THE LIGHT FACTOR IN PHOTOSYNTHESIS 1017 



But most plants require light for the formation of chlorophyll and 

 for this the wave-length of the light, its intensity, and the length of 

 exposure are of significance. Much of the earlier investigation was done 

 with methods which could yield only qualitative results. In fact, two 

 rather distinct aspects must be considered, namely, it is necessary to 

 differentiate between the formation of clilorophyll and its accumulation. 

 In the first case one would be dealing with the appearance of the first 

 traces of chlorophyll, and in this case the difficulties of accurately deter- 

 mining extremely small quantities of the pigment must be dealt with. 

 In determinations involving the accumulation of chlorophyll considera- 

 tion must also be given to the simultaneous decomposition of the pigment, 

 which may occur under a variety of conditions; particularly in the sudden 

 exposure of a plant to light of high intensity. 



There is an extensive older literature dealing with the formation of 

 chlorophyll in different portions of the spectrum (139). For this purpose 

 use has been made of optical prisms, gratings, and filters (94), but insuf- 

 ficient regard has been paid to intensity and possible destructive action 

 of light on the chlorophyll. The result of the total of this work is a 

 rather confused picture and there are few points on which there is general 

 agreement. The infra-red and extreme red end of the spectrum is 

 apparently without benefit to chlorophyll formation (47). The orange- 

 yellow rays have been generally regarded as the most effective in chloro- 

 phyll formation; also in this portion of the spectrum the destruction of 

 the chlorophyll is most pronounced. Chlorophyll formation also occurs 

 in the blue end of the spectrum, though it is not certain what the limit 

 is in the violet region. There is as yet no agreement as to whether 

 chlorophyll formation in different portions of the spectrum accords with 

 the absorption spectrum of chlorophyll. 



The complexity of the series of reactions involved in chlorophyll for- 

 mation has been emphasized by almost everyone who has given the 

 subject intensive study. It would appear that little is to be learned 

 from a consideration of light only as a factor in this process. Of equal 

 importance are the nutritive substrate in the cells containing the chloro- 

 plasts and the action of certain, as yetonly imperfectly described, enzymes. 

 Light may affect the general reaction of greening in various ways, directly 

 through a photochemical reaction of some precursor of chlorophyll and 

 indirectly through its influence on the permeability of the cells, thus 

 affecting the nutrition of the plastid. 



Several theories have been advanced in an effort to describe the 

 chemical steps involved in chlorophyll formation, notably by Lubimenko 

 (67, 68, 69, 70, 71), Noack (85), and Liro (65). Lubimenko strongly 

 supports the contention that the chloroplast pigments are intimately 

 bound to protein constituents of the plastids and consequently the forma- 

 tion of chlorophyll must also be dependent upon the protoplasmic activity 



