THE CHEMISTRY OF PHOTOSYNTHESIS 289 



This pigment acts as a photosensitizer of the substance A which prob- 

 ably has its maximum absorption in the ultra-violet. Wurmser's theory- 

 is as yet incompletely founded, but may prove of considerable value. 

 In this theory the Blackman reaction would probably be reaction II, which 

 is a dark reaction and one in which oxygen apparently plays a role. 



Ostwald ^^ has formulated a theory which has a certain similarity to 

 that of W'urmser. Ostwald assumes that the primary step in photosyn- 

 thesis consists in the formation by oxidation of a lipoid peroxide. How- 

 ever, the theory^ contains many assumptions which have little or no evi- 

 dence for support. 



Baudisch ^^ has also made some interesting theoretical deductions re- 

 lating to the mechanism of photosynthesis, more particularly in regard 

 to the synthesis of amino acids. 



The theories relative to the function of chlorophyll in the photosyn- 

 thetic process are discussed in Chapter 7. 



2. Attempts To Find in the Plant Substances Which Form the 

 Intermediate Steps Demanded by the Theories 



A very natural sequel to the construction of theories of the mechanism 

 of photosynthesis has been the desire to obtain substantiation of these 

 by finding in the plant the substances which, according to the theories, 

 are the intermediate products between carbon dioxide and carbohydrates. 

 A great many investigations have been carried out from this viewpoint. 

 Most of these are concerned with the detection of formaldehyde as a 

 means of testing the Baeyer hypothesis. We shall not discuss these in- 

 vestigations in detail ; in many of them color reactions were employed 

 as final tests for the presence or absence of formaldehyde, tests which 

 are of very questionable value. The entire method is, in fact, of very- 

 limited significance for establishing the validity of any theory. Many of 

 the older reports have been shown to be of no value on account of the 

 fact that the chemical tests employed were later shown to be not spe- 

 cific for formaldehyde or had been improperly employed. 



a. Formaldehyde. 



The results of the earlier attempts to detect formaldehyde in illumi- 

 nated leaves have been subjected to criticism by Curtius and Franzen,^° 

 Czapek ^^ and Euler ^- and were found to be unreliable. The aldehyde 

 reaction which had been obtained when the expressed juice or extracts 

 of leaves were distilled could not be ascribed to formaldehyde, but un- 



^ Ostwald. Koll. Zcit., 33, 356 (1923). 



"^Baudisch, Science. 58, 451 (1923) ; Bcr. client. Ges., 44, 1009 (1911) ; 49. 1159, 

 1176 (1916); 50, 652 (1917). 

 . '"Curtius and Franzen, Ber. chem. Ges., 45, 1715 (1912). 



"Czapek, Bot. Zeitq., 58, 153 (1900). 



''Euler, Ber. Chem. Ges., 37, 3411 (1904). Bokorny, Chem. Zeitq., 33, 1141, 

 1150 (1909). Plancher and Ravenna, Atti. Accad. Lincei [5], 13, 459 (1904). 



