THE METABOLISM OF PLANTS. 157 



are active in the decomposition of carbon dioxide. The 

 other view is that taken by Pringsheim. He urges that 

 the rays absorbed by chlorophyll are not active in the 

 decomposition of carbon dioxide, but that still the absorp- 

 tion must have some physiological significance, and he con- 

 siders this to be that the rays absorbed are such as would 

 interfere with the synthetic processes; that the chlorophyll 

 acts as a kind of filter to the rays of light which fall upon the 

 plant, allowing those to pass which promote the synthetic 

 processes and absorbing those which would be prejudicial to 

 them. As we shall discuss this view in a subsequent lecture, 

 it will suffice for the present to say that there is no evidence 

 to prove that the rays absorbed by chlorophyll have any effect 

 in diminishing the decomposition of carbon dioxide. It has 

 been further suggested that chlorophyll combines with and 

 fixes carbon dioxide just as the haemoglobin of the blood 

 combines with and fixes oxygen. But experiments made 

 with various solutions of chlorophyll shew that this is not 

 the case ; still, it is possible that what the chlorophyll cannot 

 do when extracted in solution it may be able to do when 

 it is in the living chlorophyll-corpuscle. 



Of these various views, the one which is most strongly 

 supported by experimental evidence is that of Lommel, and 

 it is therefore this one that we shall accept. The function of 

 chlorophyll may, then, be briefly stated as follows ; that it 

 absorbs certain rays of light, and thus enables the protoplasm 

 with which it is intimately connected to avail itself of the 

 radiant energy of the sun's rays for the construction of 

 organic substance from carbon dioxide and water. 



4. TJu Action of Light. 



Attention has been repeatedly drawn both in this and in 

 previous lectures to the fact that the absorption of carbon 

 dioxide and the evolution of oxygen, in other words, the 

 formation of organic from inorganic substance, can only be 

 effected by green plants when they are exposed to light. 

 This subject will not be entered upon at present ; it will be 



