CARBOHYDRATES. 51 



formation of carbohydrates in plants. 1 No synthesis is more wonderful 

 than this. It determines the whole metabolism of the plant; it forms 

 the support upon which rests the whole development or extinction not 

 only of plants but of the animal world. By means of it the great mass of 

 carbon which, in the form of carbon dioxide, is apparently withdrawn 

 from metabolism as the final product in the combustion of organic sub- 

 stances, is carried back to it; thus the sugar synthesis, or what is commonly 

 spoken of as the assimilation of carbonic acid by the organs of the plant 

 containing chlorophyll, forms an important stage in the carbon cycle. 

 The great deposits of coal, and the rocks which underlie the surface of 

 the earth in vast layers, and are to a considerable extent composed of 

 carbonates (principally of calcium and magnesium), belong to this cycle. 

 The coal is formed from plant residue which formerly thrived on the 

 carbon dioxide of the air; and by burning coal, carbon dioxide is again 

 formed, so that the cycle can again take place. Carbon dioxide found 

 combined with bases such as lime and magnesia likewise originated from 

 the atmosphere. It is temporarily removed from the cycle only to return 

 to it when, for example, it is replaced by the action of another acid, such 

 as silicic acid. Oxygen, for the greater part, also makes the cycle with 

 carbon. 



The only important source of the carbon contained in plants is, in fact, 

 as Ingenhousz 2 and then Theodor de Saussure 3 first showed, the carbon 

 dioxide of the air. It is true that it has been observed that roots can 

 take up carbonic acid from carbonates and bicarbonates in the soil, but 

 the amount thus available is very slight. Carbon dioxide is taken up 

 for the most part through the stomata (breathing-pores) of the leaves. 

 The assimilation depends within certain limits upon the amount of car- 

 bon dioxide in the atmosphere, the temperature of the leaf, and the 

 intensity of the illumination. 4 For every temperature there is a definite 

 amount of carbon dioxide assimilation; in general, the optimum lies 

 between 20 and 30 C. 



In spite of numerous studies, it is still a problem as to what is first 

 formed from the carbon dioxide. At present we understand merely the 



1 Concerning the assimilation of carbonic acid, consult the text-books on botany, e.g., 

 W. Pfeffer's " Pflanzenphysiologie, Ein Handbuch der Lehre vom Stoffwechsel und 

 Kraftwechsel in der Pflanze," published by Engelmann in Leipsic. Friedrich Czapek: 

 "Biochemie der Pflanzen," published by Fischer in Jena. 



2 Ingenhousz: "Experiments upon Vegetables," London, 1779, and "Essays on the 

 Foods of Plants and the Renovation of Soils " (1796). 



3 T. de Saussure: "Recherches chimiques sur la ve"ge"tation," Paris, 1804. 

 Edmund O. v. Lippmann: "DieChemie der Zuckerarten." T. Sachs: "Geschichte 

 des Botanik." A. Hansen: "Geschichte der Assimilation." 



4 F. Frost Blackman and Gabrielle L. C. Matthaei: Proc. Roy. Soc. London, 76, 402 

 (1905). 



