322 Biological Chemistry. 



The demonstration of the starch formation can be 

 made by partly covering a leaf with tinfoil and exposing 

 it to the light; it is then decolorized by treatment with 

 alcohol and placed in a solution of iodine in caustic potash 

 solution. The parts of the leaf which were covered with 

 the tinfoil will be coloured brown, whereas the parts 

 exposed to light will assume a blue colour due to the 

 presence of the starch. 



A series of experiments was carried out by Timiriazev 

 with the object of determining which rays were the most 

 active in causing the assimilation of carbon dioxide by the 

 plant. It was found, when leaves were exposed to different 

 parts of the spectrum, that the greatest amount of activity 

 took place in the red region, between the Fraunhofer 

 lines B and C, with a second region of maximal activity 

 in the blue region at F. These are precisely the kinds 

 of light absorbed by chlorophyll solutions, which show 

 absorption bands in these parts of the spectrum. The 

 chlorophyll appears therefore to absorb the light, which 

 forms the source of energy for the building up of the 

 complex substances in the plant. The absorption of light 

 for the purposes of synthesis is probably not the only 

 function of the chlorophyll. Recent researches of Will- 

 statter and his pupils have shown that no less than four 

 pigments are contained in the crude chlorophyll solutions 

 which are held in certain rounded bodies in the leaf known 

 as chloroplasts. These are carotin, C 40 H 56 ; xanthophyll, 

 C 40 H 56 O 2 ; chlorophyll a, C 55 H 72 O 5 N 4 Mg ; and chlorophyll 6, 

 C 55 H 70 O 6 N 4 Mg. The exact functions of these four pigments 

 have not yet been determined. 



The method of the formation of starch from carbon 

 dioxide and water vapour may be formulated in the 

 following way : 



+ 5H 2 0) = (C 6 H 10 5 ) n 



