842 Preside?it's Adchess. 



yellow and green rays are more effective in decomposing carb.onic 

 acid than blue and red, rightly enough express the observed facts ; 

 but there remains for explanation the function of the rays so 

 markedly absorbed in the colouring matter. 



The experiments here recorded are not conclusive regarding the 

 effects of colour in the reduction process. Green and yellow are 

 naturally more active than blue, because the latter is absorbed to 

 such an extent by the chlorophyll colouring matter that it is unable 

 to produce an effect, just as in photography the silver salts behind 

 an interposed green glass screen are much less sensitive to blue than 

 to yellow and green light. 



At the present time, then, notwithstanding many accurate re- 

 searches already made, the dependence of the decomposition of 

 carbonic acid upon the wave lengths of the light rays needs elucida- 

 tion. There is no doubt that, for green plants, yellow and green 

 rays are far more active for the evolution of oxygen than blue, yet 

 this is no clue to the dependence of assimilation on colour. This 

 process might equally well be stronger in the blue, for researches 

 say nothing certain on this point. Yet a priori it appears more 

 probable that blue rays have no effect, as the absorption in the 

 chlorophyll colouring matter would be a more significant adaptation 

 for the accumulation of carbon in the plant if the absorption of the 

 blue rays only enfeebled respiration without directly affecting 

 assimilation. 



Another outcome of the absorption in the chlorophyll colouring 

 matter is, that the maximum activity in the spectrum for assimila- 

 tion cannot be the same for all plants and for all brightnesses, but 

 beinf dependent on the depth of colour of the plant and on the 

 total intensity of illumination, its position must change with both 

 conditions. Herein may be found an explanation of the varying 

 determinations which have been given of the course of the assimila- 

 tion, curve in the spectrum. 



Thirdly, the constancy in volume maintained in an atmosphere 

 in which "reen plants are growing has been looked to as affording a 

 clue to the chemical -origin of the carbon compounds produced in 

 assimilation. The primary assimilation products, it has therefrom 

 been assumed, are directly derived from the carbonic acid and water 

 in the reducing process, the carbon of the former combining with 

 the elements of water to form a carbo-hydrate whilst the oxygen is 

 o-iven off. And as anatomical evidence in support of this, the 

 existence of starch in the chlorophyll-corpuscles, as well as the 

 fancied physiological importance and distribution of glucose, have 

 been quoted. The, at present, commonly received theory of assimi- 

 lation then, which considers starch and sugar as the primary products 



