THE METABOLISM OF PLANTS. 211 



when free oxygen is supplied to them, when their metabolism 

 is what we may term normal ; but Pasteur has shewn that 

 certain forms of Saccharomycetes and of Schizomycetes thrive 

 best in the absence of oxygen, and that, in certain cases, the 

 access of oxygen proves fatal to the organisms. Pasteur 

 terms plants of the former kind aerobia, of the latter anaerobia. 

 It is difficult to say whether these anaerobiotic Saccharo- 

 mycetes and Schizomycetes are distinct species, or whether 

 they are physiological varieties, that is, individuals which 

 have become adapted to a life in the absence of free oxygen : 

 Pasteur inclines to the former of these alternatives. The 

 absence of oxygen does not, however, prove immediately 

 fatal to an aerobiotic plant ; it can live for a time without 

 being supplied with free oxygen, and it appears that the 

 more lowly the organism the greater its independence in 

 this respect. Thus, de Saussure observed that seeds do not 

 germinate in an atmosphere destitute of free oxygen, and 

 that, like all other parts of highly organised plants, they die 

 within a comparatively short time. Pasteur, on the other 

 hand, found that a Mould (Mucor racemosus] was still alive 

 after having been kept for six months in a vessel containing 

 no free oxygen ; but that after being for nearly three years 

 in the vessel it was dead. 



We have now to endeavour to account for the modification 

 of the metabolism which is induced by the absence of free 

 oxygen. One conclusion from the facts before us is suffi- 

 ciently obvious, namely this, that, in the absence of oxygen, 

 the activity of those metabolic processes which we have termed 

 fermentative is largely increased. We may say, indeed, of 

 every living cell what Pasteur has said of Yeast -that in the 

 presence of abundance of free oxygen it is not a ferment, 

 and that it is only in the absence of free oxygen that it 

 exhibits those properties which have earned for it this name. 

 It is evident, too, that fermentative decomposition serves to 

 maintain the life of the plant : this is a simple inference from 

 the fact that anaerobiotic plants exist. The real difficulty 

 is to explain the significance of fermentative decomposition, 

 to determine the mode in which it contributes to the main- 



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