260 PHOTOSYNTHESIS 



garded that this reaction took place in a series of steps in which the com- 

 pounds formed showed a gradual decrease in the amount of oxygen they 

 contain and an increase in the amount of hydrogen. This was, of course, 

 an hypothesis drawn in very general terms. Liebig, it will be recalled, 

 had devoted much attention to the elementary analysis of a large number 

 of organic compounds by means of combustion. He now emphasized that 

 the synthesis of carbohydrates from carbon dioxide and water was a step- 

 reaction, starting with a reduction of carbonic acid to oxalic acid, then 

 to tartaric, malic and other aliphatic carboxylic acids. Liebig's conclusions 

 as to which acids thus formed the theoretical intermediate steps were ar- 

 rived at entirely from a consideration of their composition as to C : H : O. 



Liebig sought substantiation of his theory in the fact that in ripening 

 fruits the acidity decreases and the carbohydrate content increases. Un- 

 fortunately, Liebig had entirely misconstrued the phenomenon of oxidative 

 respiration in plants. As a consequence he also misinterpreted the pres- 

 ence of organic acids in fruits and fleshy plants, and considered that these 

 acids could disappear from the plant only through reduction. 



It was established through a number of extensive investigations ^ that 

 although decrease in the acid content of succulents was accompanied by 

 increased carbohydrate content, this was not a direct conversion of or- 

 ganic acids into carbohydrates. Rather, the carboxylic acids are broken 

 down by a process of photolysis with the liberation of carbon dioxide and 

 the latter is used in photosynthesis to form carbohydrates. The acids 

 accumulate in the night and disappear during the day. On account of the 

 complex morphological conditions in succulent plants it required consider- 

 able time to arrive at this conclusion. At the same time the fact was 

 established that similar conditions, only to a lesser degree, prevail in most 

 leaves. The accumulation of acids is apparently the result of incomplete 

 oxidation of carbohydrates; in the light the acids disappear due partly to 

 the better supply of oxygen furnished by photosynthesis and partly due 

 to a direct photolysis of the acids. 



The principal idea of Liebig's theory, that the organic acids are inter- 

 mediate products in the synthesis of carbohydrates has in the course of 

 years reappeared periodically with slight modification of Liebig's original 

 ideas. At this time may be mentioned the theories with formic acid as a 

 first reduction product. An early suggestion of this nature was made by 

 Rochleder,^ though the limited chemical knowledge of the time did not 

 permit a very clear statement. This scheme is as follows : carbon dioxide 



^ De Saussure, "Recherches chimiques stir la vegetation," 1804, p. 64. Mayer, 

 Landuf. Versuchstat., 18, 428 (1875); 21, 277 (1878); 30, 217 (1884); 34, 127 

 (1887). Kraus, Ahh. Natur. Ges. Halle, 16, 143 (1883). De Vries, Bot. Zeitg., 42, 

 336 (1884). Warburg, Untcrs. hot. Inst. Tubingen, 2, 53 (1886). Aubert, Rev. gen. 

 Bot., 2, 369 (1890); 4. 203 (1892); Ann. Sci. Nat. Bot. (7), 16, 1 (1892). Pur- 

 jewiecz, Bot. Cent., 58, 368 (1894). Gerber, Ann. Sci. nat. Bot. (8), 4, 153 (1896). 

 Astruc, Ann. Sci. nat. Bot. (8), 17, 1 (1903). Spoehr, Biochem. Zeit., 57, 93 (1913). 

 Richards, Carnegie Inst. Washington, Pub. No. 209 (1915). Hempel, Compt. rend, 

 trav. Lab. Carlsberg, 13, 1 (1917). 



'Rochleder, "Phytochemie" (1854), p. 321. 



