54 LECTURE IV. 



different conjectures have been made, and the same is true concerning 

 the action of chlorophyll. Some conceive it to have the action of a fer- 

 ment, 1 others consider it as closely related to the first assimilation product 

 assuming that it combines with the carbonic acid. 



As long as we know practically nothing concerning the formation of 

 chlorophyll, and still less about the plasma of the plant cell itself, and as 

 long as we are unable to isolate satisfactorily any of the first products of 

 the assimilation, the discussion of all such possibilities has only a relative 

 value. For the present we can restrict ourselves to those hypotheses 

 which seem plausible from a chemical standpoint. At the same time it 

 does not follow necessarily that the plant organism carries out its syntheses 

 in such a way, nor that the intermediate products which we assume are 

 actually formed in the living plant cell. Above all, the impression should 

 not be left that the assimilation of carbonic acid can take place only in one 

 direction. It is indeed possible, and in fact probable, that other primary 

 products of assimilation exist. It is undoubtedly certain that one of the 

 first products of the assimilation is a carbohydrate, whether grape-sugar, 

 starch, or perhaps a simpler sugar with fewer than six carbon atoms in the 

 molecule. At all events, and this is of greatest importance for under- 

 standing the whole metabolism of plants, and indirectly that of the animal 

 organisms, the carbohydrates stand at the center of all the synthetical 

 processes taking place in the plant organism. We shall see later on that 

 the fats, the albumins, and many other highly complicated compounds, 

 evidently originate from carbohydrates, whether by constructive or 

 destructive processes, or both acting together. 



In discussing the first synthesis produced in the plant cell we meet with 

 a highly important problem which we have already briefly touched upon. 2 

 All the carbon compounds which are either directly or indirectly produced 

 by animal or plant organisms, are optically active; that is to say, they 

 possess at least one asymmetric carbon atom. As we have already seen, 

 most of these compounds are found in nature, only in the optically- 

 active state. Now, the plant cell in carbon dioxide receives a carbon 

 compound, which does not contain an asymmetric carbon atom. The 



1 Jean Friedel (Compt. rend. 132, 1138 (1901)) has tried to support this view by 

 experiments. He brought together the glycerol extract from fresh leaves with finely 

 powdered leaves which had been rapidly and carefully dried, and found oxygen was 

 evolved by the action of light, and carbonic acid taken up. The experiments, however, 

 are not conclusive, and have not been corroborated. Cf. M. Harroy: Compt. rend. 138, 

 890 (1901). R. O. Herzog: Z. physiol. Chem. 35, 459 (1902). Again, Bach and Chodat 

 have published a great deal of work. summarized in Biochem. Zentrbl. 1, 11, 417, and 

 I, 12, 457 (1903). 



The relations are so complicated that it is hard to draw a conclusion at the present 

 time concerning the work of these scientists. 



2 Cf. p. 15. 



