LIGHT AND VEGETATION 89 



contains a very small proportion — 1 litre of carbon dioxide 

 in 3,300 litres of air. 



The following table gives the composition of dry air at 

 ground level: 



Thus argon, although it is classed among the rare atmo- 

 spheric gasses, is 31 times more abundant than carbon dioxide, 

 but it has no chemical activity. 



Most plants spreading their leaves over a considerable 

 area are well adapted by their shape to capture a sufficient 

 quantity of carbon dioxide and to absorb the luminous 

 energy which they need for many purposes, and particularly 

 for photosynthesis. 



Without making any hypothesis on the process of photo- 

 synthesis or describing any of the certainly complex inter- 

 mediate reactions, we can write the overall result in the form 

 of a very simple chemical equation. The products of photo- 

 synthesis — starch, sugars, etc. — all have a composition 

 expressed by the formula CHgO. Assimilation through 

 chlorophyll is therefore represented by: 



carbon dioxide + water — >■ carbohydrates + oxygen 

 CO2 +H2O — > CH2O + O2 



Thermochemical data show that such a reaction is 

 endothermic, which means that it cannot take place without 

 energy. C represents 12 grammes of carbon, H 1 gramme of 

 hydrogen, 16 grammes of oxygen, and the energy necessary 

 to produce the quantities shown in ^the formula is 1 12,000 

 calories (equal to 460,000 joules, or 128 watt-hours). 



The plant draws this energy from the Ught absorbed and 



