196 VNICELLULAR PLANTS. 



show a surprising diversity in the precise conditions of their 

 nutrition, and it is tlierefore difficult to make for them a 

 satisfactory general statement. As a group, however, and dis- 

 regarding for the moment certain important exceptions, they are 

 to be regarded as colorless plants living for the most part upon 

 complex organic compounds from which they derive their in- 

 come of matter and energy and wliich they decompose into 



simpler compounds poorer in poten- 

 :^: tial energy. In so doing they 

 i^l bring about certain chemical 

 :/: changes in the substances upon 

 >.V: which they act which are of the 

 vy. highest theoretical interest, and 

 ■^ sometimes of great practical im- 

 % portance. Perhaps the most pecul- 

 ■i: iar feature of the physiology of 

 % bacteria is the fact that while tliey 

 ;i:' are themselves individuallv invisi- 



• ..■ «/ 



^;v;:^:•.:.^>:•.V■:^^•:••v.•:vv/-•;:•::v;::^■^^ ble, they collectively produce very 

 Firiw!-spirnium'nnduia^ couspicuous and important changes 



bacteria deeply stained. Drawn yh their environment. For CXaui- 

 from the first photographic repre- . i , • . 



sentation of bacteria ever pub- ple, Vinegar bacteria act upon 

 lished, viz., that of Robert Koch, alcohol (in cidcr, etc.) and by a 



in Cohn's Beitmye, 1876.) \ ... i i 



process oi oxidation slowly convert 

 it into acetic acid and water, thus : — 



C,H.O + O, = C,H.O, + H,0. 



Here it is not the bacteria that are most conspicuous, but the 

 effect which they produce. It is clear that the alcohol can be 

 only one factor in the nutriment of the organism, because it 

 contains no nitrogen, and the above reaction cannot represent 

 more than a phase in the nutrition of the bacterium. That this 

 is indeed the case is proved by the fact that if the conditions be 

 somewhat changed the same bacteria may go further and convert 

 the acetic acid itself into carbonic acid and water : — 



C,H,0, + O, = 2C0, + 2H,0. 



Chemical changes of this kind in which the effect upon the en- 



