PRINGSHEIM^S RESEARCHES ON CHLoROPII VLL. Ill 



then, even with this uninterrnpteu decomposition of carbonic 

 acid, be quite impossible. The presence of" chlorophyll colour- 

 ing matter changes at once the condition to one favourable 

 for such accumulation ; for the absorption of oxygen increases 

 in the more refrangible part of the spectrum — that part 

 which is especially al)sorbed in the chlorophyll colouring 

 matter — and proportionally too to the intensity of illumina- 

 tion. Even a single layer of chlorophyll-corpuscles in the 

 cell absorbs in diffuse daylight, more or less strongly accord- 

 ing lo the depth of tlieir coloration, all the blue up to the 

 line F (fig. 28, spectrum j3), although in direct sunlight a 

 considerable portion of the blue pass through. The amount 

 of respiration in green tissues must, theiefore, decrease in 

 daylight, in consequence of their colour, and proportionally to 

 its depth; and this, not only on account of a general reduc- 

 tion 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 (fag. £9 cf) sinks in all higher 

 intensities of light below that of assimilation (fig 29 ag)^ for 

 this latter process is but slightly influenced by reduction of 

 light-inteiisity 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 effect in destruction of 

 carbonic acid. In daylight, chlorophyll colouring matter, l)y 

 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 nuitter to plants. 



Previous analytical researches have only slightly insisted 

 upon this increased respiration in daylight. In green organs, 

 as the concurrent assimilation always exceeds, except in the 

 very lowest intensities, the resj>iration, it is necessarily, in 

 light of the intensity of daylight, concealed. In spite of the 

 great oxygen-absorption proceeiling there is observed a con- 

 stant giving off of oxygen only, and iu 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 euiployed. Sometimes, indeed, in direct sunlight 

 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 



