ASSIMILATION OF CARBOX 33 



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." 1 



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

 tial energy in the plant. Krasheninnikov 2 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 

 then dried and their heat of combustion was also determined. Below are given 

 the average values of all the determinations, calculated for an area of 1 sq. m. 

 of leaf surface exposed to the light. 



Increase in dry weight 3 . 51 g. 



Increase in carbohydrates 2 .46 g. 



Increase in carbon 1 . 58 g. 



Increase in heat of combustion i5>35° g.-cal. 



Amount of carbon dioxide decomposed 5 . 626 g. 



From the data of this experiment Krasheninnikov calculated that there was 

 an increase of from 2.2 to. 3.6. g.-cal. for each gram of carbon dioxide decom- 

 posed." 



It is also desirable to know what proportion of the radiant energy falling 

 upon the leaf is assimilated. The first calculation bearing upon this question 

 was made by Becquerel, 3 with the following results, which represent the yearly 

 amounts of assimilation for three different types of vegetation, per hectare 

 (2.5 acres). 



1 Mayer, Adolf Eduard, Lehrbuch der Agrikulturchemie. 5 Aufl. Heidelberg, 1001-1902. P. 35. 



; Krascheninnikoff, 1001. [See note 1, p. 32.] 



3 Becquerel, Alexandre E., La lumiere, ses causes et ses effects. Paris, 1 867-1 868. 



u On alterations in the areas of leaves when the latter are transferred from shade to sun- 

 light, which may possibly have some influence on the magnitudes of such values as these, 

 see: Thoday, D., Experimental researches on vegetable assimilation and respiration. V. A 

 critical examination of Sachs' method for using increase of dry weight as a measure of 

 carbon dioxide assimilation in leaves. Proc. Roy. Soc. London B82: 1-55. 1909. — Ed. 

 3 



