32 PHYSIOLOGY OF NUTRITION 



According to Krasheninnikov 1 a definite relation holds between the amount 

 of carbon dioxide decomposed and the concomitant increase in dry weight, as is 

 evident from the following average values: for a square meter of leaf surface the 

 amount of carbon dioxide decomposed was 2286 cc. or 4.48 g., while the corre- 

 sponding increase in dry weight was 2.94 g. The increase in dry weight for 

 each weight unit of carbon dioxide decomposed was found to have the values 

 given below, for the different plant forms considered. 



Bamboo 0.60 



Cherry-laurel о . 60 



Sugar cane 0.67 



Linden 0.74 



Tobacco о . 68 



It is seen that this ratio appears to be fairly constant. The formation of a 

 carbohydrate with the composition Ci 2 H 22 0ii (like cane sugar) would give 

 this ratio a value of 0.64. 



Investigations upon the first products of photosynthesis agree with plant 

 analyses in showing that an assimilation of water occurs simultaneously with 

 that of carbon dioxide. In every green plant the formation of organic substance 

 in sunlight is accompanied by assimilation of carbon, hydrogen and oxygen. 

 The bulk of the dry weight of the plant is due to these three elements; this dry 

 weight is made up of about 45 per cent, carbon, 42 per cent, oxygen, 6.5 per cent, 

 hydrogen, 1.5 per cent, nitrogen, and 5 per cent, mineral constituents. Thus 

 plants obtain more than 90 per cent, of their dry weight from the carbon dioxide 

 of the air and the water of the soil. 



§7. Assimilation of Solar Radiant Energy by Green Plants.— We have 

 already seen that green plants are able, with absorption of sunlight, to build up 

 combustible organic compounds out of non-combustible inorganic substances. 

 The chloroplasts of green plants furnish conditions for this process. Animal 

 heat and movement, the heat of fuels, the work of steam engines, are all due to 

 the freeing of the radiant energy of the sun which was previously fixed by the 

 chloroplasts. 



Julius Robert Mayer stated very clearly the role of green plants when he 

 said: 



Nature has set for herself the task of seizing the sunlight in its flight, as it streams upon the 

 earth, and of accumulating the most swiftly moving of all forms of energy by transforming it 

 into a potential state. To accomplish this purpose she has covered the surface of the earth 

 with living organisms that absorb sunlight into themselves and thus generate a permanent 

 store of potential chemical energy. These organisms are plants, and the plant world forms a 

 reservoir in which the fleeting rays of light are caught and cleverly hoarded for future use. 2 



The following interesting anecdote is taken from the biography of the engi- 

 neer Stephenson, and shows that he also was well acquainted with this role 

 played by plants. 



On Sunday as people were returning from church, with Stephenson and Buckland among 



1 Krascheninnikoff, Th., Ansammlung der Sonnenengergie in den Pflanzen. Moskow, 1901. [Russian. ]• 



2 Mayer, Julius Robert, Die Mechanik der Wärme. P. 34. Leipzig, 1911. (Ostwald's Klassiker no. 

 180.) 



