President's Address, 341 



is only the excess of oxygen exhaled over what is inhaled. Both 

 processes, that of exhalation and that of inhalation of oxygen, are 

 differently affected by light-intensity and colour, and if they are not 

 distinguished from one another in a research, the determination of 

 the amount of oxygen exhaled fixes only approximately the relation 

 between respiration and assimilation. It is possible in this way to 

 determine only the intensities in which one or other process pre- 

 dominates, but not the amount of increase or decrease of decomposi- 

 tion of carbonic acid. The amount of oxygen exhaled, as has 

 already been pointed out, is not an exact measure of the decomposi- 

 tion of carbonic acid, for an increase in brightness of illumination 

 may bring an increase in decomposition of carbonic acid with an 

 apparent decrease in the amount of oxygen given off, and in this 

 lies the explanation of the smaller amount of gas evolution in direct 

 sunlight than in bright diffuse daylight. 



If now the exact carbon gain be sought instead of the amount of 

 decomposition of carbonic acid in light, the oxygen exhalation 

 again only gives an approximate result so long as the quantitative 

 relation of the carbonic acid expired to the oxygen inhaled in varying 

 intensities is unknown. The question is still further complicated 

 by the unequal possession by the plants of chlorophyll, which must 

 exercise an equally varying influence upon respiration and assimila- 

 tion. 



Secondly, as to the relative energy of the different rays of the 

 spectrum in assimilation. This cannot be determined by the 

 amount of gas given off in the different colours, because the absorp- 

 tion in the chlorophyll colouring matter, according to the screen 

 theory here set forth, must modify the result. All accurate experi- 

 menterSj from Daubeny and Draper to Sachs and Pfeffer, agree in 

 showing that the greatest activity for evolution of oxygen by green 

 tissues resides in the rays of middle refraction in the spectrum. 

 Objections urged against this statement, and these come mainly 

 from physiologists, who adopt a purely physical theory of assimila- 

 tion, are essentially theoretical, based upon the idea that the colour- 

 ing matter is the seat of light activity. Thus,- Lommel considered 

 that light action must be dependent on the degree of completeness 

 of absorption of the rays and on their energy as measured by their 

 heat-eflfect or mechanical intensity, and concluded that the chief 

 activity must lie in the absorption bands of the chlorophyll colour- 

 ing matter, that is, in the red, because the blue, on account of their 

 small mechanical intensity, could have no effect. Experiment does 

 not, however, confirm this idea ; and it were nearer the truth had 

 the seat of activity been looked for in the cell-contents outside the 

 chlorophyll colouring matter. Those physiologists who hold that 



