THE RESPIRATION OF PLANTS 467 



is about equal to that of CO., evolved, but if oily seeds be examined 

 during germination, more oxygen is taken in in proportion to 

 the CO., given out. Various observers have shown that in certain 

 cases, succulent leaves, or the phylloclades of a Cactus, ar 

 capable of absorbing oxygen without the simultaneous evolution 

 of CO., at all. Nor is the oxygen absorbed without entering 

 into chemical combination, for it cannot be extracted by the air- 

 pump. Conversely CO., may be thrown off from a plant without 

 any simultaneous absorption of oxygen. If a seed be germinated 

 in a vacuum over a column of mercury, CO., is found to be 

 evolved. Ripe fruits have been found to give off CO., in an 

 atmosphere quite devoid of oxygen. 



Again, it is found that the ratio of oxygen absorbed to CO., 

 exhaled varies according to the temperature at which the experi- 

 ment is conducted. Evidently the two processes are not 

 directly dependent upon each other. 



It is evident from the foregoing considerations that the vital 

 activity of protoplasm is somehow associated with the two pro- 

 cesses. In the absence of oxygen it gradually ceases, the living 

 substance being in fact slowly asphyxiated. During its life one 

 of the manifestations of its metabolism is the formation and 

 exhalation of the two fairly simple compounds CO., and water. 

 To ascertain what is the true relation of the two processes, it is 

 necessary to look closely at the nature of the chemical changes 

 going on in the protoplasm itself, or at what we have called its 

 metabolism. 



Respiration in the strict sense is, therefore, a process going 

 on in the living substance itself, or rather is the expression of 

 the beginning and end of a series of cormplex changes in which 

 the molecules of the living substance are involved. The details 

 of the absorption of the oxygen by the plant from its environment 

 and the ultimate evolution of the C0._, and water from the plant 

 body should rather be regarded as the mechanism of respiration 

 than respiration itself, which is a function of the living substance 

 only. A distmction is often made by some writers between the 

 two, the latter being spoken of as intrainolecular respiration. It 

 seems better on the whole to consider the latter phenomenon 

 only under the name of respiration. 



We see, then, that the two processes are not immediately 

 connected in the sense of the CO., coming at once from the direct 

 oxidation of carbon, but that they are ultimately associated, 

 though separated in time by a series of chemical changes taking 

 place in the living substance. 



H H 2 



