80 ELEMENTARY AGRICULTURAL CHEMISTRY 



transpiration, by which the surplus water taken in ^by the 

 roots is got rid of by evaporation. 



The chemical change so characteristic of plant-life (the 

 absorption of carbon dioxide and the elimination of oxygen), ie 

 an endothermic reaction — i.e., one which requires energy to 

 bring it about. The necessary energy is derived from light, 

 for it is only in the presence of light that the change takes 

 place. The light is absorbed by the green colouring-matter of 

 the leaves, chlorophyll, and it has been shown that it is just 

 that kind of light which is most absorbed by chlorophyll — 

 red light — which produces the largest amount of assimilation 

 of carbon dioxide. The process takes place in spaces below the 

 outer covering (the epidermis) of the leaf, the carbon dioxide 

 entering these spaces by diffusion through the stomata, minute 

 apertures which are exceedingly numerous on all leaves, 

 particularly on their under surfaces. The nature of the change 

 by which carbon dioxide is absorbed and oxygen evolved by 

 green leaves in sunlight is not thoroughly known. It has been 

 suggested that the first step is the formation of formaldehyde, 

 CH,0, by the union of carbon dioxide and water and the 

 evolution of oxygen — 



COa + H,0 = CHp + 0, 



— and that the formaldehyde then immediately polymerises, i.e.; 

 several molecules combine together, to form sugar : 



6CH,0 = C,H„0,. 



However, the matter cannot be said to be definitely settled. 

 It appears probable that in many plants, cane sugar, C^gH^^Oji, 

 is first formed, and when its concentration in the sap attains a 

 certain value starch granules begin to form. The change 

 is empirically a very simple one, but how it occurs is not 

 known : 



Ci,H,,0„ = 2C,H,,0, + H,a 

 Cane sugar Starch 



