements carbon and nitrogen, and of the role played therein by bacteria, 

 would be imperfect and inaccurate if no mention were made of the part 

 taken in the process by other micro-organisms. The great importance of 

 bacteria in medicine and in many technical operations has caused them to 

 receive more attention than other groups, and as a result we too often 

 regard them as being the only agents in the disintegration of dead organic 

 matter. This idea is incorrect, for although precise investigations are 

 wanting there can be no doubt that many other protozoa found in abun- 

 dance where putrefaction is going on, ciliate and flagellate infusoria and 

 amoebae, also take part in the work of destruction. They are not always 

 merely saprophile ; they are saprogenic, and assist in the breaking-up of 

 the molecules of organic substance. 



Nor should we forget that the mould-fungi and all other plants devoid 

 of chlorophyll are entirely dependent for food on decaying organic matter, 

 the carbon of which they help to dissipate by their respiratory processes. 

 They furthermore convert stable and resistant bodies (such as wood) into 

 the easily decayable substance of their own cells. Not a dead twig or 

 trunk in the forest that does not bear minute Pyrenomycetes silently and 

 imperceptibly eating up the wood, to be in their turn disintegrated and 

 dissipated by the putrefactive bacteria to which sooner or later all fungi 

 fall a prey. 



Thus the final destiny of all living substance is sooner or later, directly 

 or indirectly, to become food for bacteria, and it would seem as though the 

 earth must at last be suffocated by their numbers. But, apart from the fact 

 that large quantities of bacteria are consumed by other protozoa (infusoria 

 and amoebae are often crammed full of them), it must be remembered that 

 each bacterial cell lives only for a short time and leaves but a limited if 

 large number of descendants, which in their turn die off and are consumed 

 by their kindred, only to enter again into new cycles of chemical change. 



