THE METABOLISM OF PLANTS. 2O/ 



We are now in a position to consider the curious ab- 

 normal case afforded by de Saussure's Opuntia, He found 

 that he could not extract, by means of the air-pump, any 

 appreciable quantity of either oxygen or carbon dioxide from 

 a piece of stem which had absorbed about 80 c.c. of oxygen 

 from the air contained in a receiver. From this it appears 

 that the absorbed oxygen was chemically fixed in the plant, 

 and that the oxidised substance had not undergone any 

 decomposition attended by evolution of carbon dioxide. It 

 is possible to conceive, however, that carbon dioxide may 

 have been formed and retained in the plant in some sort 

 of loose chemical combination, but it is probable, had this 

 been the case, that some considerable amount of this gas 

 would have been given off to the air-pump. We are appa- 

 rently bound to admit that no carbon dioxide was formed. 



In endeavouring to account for this remarkable fact, the 

 explanation which naturally suggests itself is that the absorbed 

 oxygen is retained by the protoplasm as what we have 

 termed intramolecular oxygen. This explanation would, 

 however, involve the assumption that the decomposition of 

 the protoplasm is arrested, inasmuch as no carbon dioxide is 

 evolved. But there are no grounds for such an assumption. 

 The only alternative is to assume that in this instance de- 

 structive metabolism is unaccompanied by an evolution of 

 carbon dioxide. It has been ascertained by Mayer, who has 

 carefully investigated this matter, that there is a consider- 

 able increase in the amount of organic acid in the organ 

 during the experiment, and this is confirmed by an obser- 

 vation of Deherain's. It seems, then, that we have before us 

 a case in which the decomposition of protoplasm is accom- 

 panied by the formation not of carbon dioxide, but of a more 

 complex acid instead. 



The acid which Mayer found in the succulent leaves of the Crassu- 

 laceous plants with which he experimented appears to be an isomer of 

 malic acid, whereas in the Opuntia it is oxalic acid, according to Dehdrain. 



We will now pass on to consider those metabolic processes 

 which we have termed fermentative. Of these, some are 

 essentially dependent upon the presence of oxygen, whereas 



