LIFE AND LIGHT 23 



is burnt and so completely oxidised in the air, is present only to the 

 extent of between three and four parts in ten thousand of the air. 

 The living cell takes up all its materials in solution, and if we regard 

 the carbon dioxide as in solution in the atmosphere we gain a clear 

 idea of the enormous dilution at which plants take this their most 

 important raw material. 



Since the molecular weight in grammes of any gas occupies at 

 atmospheric pressure and standard temperature a volume of about 

 22 litres, and air contains 4 litres of carbon dioxide in 10,000, it 

 follows that 22 litres, or the molecular weight in grammes, will be 



22 



contained in X 10,000 =55,000 litres. That is to say, the atmo- 

 sphere is approximately a 55 ,J 00 molecular solution, and at this 

 exceedingly dilute strength plants take in all their supply of carbon. 



The supply of carbon dioxide is taken in only by the green parts 

 of the plants, especially the leaves, and by these it is taken up with 

 the greatest avidity for it has been shown that a 7-5 per cent, solu- 

 tion of sodium hydrate only absorbs carbon dioxide at about five 

 to six times the rate of an equal surface of green plant leaf in the 

 sunlight. Brown and Escombe found that the comparative rates 

 of absorption of strong alkali and leaf were less than double. 



In the absence of light this rapid absorption abruptly ceases, 

 and instead there is given out an excess of carbon dioxide arising 

 from the oxidation processes of life going on in the -living cells of 

 the plant. 



It is only when the energy of the light at sufficient intensity 

 (which here corresponds to the potential or pressure factor of this 

 form of energy) is poured upon or into the energy machines existing 

 in the green cells that chemical energy begins to be formed by trans- 

 formation from the energy of the light, and this process must not 

 by any means be confused with the output of carbon dioxide due to 

 respiration of the plant, which goes on continuously and all over the 

 plant whether it is in sunlight or in darkness. 



In the leaf, in sunlight, the synthetic processes mightily out- 

 balance the respiratory or oxidation processes, and the result is 

 rapid disappearance of carbon dioxide with simultaneous appearance 

 of oxygen in equal or somewhat greater volume. 1 In feeble light 



1 The reverse of what is called the animal respiratory quotient occurs 

 in plant syntheses, in that on the whole more oxygen is produced than carbon 

 dioxide taken up, the excess oxygen coming from water. In the green leaf 



