THE GENERAL CONDITIONS OF LIFE 



287 



these plants are the exact reverse of animals : they take up 

 carbonic acid and give off oxygen. So long as the sunlight acts 

 upon their green leaves, they need no oxygen. A green plant, 

 therefore, can be kept alive in a space free from oxygen, if it be 

 allowed to stand in the light and receive carbonic acid. But this 

 taking-in of carbonic acid and giving-out of oxygen is not the 

 plant's respiration. In reality, as 

 has already been seen, 1 the plant 

 like the animal inspires oxygen and 

 expires carbonic acid. This fact is 

 simply disguised by the process of 

 assimilation. During the night, how- 

 ever, when assimilation ceases in the 

 darkness, the plant inspires oxygen 

 and expires carbonic acid ; and, if it 

 be cultivated in a closed space, it 

 lives during the night upon the 

 oxygen that it has set free during 

 the day by the cleavage of the car- 

 bonic acid that it has taken in. The 

 process of assimilation of carbonic 

 acid is, therefore, to be sharply 

 separated from that of respiration. 

 The two phenomena are entirely 

 distinct from one another. 



But in a peculiar kind of organ- 

 isms, the so-called Anaerobia, the 

 relations are even much less clear 

 than in the plants. The Anaerobia 

 are organisms, belonging chiefly to 

 the Bacteria, that can continue to 

 live with complete absence of oxygen. 

 Many of them even perish when they 

 come in contact with free oxygen. 

 Since Pasteur, the father of Bac- 

 teriology, first asserted the reality 

 of such rare beings, their actual 

 existence has frequently been 

 doubted, but there is no longer any 

 question of the correctness of this 



claim. Thus, e.g., the bacteria of symptomatic anthrax and of 

 tetanus grow anaerobically (Fig. 131). So, also, the vibrios of 

 cholera are able to live admirably in alkaline nutrient media with 

 absence of air ; under these conditions they increase rapidly in the 

 intestine, where scarcely a trace of pure oxygen exists. This fact 

 is the more remarkable since when brought into contact with air 



1 Cf. p. 173. 



FIG. 131. A, Culture of the bacteria of 

 symptomatic anthrax. (After Migula.) 

 The spherical colonies lie in the interior 

 of nutrient gelatine excluded from the 

 air. J3, Culture of the bacteria of 

 tetanus. The bacteria have liquefied 

 the lower part of the nutrient gelatine 

 in the test-tube and have formed a 

 bubble of gas, which lies at the upper 

 end of the liquefied mass. They have 

 grown only in the lower parts of the 

 test-tube, separated from the air by a 

 thick layer of gelatine. 



