ON VITAL PRODUCTION 83 



in the dark for some time, while within a few minutes of its 

 exposure to sunlight starch accumulates in the cells. 



It is difl&cult to convince the non-chemical reader what a very- 

 extraordinary thing this process of photo-synthesis of starch by 

 the green plants must be. Let him note that on the one hand 

 there is water and carbonic acid, and on the other there is dex- 

 trose and finally starch. The chemical equation is (probably) : 



6CO2 + 6H2O = C'cHiaOe + 6O2. 

 Carbonic acid Water Dextrose Oxygen 



Then the dextrose is converted into starch (CeHioOg)^, and the 

 latter gathers in the cells of the leaf till it is required by the 

 plant, when it passes into solution as dextrose, and is removed 

 from the leaf by the circulating sap juices. This is the process 

 of photo-synthesis. Look, however, at the first equation and 

 read it from right to left. There is dextrose and oxygen. 

 Dextrose is a highly combustible substance, and when it burns 

 it combines with oxygen to form CO2 and OHg, with the libera- 

 tion of a large quantity of energy in the form of heat. What 

 actually occurs in the plant, however, is represented by reading 

 the equation from left to right, and this shows that when the 

 CO2 and OH2 are synthesised in the plant to form dextrose, the 

 same quantity of energy must be absorbed as is liberated when 

 dextrose burns in oxygen to form COg and OH2. 



The former kind of reaction — an endothermic one — can be 

 made to occur. A chemist can decompose CO2 into C and 0, as, 

 for instance, when a burning piece of magnesium ribbon is placed 

 in a jar containing CO2. But the CO2 is only decomposed into 

 carbon and oxygen if a large quantity of energy is supplied in 

 the form of the heat liberated by the burning magnesium, and 

 so the decomposition only occurs at a relatively high tempera- 

 ture. The following statement is very important in all such 

 discussions as these: reactions like that in which CO2 is decom- 

 posed into C and O2 only occur when a compensatory energy 

 transformation is brought about — that is, when at least as much 

 energy is supplied from outside as would be yielded if the reaction 

 went in the opposite direction. Such reactions — an endothermic 

 process coupled with a compensatory one — do not occur of 

 themselves. When they do occur, the coupling is the consequence 

 of some outside agency. 



In the green plant, however, it is precisely this coupled reaction 



