INTERNAL TEMPERATURE OF LEAVES. 257 



Pringsheim , and would seem to indicate that the destructive 

 effect of light on chlorophyll had been over-estimated by him. 



Engehnann (5), 1883, using the bacterial method, showed in 

 opposition to Pringsheim that the green colouring matter, and 

 that only, did the work of assimilation. He went onto show 

 that in all coloured cells red, green, blue-green, and yellowish- 

 bro'wn there is a general paraUellism between absorption of 

 light and assimilation. This he did by investigating the 

 assimilation of blue-green, brown, and red algas by the bacterial 

 method. He even conjectured, though his work does not here 

 afford a sufficient foundation for proof, that assimilatory value 

 A = n E where E is the actual energy of a wave-length and 

 n = co-efficient of absorption of that particular wave-length. 



Reinke (26), 1885, attempted to trace the relation between 

 absorption of the different parts of the spectrum and the 

 destruction (oxidation) of the colouring matter by these parts 

 of the spectrum. He used his well-known spectrophore to 

 correct for unequal dispersion. His figures showed that the 

 amount of destruction of chlorophyll for the different wave- 

 lengths showed the same sequence of effects as the course of 

 absorption of these different wave-lengths by the chlorophyll. 



It seemed to be established by these researches that the 

 effects of the different parts of the spectrum on the chlorophyll, 

 whether in causing assimilation or oxidation, varied in 

 intensity according to the absorption by the chlorophyll of 

 these particular wave-lengths. It followed that rays not 

 absorbed by the chlorophyll did not have any effect on it. 

 When, therefore, Engelmann (6) in 1887 showed by the use of 

 the microspectral-photometer that the course of absorption of 

 the rays of the different wave-lengths in anthocyan was almost 

 exactly complementary to that in chlorophyll, the following 

 argument arose : — If the anthocyan only absorbs those colours 

 which are not absorbed by the chlorophyll and allows the 

 others to pass unchanged, how can it have a screening effect ? 

 To be of any use as a protection against chlorophyll destruc- 

 tion it should absorb some of those rays which would otherwise 

 be absorbed by the chlorophyll, for it is these which produce 

 the destructive effect. ReaUy to protect the chlorophyll. 

 Engelmann pointed out, a green screen would be the best, as it 



