Ch. IV, S] RESPIRATION OP PLANTS 173 



use the air, and even are fatally affected by its presence), they 

 break up the molecules of organic substance, and therefrom 

 obtain materials for their respiration and growth, simul- 

 taneously forming various by-products. Usually they re- 

 lease carbon dioxide in their respiration or fermentation, 

 but sometimes they form otlicr gases which happen to possess 

 the offensive odors familiarlj^ associated with decay. Some 

 kinds of organisms start the decay, which other kinds con- 

 tinue ; and thus in successive steps the most complicated 

 organic substances are reduced gradually back to the car- 

 bon dioxide, water, nitrogen, and mineral matters from 

 which they were originally constructed. There is always 

 at each stage some kind of organism ready to utilize any po- 

 tential energy remaining in the organic substance, until ' 

 finally all is exhausted. This is the ultimate fate of all or- 

 ganic substance, which, formed photosynthetically in 

 leaves from carbon dioxide and water and mineral salts, is 

 converted back to those substances, either by the respiration 

 of the higher plants and animals themselves, or else by the 

 respiration of the micro-organisms of decay. Thus the 

 carbon dioxide, water, nitrogen, and mineral matters with- 

 drawn from the general circulation of nature and locked up 

 for a time in the substance of plants and animals are all re- 

 turned in time to their source. But on this -withdrawal and 

 return hinge the visible phenomena of life. 



The formation of incidental by-products by Molds and 

 Bacteria in their respiration and growth has important prac- 

 tical consequences to other organisms. Thus parasitic Fungi 

 produce such by-products, whereby their host plants are poi- 

 soned and damaged, often to complete destruction. The 

 Bacteria associated with disease often produce violently poi- 

 sonous products, collectively called toxins ; and it is these sub- 

 stances, and not any direct injury done by the Bacteria them- 

 selves, which cause death from bacterial diseases in man, other 

 animals, and some plants. This fact underlies the methods 

 of combating those diseases by use of antitoxins and the like. 



