Chap, viii Respiration 423 



apparent when Laskowski showed in 1874 that water, as 

 well as carbon dioxide, is formed in the respiratory processes, 

 and consequently that the amount of oxygen absorbed is 

 insufficient for the complete combustion which the theory 

 requires. 



Another blow to the theory was given by the reinvestiga- 

 tion of some old experiments of De Saussure on Opuntia, 

 in which he found that under certain conditions it exhales 

 no carbon dioxide, though it may be supplied with oxygen. 

 DeheYain confirmed this statement in 1874, and showed 

 that the plant under these conditions accumulates a large 

 quantity of oxalic acid. Mayer, the next year, showed 

 that certain succulent Crassulaceous plants store a con- 

 siderable amount of malic acid in their leaves, and that, 

 as in the case of Opuntia, this is attended by the cessation 

 of the exhalation of carbon dioxide. These experiments 

 pointed clearly to such a modification of the respiratory 

 processes as is incompatible with its being a combustion. 



Yet the ultimate dependence of the carbon dioxide upon 

 a supply of oxygen appeared from many researches to be 

 undeniable. Broughton, in 1870, and Wortmann, in 1880, 

 showed that when seedlings are deprived of oxygen, their 

 output of carbon dioxide becomes less. The latter observer 

 confined his seedlings alternately in air and in hydrogen, 

 taking measurements of the carbon dioxide every half-hour 

 under conditions of such an alternation of environment. 



Though the idea of respiratory combustion has been 

 proved to be untenable in the form in which it was stated 

 by Sachs and his immediate successors, it has nevertheless 

 not disappeared. Many writers have endeavoured to evade 

 the difficulties it involves and still maintain the essential 

 accuracy of the term by substituting the expression physio- 

 logical combustion for the older one. It seems, however, 

 rather to introduce fresh difficulties into our conception of 

 the process than to elucidate it. 



