ACTION OF LIGHT ON VEGETATION. ' 68 1 



ascertained w hich rays of the sunlight cause the fluorescence ; the red begins a Httle 

 to the left of the line B of the solar spectrum, and stretches, although varying in inten- 

 sity, over the violet end. On the dark-red ground are seen seven intensely red bands, 

 each corresponding exactly both in position and in strength to an absorption-band 

 in the spectrum of chlorophyll. If the fluorescence caused by the solution of chloro- 

 phyll is itself observed through a prism, it is seen to consist only of red rays, the 

 refrangibility of which coincides with the strongest absorption-band of chlorophyll 

 between B and C. Every ray produces by fluorescence only such as correspond in 

 their refrangibility to the absorption-band /. Whether the chlorophyll contained in 

 living cells is subject to the same fluorescence is not certain, from the imperfect 

 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- 

 grains 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 facts ^ : — 



' 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. 667, 668, shows that this 

 theoretical reasoning is incorrect. If Lommel's hypothesis were con-ect, 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 Q.c. p. 584) that 'this 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 (see p. 679). In the former there is however no evolution of oxygen (and ap- 

 parently also no oxidation) ; and there is nothing to justify the supposition that the 

 same rays which the colouring matter of chlorophyll absorbs 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, 



1 Lommel, Pogg. Ann. Vol. 143, p. 581 et seq. 



2 Muller, (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 Pfeflfer, Bot. Ze.t. 1872, No. 23 

 el seq. 



■^ See also what I said on this subject seven years ago in my Experimental Physio.ogy, p. 2^7. 



