56 LECTURE IV. 



by leaves of the Oleacese, dulcite by those of the Evonymus; and interest- 

 ingly enough it has been found in the case of all the plants examined that 

 the assimilation takes place with those carbohydrates which they contain 

 normally as reserve-substances. 



The fact that the volume of carbon dioxide absorbed is equal to that of 

 the oxygen evolved * has been cited to prove that a carbohydrate must be 

 the first assimilation product. 2 Again, the relations of the heat effect 

 observed coincides very satisfactorily with this assumption. On the other 

 hand, it must be remembered that it has not been possible to carry out 

 exact measurements in this direction. Side by side with the assimilation 

 of carbon dioxide there is a constant absorption of oxygen and production 

 of carbon dioxide. Both processes stand in a certain relation to one 

 another, although the nature of this has never been determined. It has 

 been repeatedly asserted that oils and fats may appear as the first pro- 

 ducts of assimilation. Inclusions of oil have been observed frequently 

 in the chromatophores of certain plants; for example, in the Musacese, 

 Cactacese, alga?, and especially the Vaucheria. 3 By the decomposition of 

 the fat into glycerol and fatty acids, and the simultaneous partial reduc- 

 tion or oxidation of these cleavage products, sugars could be formed from 

 the glycerol, and vegetable acids from the fatty acids. It was soon evi- 

 dent, however, that these fats and oils were not primary products of 

 assimilation, but rather reserve-substances, the formation of which can 

 be readily traced back to carbohydrates. Equally untenable proved 

 Liebig's 4 hypothesis that the vegetable acids were formed as the first 

 assimilation products from which the carbohydrates were obtained 

 secondarily. At present, there are no known observations which are 

 contrary to the assumption that the carbohydrates represent the entrance 

 of the carbon from carbon dioxide into the general metabolism of the plant. 



It is now a question as to how we shall explain this synthesis of carbo- 

 hydrates d-glucose, for example from carbon dioxide and water. We 

 have already mentioned 5 one hypothesis, namely, the assumption of 

 Baeyer 6 that carbon dioxide by reduction is changed into formaldehyde, 

 and by the condensation of the latter a sugar is formed: 



C0 2 + H 2 O 

 Baeyer originally assumed that the carbon dioxide absorbed was first 



1 Boussingault: Compt. rend. 53, 862 (1861), and Holle: Flora, 118 (1877). 



2 This might take place as follows: 6 CO 2 + 6 H 2 O = C 6 H 12 O r> + 6 O 2 . 



3 Paul Fleissig: "Ueber die physiologische Bedeutung der olartigen Einschliisse in 

 der Vaucheria," Inaug. Diss. Basel, 1900. 



4 J. Liebig: Ann. 46, 58 and 66 (1843). 



5 Cf. p. 15. 



6 Ber. 3, 63 (1870). 



