SIGNIFICANCE OF ASSIMILATION 79 



investigated even the relative amounts of the four con- 

 stituents are nearly constant. 



Chlorophyll absorbs light and, in some way, enables a 

 fraction of the energy thus obtained to be utiUsed in reducing 

 carbon dioxide in solution. There results a simple com- 

 pound, the first stage in the building up of organic matter ; 

 what this first product is we do not certainly know, though 

 the evidence suggests that the classical hypothesis of Bayer, 

 which identifies it with formaldehyde, is correct. What- 

 ever its nature this first product never accumulates, but 

 changes immediately into higher molecular compounds, 

 and gives rise to sugars (glucose, fructose, and saccharose) 

 and to starch. The exact sequence of the formation of 

 these has not been finally determined. It is possible that 

 the synthesis of the nitrogenous organic compounds, 

 culminating in the proteins, also starts with reactions 

 between the earlier, transient products of photosynthesis 

 and the nitrates which are carried to the leaf cells in the 

 transpiration current. The oxygen liberated by the reduc- 

 tion of the carbon dioxide leaves the plant as free gas. 



The whole process is referred to as carbon (or better, 

 carbon dioxide) assimilation, or 2iS photosynthesis . The former 

 term regards the raw material, the latter the end products, 

 but both are in common use for the whole process. 



The importance of photosynthesis for the plant, and for 

 fife in general, needs no emphasis. There are colourless 

 organisms — among the bacteria — which carry on the reduc- 

 tion of carbon dioxide in absence of light, using chemical 

 energy, but their eff"ect is relatively negligible. The organic 

 matter of which the green plant is formed, and from the 

 oxidation of which it derives energy, the organic matter 

 which parasitic plants obtain from green ones, and all the 

 organic food of the animal kingdom is synthesised in the 

 first place from carbon dioxide and water through the action 

 of light on chlorophyll. The major part of organic matter 

 available on the land surface is the product of the flowering 

 plants. Ruskin's " vast family of plants, which, under 

 rain, make the earth green for man, and under sunshine 



