§ VI.] Conditions of vegetation. Oxygen. 55 



from contact with the air, did not contain a single living germ. A 

 few similar plugs which were only six years old contained germs 

 still capable of development. Duclaux' interpretation of these 

 facts may be correct, but it requires further proof, since we are 

 dealing with matters in which many other things besides the 

 supply of oxygen may have been unequal. Above all things it 

 is necessary in these questions that experiment should be made, 

 not with collective Bacteria, that is, with mixed masses which are 

 possibly or certainly undetermined, but always with a single 

 definite species. 



Oxygen is taken up as material for respiration or breathing, 

 oxygen-breathing, to use a more precise term, carbon-dioxide 

 being at the same time given off. Water, except in some cases 

 which will be mentioned presently, serves as the agent and 

 medium of the chemical processes of the metabolism. Neither 

 of these bodies is properly a nutrient substance, that is, a sub- 

 stance from which carbon-compounds, the constructive material 

 for growth and cell-formation, are produced. 



With respect to the true nutrient substances which therefore 

 supply building-material we must assume in the case of the few 

 green Bacteria, if they really contain chlorophyll, that accord- 

 ing to the analogy of all other plants containing chlorophyll, 

 they assimilate carbon as their food and give off oxygen. 

 Engelmann (26) has ascertained that a small portion of oxygen 

 is given off by his Bacterium chlorinum, and this supports the 

 assumption, while the employment of water also as a food- 

 material in the case of these forms, as in all other plants con- 

 taining chlorophyll, would also be probable. 



The Bacteria containing no chlorophyll, which are by far the 

 greater number and almost the only ones which concern us 

 at present, require, like all cells and organisms that are devoid 

 of chlorophyll, carbon-compounds previously formed else- 

 where for the supply of their carbon, and do not assimilate 

 carbon-dioxide. The nitrogenous food-material may be furnished 

 both by previously formed organic and also by inorganic sub- 



