198 UNICELLULAR PLANTS. 



been said already to prove tliat the bacteria are plants, for only 

 plants can live upon inorganic food. But if the ex23eriments 

 just referred to are correct, bacteria are not only plants, but, in 

 spite of their lack of chlorophyll, some at least appear to be 

 able, like green plants to manvfacture their own food out of 

 the raw materials of the morganic world. The importance of 

 this fact in studies of the genealogy of organisms is very great, 

 for we are no longer obliged to suppose all chloroj^hylless plants 

 to be degenerate forms. They may have been the primitive 

 forms of life. 



As was the case with yeast and Protococcus^ it is extremely 

 difficult to make any precise statement concerning the income or 

 outgo of bacteria. It is believed, however, that the income 

 always includes salts and water, and the outgo C02,H20 and 

 some nitrogenous compound or, possibly, free (dissolved) nitro- 

 gen. In more favorable cases the income appears to include 

 proteids, fats, and carbohydrates or their equivalents. Sugar is 

 freely used under some circumstances ; and fats (when saj^onified) 

 and proteids peptonized, or otherwise altered, might readily be 

 absorbed. It is probable that soluble ferments are excreted by 

 the bacteria, which dissolve, and make absorbable, solid matters, 

 such as meat or white of ^^^ ; and if this is true, bacteria exhibit 

 a kind of external digestion. However this may be, it is certain 

 that bacteria can live and multiply upon an amount of food ma- 

 terials so small as almost or quite to elude chemical analysis ; and 

 it is fair to say that they are among the most delicate of all 

 reagents. 



It must not be inferred from what has been said above that bacteria are 

 always oxidizing agents. Broadly speaking and in the long run they are 

 such, and in this respect they resemble animals. Like the latter they are 

 unable (because of want of chlorophyll) to utilize solar energy, and there- 

 fore must obtain their energy by oxidizing their food. Yet under certain 

 circumstances bacteria act as reducing agents, as, for example, when they 

 reduce nitrates to ammonia. This action only takes place, however, in 

 the presence of organic matter, and appears to be merely an incidental 

 effect, the oxygen of the nitrate being needed for the oxidation of carbon. 

 What at first sight appears to be an exception, therefore, proves in the end 

 to be a part of a general law that bacteria, like animals, are oxidizing 

 agents, are dependent for their energy upon the potential energy of their 

 foods, and are unable to utilize solar energy (p. 104). 



