496 BIOLOGY OF THE MICRO-ORGANISMS. 



to build up new tissue. Hence they cannot exist in 

 pure water containing only mineral substances ; they 

 grow, on the contrary, only on dead organic matter, rich 

 in carbon and nitrogen, especially on dead plants and 

 animals; or they live as parasites, withdrawing from 

 their vegetable or animal hosts the organic substances 

 necessary for their life and growth. 



Teleological From these facts we can at once understand the signi- 

 lower fungi, ficance of the fungi in the economy of nature. In order 

 to supply the necessary simple nutrient materials to the 

 chlorophyllous plants, it is essential that the sub- 

 stance of dead plants should be continually broken up 

 and reduced to these simple compounds. 



The whole of the vegetation, which is yearly produced 

 and again dies, must be so altered in a relatively short 

 time that water, carbonic acid, and ammonia may be 

 again formed from albumen, carbo-hydrates, cellulose, 

 &c. ; and it is only under these circumstances that a 

 constant and increasing renewal of vegetation is con- 

 ceivable. 



A portion of this destructive work is performed by 

 the animal organism ; the animal cells split up the 

 vegetable materials which have been taken in, and pre- 

 pares them for oxidation. The energy which had be- 

 come stored up in the complex chemical compounds of 

 the plant, from the conversion of the energy of the light 

 rays into a chemical energy by the aid of the chlorophyll, 

 is used up by the animal organism, and employed for 

 the production of heat, and for the various functions of 

 the body. This consumption of vegetable materials by 

 animals is, however, by no means sufficient to equalise 

 the production of vegetable substances and to keep the 

 amount of the simple nutrient materials of plants at such 

 a level as to suffice for the constant nutrition and growth 

 of new vegetation. It is evident that some other factor 

 must come into play in the economy of nature by which 

 a much more extensive destruction of dead vegetable 

 substances and a much greater formation of carbonic 

 acid and ammonia is brought about than by the vital 

 processes of animals ; and this necessity has become 



