CHLOROPHYLL FORMATION 433 



cursor than to assume that chlorophyll b is formed from chlorophyll a 

 [Smith (1949b); cf. Seybold (1942); for criticism of this hypothesis see 

 Aroiiotf (1950)]. 



To account for the formation of the two chlorophylls, Seybold (1948- 

 1949) has suggested the possible participation of two protochlorophylls, 

 protochlorophylls a and b. It is entirely possible that protochlorophyll b 

 is formed prior to chlorophyll b, but so far no evidence for the existence 

 of the b component in dark-grown unilluminated seedlings of the angio- 

 sperms has been presented, and its absence is attested by the failure of 

 chlorophyll b to be formed in the initial stages of illumination of etio- 

 lated leaves. 



The greening of conifer seedlings in the dark has been ascribed to the 

 action of a "chlorophyll agent." Although it is reasonable to hypothe- 

 size that this substance is formed in the endosperms of germinating seed- 

 lings and diffuses into the embryos, thereby causing them to green, it is 

 not excluded that, at the surface of contact between the two organs, a 

 substance is formed which diffuses into the embryo and causes it to green. 



But illumination of the extirpated embryo also brings about greening. 

 The question then arises whether illumination produces a "chlorophyll 

 agent" and thereby indirectly causes greening or whether light acts 

 directly on the chlorophyll precursor to cause greening. Since the coni- 

 fer seedlings do not continue to green when returned to darkness and 

 since the effect of light is distinctly localized in the angiosperms, it seems 

 very doubtful that light acts indirectly in greening by producing a 

 "chlorophyll agent." It is much more Ukely that light acts directly on 

 the chlorophyll precursor. 



There seems to be no essential difference in the chlorophylls formed 

 or in the nature of the transformations accomplished in the dark and in 

 the light, because in the conifer seedlings protochlorophyll, chlorophyll a, 

 and chlorophyll b have all been identified. 



Now we come to the precursors of protochlorophyll. Little is actually 

 known of these in the direct series of reactions leading to protochlorophyll. 



Only a limited quantity of protochlorophyll is accumulated in dark- 

 grown leaves before they are illuminated and in leaves that are returned 

 to the dark after they have been illuminated for a brief period. Noack 

 (1934) has suggested that this limit results from an equilibrium or steady- 

 state condition between protochlorophyll and its precursors; that by a 

 short irradiation of the leaves protochlorophyll is removed to form chloro- 

 phyll; and that, when the leaves are returned to the dark, the equilibrium 

 condition is restored by the formation of more protochlorophyll from its 

 precursors. 



An alternative possibility also exists: that illumination of the dark- 

 grown leaves, besides converting protochlorophyll to chlorophyll, simul- 

 taneously produces a precursor of protochlorophyll which is transformed 



