Ch. hi, 3] 



SYXTHESIS OP FOOD 



21 



fore the sugar, increases in quantity under action of light. 

 Indeed so exactly quantitative is this relation of light to 

 starch-formation that, v^ith certain practical precautions, 

 one may apph' a photographic negative to a leaf, and after 

 exposure to light develop a very fair positive "blue-print" 

 of the picture ^Yith iocUne. 



The increase of the grape sugar in light is found bj' ex- 

 periment to add weight to the 

 plant. Therefore the sugar must 

 represent not a transformation of 

 material already present, but a new 

 construction out of materials dra-wn 

 from outside the plant ; and all 

 research confirms this conclusion. 

 Further, suitable tests always show 

 that its formation takes place only 

 in light and only in green tissues, 

 which never occur away from the 

 Ught. Its production indeed is the 

 particular primary function of the 

 chlorenchyma, wherever found, 

 whether in leaves, stems, or other 

 parts, — the leaves being organs 

 adapted to spread chlorenchyma to 

 light. The formation of the sugar 

 being thus a process of synthesis 

 under action of hght, is knowTi as 



PHOTOSYNTHESIS. 



What now are the materials from which the grape sugar 

 is constructed ? 



The chemical formula of grape sugar is CeHioOe, which 

 means of course that its molecule is composed of six 

 atoms of carbon, twelve of hydrogen, and six of oxygen. 

 Now the proportions HioOe in this formula recall the 

 familiar HoO, suggesting that water may be the source 

 of that part of the sugar, at least of its hj^drogen ; and 



Fig. 4, — A leaf treated 

 ■ttdth iodine after cxpo-sure to 

 light under the screen of Fig. 

 3; Xi- The black shading 

 represents dark blue in the 

 actual leaf. 



