KINETIC ANALYSIS OF PHOTOPERIODISM 467 



pyrrole rings are not closed into a major ring, such as biliverdin, and 

 the algal pigment, phycocyanin. It would seem to be significant, also, 

 that the most sensitive photoperiodic systems having this type of action 

 spectrum are those including only the chlorophyllous plants, and the 

 photoreceptors are always potentially green tissue. We are not aware 

 of any photoperiodically red, far-red sensitive system that does not 

 contain chloroplasts. Since the action spectrum appears to fit a tetra- 

 pyrrolic type of structure, it is reasonable to assume, for the present at 

 least, that the site of the photochemical reaction is most likely in the 

 chloroplast and involves possibly either precursors to protochlorophyll 

 or a chlorophyll analog. While much work has been done on the 

 intermediate steps of chlorophyll synthesis in the algae by Granick 

 et al. (1953) and others, there appears to be a considerable gap 

 in knowledge of the steps between protoporphyrin 9 and its precursors. 

 It is possible that these precursors are porphyrins which attain ex- 

 tremely low equilibria concentrations as would be indicated by the 

 lack of in vivo absorption in this region and that, since the half-life of 

 these compounds is very short, they have not been observed by direct 

 chromatographic technics. The recent work of Krasnovskii et al. 

 (1957) which shows a photoreversible reduction of chlorophylls in 

 the red and far red also is conducive to much speculation as to the in- 

 volvement of a chlorophyll or one of its analogs as the photoreceptor. 



Region 2 includes the complex cell growth systems that respond 

 to the photoreactions and to all the other external physical and 

 chemical factors of the environment. These include growth responsivity 

 in stem length, leaf development, anthocyanin synthesis, and other 

 activities of the organism. These systems are not refractory, their 

 response is graded, and the magnitude of the response has little rela- 

 tionship to the phase of the applied energy. 



It is System 3 which is closely coupled to the clock mechanism and 

 which is relatively refractory as compared with the other systems. 

 Flower bud initiation in plants is relatively uninfluenced by physio- 

 logical levels of most physical and chemical factors except temperature 

 in certain classes of plants and day length in others, or a combination 

 of the two. Recent work has demonstrated that also in certain classes, 

 especially the long-day plants, gibberellin is capable of inducing 

 flower bud initiation in an unfavorable photoperiod. But the evidence 



