172 VEGETABLE PHYSIOLOGY 



together with carbon dioxide. There are two kinds of 

 these bacteria, one of which oxidises ammonia to nitrous 

 acid and the other converts this into nitric acid. They 

 grow freely in the soil and multiply with considerable 

 rapidity, the result being the formation of certain quanti- 

 ties of organic substance. They cause the carbon dioxide to 

 enter into combination, this gas being normally the only 

 source of their supply of carbon. They possess no chlorophyll 

 and consequently cannot utilise directly the radiant energy 

 of the sun. Their energy is apparently derived from the 

 oxidation of the nitrogenous compounds which they attack. 

 Nothing is known at present of the steps by which the 

 synthesis of the organic matter takes place. 



A process which at first appeared to involve a 

 mechanism resembling that of the chlorophyll apparatus 

 was discovered some years ago by Engelmann. Certain 

 bacteria which contain a purple pigment were found to 

 possess the power of photosynthesis. The pigment was 

 thought to be allied to chlorophyll and to possess the same 

 power of absorbing and utilising the radiant energy of 

 light. Recent researches make it probable that, like the 

 red seaweeds, these organisms contain a certain amount of 

 chlorophyll, together with the purple pigment. 



Saprophytic and parasitic fungi, w r hich contain no 

 chlorophyll, have no power of photosynthesis. They are 

 compelled to absorb their carbohydrates from the medium 

 in which they grow, and they take them in chiefly in the 

 form of sugar. Parasitic phanerogams depend upon a 

 similar source of supply. 



