v] THE SOURCES OF FOOD 125 



sunlight, which is employed by the chlorophyll it con- 

 tains; while the nitrifying bacteria can do all that 

 a green plant can do, in the absence of chlorophyll, 

 and in the dark. Of all organisms theirs is the 

 simplest and most profound mode of nutrition. We 

 have to invent a new term to describe it and we call 

 it prototrophic. 



There remains the sulphuretted hydrogen and the 

 phosphoretted hydrogen which are produced during 

 decomposition or during excretion. These also are 

 compounds which are not entirely degraded and still 

 possess energy in that they can be oxidised. The 

 phosphoretted hydrogen is at once oxidised in the air 

 to phosphoric acid, and the sulphuretted hydrogen 

 may also be directly oxidised. But wherever in the 

 sea, or in fresh water, there is sufficient of this gas 

 there are also red or colourless sulphur bacteria, and 

 both of these forms of microbes are able to use the 

 gas, which is poisonous to the higher organisms, as a 

 source of food-stuff. A mere trace of carbonaceous 

 food in the form of some simple compound, such as 

 formic acid for instance, and a trace of nitrogenous 

 substance in the form of ammonia are sufficient for 

 the renewal of their bioplasm, and in place of the 

 soluble sugar, which is assimilated as respirable 

 material by the higher organisms, these bacteria can 

 assimilate the sulphuretted hydrogen, oxidising its 

 hydrogen to water, and setting free the sulphur in 



