1018 BIOLOGICAL EFFECTS OF RADIATION 



of the cell. It is, therefore, primarily the quantity of oxidizing enzymes 

 and the intensity of oxidations which influence directly the accumulation 

 of chlorophyll in the plastids. 



The precursors of chlorophyll are considered to be colorless substances 

 which in a series of steps are converted, apparently through oxidation, 

 to the colored compounds. Unfortunately considerable confusion exists 

 concerning the nomenclature of these hypothetical precursors of chloro- 

 phyll and clarification of the subject can probably not be attained until 

 intensive chemical investigations on these substances have been com- 

 pleted comparable to those which have been made on chlorophyll itself. 

 Some important results in this field have already been obtained by 

 Noack and Kiessling (85). Lubimenko considers that chlorophyll arises 

 in several steps: through catalytic oxidation of the colorless leucophyll 

 there is formed chlorophyllogen, a pigment having an absorption spec- 

 trum resembling that of chlorophyll. In some of the algae, the young 

 sporelings of ferns and bryophytes, and in some of the conifers chlorophyl- 

 logen is converted into chlorophyll without the action of light. Seedlings 

 of angiosperms, in which the system of nourishing the embryo is more 

 complex, require light for the formation of chlorophyll from chlorophyl- 

 logen. This reaction, according to Lubimenko, is most rapid in the red 

 end of the spectrum, the blue rays are next in activity, while the green 

 rays have little effect. Under the influence of various physical and 

 chemical agents the cell proteins are precipitated and chlorophyllogen 

 gives rise to another pigment, protochlorophyll, having an absorption 

 spectrum in which the bands are displaced toward the violet as compared 

 with chlorophyll. From the experiments of Lubimenko it seems probable 

 that light also influences the enzymatic reactions which give rise to the 

 formation of leucophyll and that light also induces the decomposition of 

 chlorophyll already accumulated. As a result of this extremely complex 

 situation it is not surprising that it has been difficult to obtain concordant 

 opinions, that every species seems to possess a different optimum light 

 intensity for chlorophyll formation, and that this optimum varies with 

 the age and previous treatment of the plant. 



While Lubimenko and also Liro consider that protochlorophyll is of 

 the nature of a postmortal product of a chlorophyll precursor, Noack (85) 

 regards protochlorophyll as the immediate precursor of chlorophyll. 

 Lubimenko bases his conclusion on the fact that he could not identify the 

 presence of the protochlorophyll absorption spectrum in living plants. 

 Noack believes that this was due to the fact that the concentration of 

 protochlorophyll in living leaves is exceedingly small. He claims to 

 have obtained the characteristic red absorption band of protochlorophyll 

 by means of special methods even in living leaves. But, he points out, 

 the concentration of protochlorophyll in leaves is so very small that 



