398 RADIATION BIOLOGY 



to all the other terms used in this connection. The precursor is a colored 

 compound; therefore the name "leukophyll" is a misnomer. "Chloro- 

 phor" connotes a chlorophyll-carrying substance rather than a direct 

 pigmented precursor. The carotenoid pigments cannot be, from what 

 we now know, the direct and complete progenitor of chlorophyll. Proto- 

 phyllin turns green in the presence of air and the absence of light and 

 does not possess spectroscopic properties that agree with those of the 

 constituent of etiolated leaves which forms chlorophyll. " Chlorophyllin 

 5" is a term that assumes the precursor to be a chlorophyll (chlorin 

 structure), which from chemical evidence appears to be erroneous. The 

 terms "protochlorophyll" and "chlorophyllogen," as they have been 

 applied, connote properties and reactions that agree well with the proper- 

 ties and reactions of the chlorophyll precursor as we know it today. It 

 might be more precise to replace the term "protochlorophyll" with 

 "protochlorophyll a," but to postulate a perfected precursor for chloro- 

 phyll b in etiolated leaves goes beyond and even contradicts the evidence 

 we have at present. 



A distinction between the terms "protochlorophyll" and "chloro- 

 phyllogen" is called for. These two terms are not synonymous, as 

 Lubimenko assumed, but they bear a close relation to each other, the 

 same relation that extracted chlorophyll bears to chlorophyll in the leaf 

 (cf. Lubimenko, 1928, pp. 89-90). Our present knowledge concerning 

 chlorophyll indicates that its state in the chloroplast is different from 

 what it is after extraction with organic solvents. To chlorophyll in its 

 natural state various names have been applied, e.g., "chloroplastin," 

 "photosynthin," "phyllochlorin" (cf. Mackinney, 1940). The concept 

 common to all these terms is that chlorophyll in the plant is combined 

 with some carrier — this is the complete natural pigment. By extraction 

 with organic solvents, chlorophyll is dissociated from the carrier and 

 becomes dispersed in true solution. The same difference applies to proto- 

 chlorophyll and chlorophyllogen: protochlorophyll is the extracted pig- 

 ment; chlorophyllogen is the pigment combined with a carrier in the plant 

 material. 



It is evident that a term is needed to designate the complete pigment 

 as it exists in its natural state and without prejudice as to the nature of 

 its combination or function. For this concept we suggest the general 

 term "holochrome''^ (Gr. holos, whole, -\- chroma, color). Such a term 

 has analogous usage in the terms "holoenzyme," "holocellulose," "holo- 

 phytic," and "holozoic." The adjective would be "holochromatic." 

 Thus we have chlorophyll holochrome to designate chlorophyll in its 

 natural state and protochlorophyll holochrome to designate protochloro- 

 phyll in its natural state — and this is chlorophyllogen. 



1 The authors are indebted to Prof. Raymond D. Harriman, Department of Clas- 

 sics, Stanford University, for his helpful advice in selecting the terms "holochrome" 

 and "holochromatic." 



