156 LECTURE IX. 



The views which have been held as to the probable rela- 

 tion between the presence of chlorophyll and the formation of 

 non-nitrogenous organic substance in the plant are so numer- 

 ous, that it will be possible only to mention the more impor- 

 tant of them here. The first to be considered, which we may 

 term the chemical theory and of which Sachsse is the prin- 

 cipal exponent, is that chlorophyll is actually converted into 

 starch, that it is a substance intermediate between carbon 

 dioxide and water on the one hand, and starch on the other. 

 The reasoning upon which this theory is based is very compli- 

 cated and abstruse, and cannot be regarded as conclusive; 

 besides, it is contradicted by Sachs' observation that etiolated 

 plants turn green when exposed to a light in an atmosphere 

 which contains no carbon dioxide, that is, that chlorophyll is 

 formed under circumstances which render impossible the 

 decomposition of carbon dioxide. Then there is what we 

 may term the physical theory which seeks to connect the 

 function of chlorophyll with its absorption-spectrum. This 

 theory is held from two exactly opposite points of view. 

 Lommel and N. J. C. Miiller argue that, according to the 

 principle of the conservation of energy, the rays of light which 

 are absorbed by chlorophyll, that is, more especially the red 

 and the blue, must be converted into some other form of 

 energy, and they conclude that these rays supply the energy 

 necessary for the decomposition of carbon dioxide and water. 

 The correctness of this view is confirmed by TimiriazefT and 

 by Engelmann who find that the decomposition of carbon 

 dioxide by green plants is most active in those parts of the 

 solar spectrum which correspond to the more conspicuous 

 absorption-bands of the chlorophyll-spectrum. It has indeed 

 been found by Draper and by Pfeffer, that the most active de- 

 composition of carbon dioxide takes place when green plants 

 are exposed to yellow light, that is, to rays of the spectrum 

 which are not absorbed by chlorophyll: but this result is not 

 contradictory of those mentioned above ; the discordance is 

 due simply to physical conditions which will be discussed 

 hereafter in connexion with the action of light. It appears, 

 then, that the rays absorbed by chlorophyll are those which 



