196 UNICELLULAR PLANTS. 



show a surprising diversity in the precise conditions of their 

 nutrition, and it is therefore 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 which they decompose into 

 simpler compounds poorer in poten- 

 tial energy. In so doing they 

 bring about certain chemical 

 changes in the substances upon 

 which they act which are of the 

 highest theoretical interest, and 

 sometimes of great practical im- 

 portance. Perhaps the most pecul- 

 iar feature of the physiology of 

 bacteria is the fact that while they 

 are themselves individually invisi- 

 . ble, they collectively produce very 



wl'iot-spwuum'unduu!' Spiral conspicuous and important changes 

 bacteria deeply stained. Drawn m their environment. For exam- 



from the first photographic repre- . . 



sentation of bacteria ever pub- pie, vinegar bactena act upon 



viz that of Robert Koch, a^ho] ( m c id er , etc.) and by a 



in Cohn a Beitrayc, 1876.) * 



process of oxidation slowly convert 

 it into acetic acid and water, thus : 



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 cae 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 : 



4 O, + O 4 = 2CO, + 2H,O. 

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



