336 President's Address. 



especially in tlicj brighter light, greatly exceed in amount the 

 assimilation. An accumulation of carbon would then, even with this 

 uninterrupted decomposition of carbonic acid, be quite impossible. 

 The presence of chlorophyll colouring matter changes at once the 

 condition to one favourable for such accumulation ; for the absorp- 

 tion of oxygen increases in the more refrangible part of the spec- 

 trum — that part which is especially absorbed in the chlorophyll 

 colouring matter — and proportionally too to the intensity of illu- 

 mination. Even a single layer of chlorophyll-corpuscles in the cell 

 absorbs in diffuse daylight, more or less strongly according to the depth 

 of their coloration, all the blue up to the line F, although in direct 

 sunlight a considerable portion of the blue pass through. The 

 amount of respiration in green tissues must, therefore, decrease in 

 daylight, in consequence of their colour, and proportionally to its 

 depth ; and this, not only on account of a general reduction of 

 illuminating power effected upon the whole spectrum, but specially 

 through the selective absorption of the rays most refrangible and 

 most active in respiration, which is characteristic of the chlorophyll 

 colouring matter. In this way the respiration curve sinks in all 

 higher intensities of light below that of assimilation, for this latter 

 process is but slightly influenced by reduction of light-intensity 

 through the colouring matter, because it already has nearly reached 

 its maximum in daylight of medium intensity, and also because the 

 blue rays absorbed in the colouring matter are of less efiect in 

 destruction of carbonic acid. In daylight, chlorophyll colouring 

 matter, by reducing the amount of respiration, allows assimilation 

 to surpass it in amount, and thus enables an accumulation of 

 carbon compounds to take place ; and in thus diminishing the 

 respiration of green tissues in light lies the value of green colouring 

 matter to plants. 



Previous analytical researches have only slightly insisted upon 

 this increased respiration in daylight. In green organs, as the con- 

 current assimilation always exceeds, except in the very lowest 

 intensities, the respiration, it is necessarily, in light of the intensity 

 of daylight, concealed. In spite of the great oxygen absorption 

 proceeding there is observed a constant giving off of oxygen only, 

 and in order to make the increase of respiratory action evident by 

 the accumulation of carbonic acid resulting from it, assimilation 

 must either be suppressed or light (direct sunlight) of greater 

 intensity must be employed. Sometimes, indeed, in direct sun- 

 light the increment of respiration may be recognised, not by the 

 accumulation of carbonic acid, but by the lessening of amount of 

 the oxygen given off. Often observed, this fact has been mis- 

 interpreted. Famintzin, for example, took this as proof of the 



