404 THE PIGMENT SYSTEM CHAP. 15 



is, nevertheless, correct, and if so, whether this effect is related to, or 



independent of, the sun or shade character of the plants, remains to be 



seen. 



The hypothesis of Smith (c/. page 389) that the two chlorophylls are 



parts of a stoichiometric complex, in which three molecules of chlorophyll 



a are associated with one molecule of chlorophyll b, finds no support in 



analytical data. 



2. Protochlorophyll 



Seeds and etiolated plantules (i. e. seedlings of the higher plants 

 sprouted in darkness) are sometimes faintly green, although they contain 

 no chlorophyll. They begin to turn green immediately upon exposure 

 to light; and it has been assumed that the pale green substance contained 

 in them is a "chlorophyll precursor" capable of rapid conversion into 

 chlorophyll. Monteverde (1893) prepared alcoholic extracts of this 

 compound and called it protochlorophyll. The best known source of 

 protochlorophyll is pumpkin seeds. Noack and Kiessling (1929, 1930, 

 1931) proved its chemical similarity to chlorophyll (the presence of 

 magnesium and phytol). Seybold (1937) obtained from squash seeds 

 two chromatographic fractions, one bluish-green and one pure green, 

 and he interpreted these as the protochlorophylls a and h. Fischer, 

 Mittenzwei, and Oestreicher (1939) prepared, by partial synthesis, a 

 crystaUized compound which proved to be identical with a derivative 

 of natural protochlorophyll. Fischer was thus able to identify proto- 

 chlorophyll as an oxidation product of chlorophyll differing from it only 

 by two hydrogen atoms (c/. page 445). 



This result is significant for speculations as to the role of protochloro- 

 phyll in nature. Preisser assumed, as early as 1844, that chlorophyll is 

 formed in young plants by the oxidation of a precursor; and this concept 

 has been further developed by Monteverde and Lubimenko (1911, 1913) 

 and Lubimenko ( 1927, 1928) . The realization that protochlorophyll is an 

 oxidation product of chlorophyll eliminates it, according to this theory, 

 as a chlorophyll precursor. Lubimenko had postulated, as early as 1928, 

 that protochlorophyll is not a precursor of chlorophyll, but a by-product 

 of its synthesis, for which he suggested the following scheme: 



higher plants 



^protochlorophyll 



(dark) 



higher plants Gight) 



^chlorophyll 



lower plants 

 (dark or light*) 



According to this scheme, protochlorophyll is formed only in the dark, 

 when chlorophyllogen cannot be converted into chlorophyll. 



* See page 430. 



