260 LECTURE XIII. 



We will now pass on to consider the relation of light to 

 the destructive metabolism of plants. 



Inasmuch as the respiration of a plant affords an indica- 

 tion of the activity of its destructive metabolism, we will 

 enquire, in the first instance, if it has been discovered that 

 respiration proceeds more or less actively in light than in 

 darkness. With regard to the absorption of oxygen, it ap- 

 pears from the researches of Mayer and von Wolkoff and 

 theirs seem to be the only direct observations bearing upon 

 this point that the absorption of oxygen by seedlings of 

 various plants (wheat, Buckwheat, Tropseolum), is slightly 

 more considerable in light than in darkness. Similar results 

 have also been obtained by Pauchon. From this we may 

 infer that light does not promote the taking up of oxygen 

 by the living protoplasm, for, were this the case, the increase 

 in the amount of oxygen absorbed would be much more 

 marked : the slight increase in the amount of oxygen ab- 

 sorbed in the light is probably to be ascribed to the influence 

 of light in promoting the oxidation or oxidative decom- 

 position of any readily oxidisable substances which may 

 be present in the cells. To these latter points we shall return 

 in a short time. Coming now to the other factor in respira- 

 tion, the exhalation of carbon dioxide, we find that, so far as 

 plants which do not possess chlorophyll are concerned, there 

 is no evidence that light has any material influence upon it. 

 Cahours found a slight increase in the amount of carbon 

 dioxide exhaled by flowers when exposed to light, and Drude 

 observed in his experiments with Monotropa that in many in- 

 stances more carbon dioxide was evolved during the night 

 than during the day. Detmer and Wilson, however, both 

 failed to find that the exhalation of carbon dioxide in light 

 is more active than it is in darkness. In the case of a 

 plant which possesses chlorophyll, it is difficult to determine 

 directly the effect of light upon the exhalation of carbon 

 dioxide on account of the decomposition of carbon dioxide 

 which is of course going on in the chlorophyll-corpuscles. 

 It would appear at first sight from Borodin's researches that 

 the exhalation of carbon dioxide by green plants is much 



