PROGRESS IN PHYSIOLOGY 119 



Progress in Physiological Problems 



Leaving these important investigations for the present, 

 let us collect the various threads of research in other 

 departments, more especially in physiology, as we left 

 them in a previous lecture. 



While Liebig and Boussingault were replacing the 

 " humus theory " with sounder views on the relationship 

 of the soil to the plant, other workers had been trying to 

 solve the even more difficult problem of photosynthesis 

 or, as it was then called, carbon assimilation. As far 

 back as 18 19 Peletier and Caventou had introduced the 

 term " chlorophyll " to indicate the green pigment in 

 plants, as to the significance of which so little was as yet 

 known. The influence of sunhght on the gaseous exchange 

 between the plant and the atmosphere had been studied 

 by the investigators of the concluding years of the 

 eighteenth century and by De Saussure in the beginning 

 of the nineteenth, but in 1834 the chemist Dumas tried 

 to carry the matter a step farther by attempting to 

 differentiate between the effects of the different rays of 

 which white hght is composed, and hazarded the sugges- 

 tion that the more refrangible blue rays — as being those 

 most markedly absorbed by chlorophyll — were those 

 chiefly concerned in the photosynthetic process, an 

 unfortunate guess as it turned out. This new hne of 

 enquiry was the direct outcome of Brewster's discovery 

 that an alcohoHc solution of chlorophyll gave a very 

 characteristic absorption spectrum, and this fact, coupled 

 with Dumas' speculation, led Daubeny, in 1836, to ex- 

 periment on plants grown behind coloured glass screens. 

 He found that little or no assimilation took place in plants 

 exposed to blue and violet light, and expressed the opinion 

 that the most efficacious rays were those in the yellow 

 region of the spectrum. About the same time Dutrochet 

 introduced the method, so frequently employed in later 

 years, of determining the activity of carbon dioxide 



