ACTION OF LIGHT ON VEGETATION. 761 



observations at present made ; but it is probable, from the absorption-phenomena and 

 their connection with fluorescence. 



The question whether the absorption-bands of the spectrum of the colouring- 

 matter of chlorophyll have any causal connection with the function of the chlorophyll- 

 granules in decomposing carbon dioxide has recently been answered by Lommel in the 

 affirmative, on purely theoretical grounds, in support of which he brings forward the 

 following statements ^ : — 



* The most efficacious rays in promoting assimilation in plants are those which 

 are most strongly absorbed by chlorophyll, and which at the same time possess a 

 high mechanical intensity (heat-action) ; these are the red rays between B and C 

 But a glance at the carefully prepared tables given at pp. 745-6, shows that this 

 theoretical reasoning is incorrect. If Lommel's hypothesis were correct, the evo- 

 lution of oxygen would be seen, on observing the solar spectrum, to attain its 

 maximum between B and C^, which however, as Pfeffer has shown, is by no means 

 the case. The second of Lommel's statements is : — ' The yellow rays can produce 

 only a small effect notwithstanding their considerable mechanical intensity, because 

 they are absorbed only to a small extent ; and the same is the case with the orange 

 and green rays.' This statement is again entirely opposed to observation ; for it 

 is these very rays that are the most efficacious in promoting evolution of oxygen. 

 Lommel says indeed (/. c. p. 584) that Uhis inference is incorrect;' it is however no 

 inference, but the result of actual observation. That the light which has passed through 

 a solution of chlorophyll causes only an inconsiderable evolution of oxygen is easily 

 explained when it is recollected that even the yellow is considerably weakened in 

 the spectrum of chlorophyll. But according to Lommel's theory there ought to be 

 no evolution of oxygen at all when light has passed through a solution of this kind 

 if it shows the absorption-bands very dark, since those rays which according to him 

 are alone efficacious are wanting. 



There is however no need for this direct contradiction; for a correct estimate 

 of known facts leads to the conclusion that it cannot be .those rays which are ab- 

 sorbed by the colouring matter of chlorophyll that cause the evolution of oxygen; 

 for the rays absorbed in such a solution are the same as those absorbed in a green 

 leaf. In the former there is however no evolution of oxygen (and apparently also no 

 oxidation) ; and there is nothing to justify the supposition that the same rays which 

 are absorbed by chlorophyll in solution without causing evolution of oxygen should cause 

 it in the living leaf. It must certainly be right to suppose, as a necessary result of the 

 principle of the conservation of energy^, that the rays which are efficacious in causing 

 evolution of oxygen must be absorbed, inasmuch as they perform chemical work; but 



^ Lommel, Pogg. Ann. vol, 143. p. 581 e( seq. 



^ Miiller (Botan. Beobachtungen, Heidelberg 1871, Heft I) has adduced a great array of 

 figures in support of this conclusion. But any one who knows how such observations should be 

 made knows also what value is to be attached to these. See also Pfeffer, Bot. Zeit. 1872, No. 23 

 et seq. 



^ See also what I said on this subject long ago in my Experimental Physiology, p. 287. 

 [Pringsheim has made a series of researches on Chlorophyll (Monatsber. d. k. Akad. zu Berlin, 

 1874-1881 ; Jahrb. f. wiss. Bot. XII, 1881 ; see also Nature, vols. XXI, XXIII, and Quart. Journ. 

 Micr. Sci. 1882), and has come to the following conclusions with respect to its function: (i) that 

 the rays absorbed by it are not those which promote assimilation ; (2) that, on the contrary, these 

 rays promote the respiration of the protoplasm ; (3) that it is the protoplasm of the chlorophyll- 

 granule, and not the chlorophyll, which decomposes the absorbed carbonic acid and forms organic 

 compounds; (4) that the energy for this purpose is derived from light, but it has not yet been 

 ascertained which rays are absorbed by the protoplasm ; (5) that the function of the chlorophyll 

 in the process is protective, that is, that it absorbs the rays which would promote respiration in 

 the chlorophyll-granule and this renders it possible for the synthetical processes to take place 

 within it.] 



