396 RADIATION BIOLOGY 



long periods of illumination is characteristic of the chlorophyll-forming 

 process. 



Short periods of illumination produce only chlorophyll a, but con- 

 tinued irradiation produces chlorophyll b as well. Once chlorophyll b 

 has appeared, the two chlorophylls increase in constant proportion to 

 one another (see Fig. 7-18). 



The presence in dark-grown leaves of a pigment, protochlorophyll, 

 whose rapid disappearance on illumination is accompanied by the simul- 

 taneous appearance of chlorophyll, suggests that this pigment is the 

 immediate chlorophyll precursor, about the reality of which there has 

 been much controversy. 



In contrast to the angiosperms, which form chlorophyll only in the 

 light, many other classes of plants form chlorophyll without light. When 

 cultivated in the dark, many algae grow and retain their greenness, and 

 some ferns put out new fronds that are green. But perhaps the most 

 interesting plants possessing this property are the conifers, which, when 

 grown from seed in the dark, produce seedlings with intensely green 

 cotyledons. 



Different groups of plants produce different chlorophylls and different 

 relative proportions of the chlorophylls. There are genetic mutants that 

 contain no chlorophyll, only a little chlorophyll, chlorophyll distributed 

 in bizarre patterns, or chlorophyll-like compounds but no chlorophyll. 

 An understanding of the causes of these variations would go far toward 

 clarifying the mechanisms of chlorophyll formation and chlorophyll 

 accumulation. 



But analysis of all these aspects of chlorophyll formation and accumu- 

 lation is beyond the scope of this review even if sufficient facts were avail- 

 able for such an analysis. Therefore only such selected aspects will be 

 presented as will document and summarize the present concepts concern- 

 ing the biosynthesis and accumulation of chlorophyll. 



2. PRECURSORS OF CHLOROPHYLL 



Inasmuch as we are endeavoring to understand the path of hio.sy a thesis 

 of chlorophyll, our attention naturally turns to the precursors of chloro- 

 phyll, especially to the immediate precursor of chlorophyll. The exist- 

 ence of such a specific precursor was postulated by Sachs as early as 1859. 



Ideas concerning the formation of chlorophyll have centered around 

 the concepts concerning the genesis, nature, and transformations of the 

 chlorophyll precursor. These ideas have taken many forms and have 

 been epitomized in the names given to this substance. The chief names 

 applied to this precursor, along with a concise statement of the properties 

 implied by each name, are presented here. 



1. Leukophyll. Sachs (1859) applied the name "leukophyll" to what 



