ASSIMILATION OF CARBON 33 



§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. 1 



The following interesting anecdote is taken from the biography of the engineer 

 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 

 them, the whole company stopped upon the terrace beside Drayton Castle to watch a railway 

 train as it vanished rapidly in the distance, with a trail of white smoke behind it. 



"Well, Buckland," said Stephenson as he turned to the famous geologist, "Answer me a 

 question, not a very easy one, perhaps. Can you tell me what sort of force it is that drives 

 yonder train along?" 



"Well," answered the geologist, "I should think that the force was one of your great 

 engines." 



"Yes but what moves the engine?" 



"Why, one of your Newcastle engineers, of course." 



"No, sunlight." 



"How can that be?" asked the doctor. 



"I assure you it is nothing else," replied the engineer. "It is light that has lain stored in 

 the earth for many thousands of years; the light absorbed by the plant during its growth is 

 essential for the condensation of carbon, and this light, which has been buried in the coal 

 measures for so many years, is now unearthed and, being freed again as in this locomotive, 

 serves great human ends." 2 



Along with the accumulation of starch there occurs also a storage of poten- 

 tial energy in the plant. Krasheninnikov 3 was able to demonstrate this rela- 

 tion by direct experiment. Half-leaves were removed from the plant and their 

 areas were measured, after which they were dried and burned, to determine the 

 heat of combustion of their dry substance. The remaining half-leaves, also 

 removed from the plant but still alive, were exposed to light for a time, and the 

 amount of carbon dioxide which they decomposed was measured. They were 



1 Mayer, Julius Robert, Die Mechanik der Warme. P. 34. Leipzig, 191 1. (Ostwald's Klassiker no. 

 180.) 



2 Mayer, Adolf Eduard, Lehrbuch der Agrikulturchemie. 5 Aufl. Heidelberg, 1901-1902. P. 35. 



3 Krascheninnikoff, 1901. [See note 4, p. 32.] 



3 



