Borthnick et al. — 77 — Wave Length Dependence 



is due to the high probability of transition from the blue to the red excited 

 levels. Energy derived from the long lived red states alone is thought to be 

 effective in photosynthesis. 



Another interpretation of the action spectra is that the effective pigment 

 absorbs strongly only in the yellow-red. This would require a blue pigment 

 unless intense photoperiodically-ineflfective absorptions are present else- 

 where in the visible portion of the spectrum. The pigment, chlorophyll or 

 unknown, could act by transfer of absorbed energy to a reaction underly- 

 ing formation of the effective compound in the leaf. Such reactions are 

 well known and are said to be photosensitized. Products leading to the 

 active substance or the substance itself must remain for several hours within 

 the leaf cells in which they were formed, possibly closely associated with 

 chloroplasts, because brief periods of irradiation after several hours of 

 darkness have elapsed would not otherwise be effective. 



Action spectrum curves for the photoperiodic response of several more 

 plants are needed to increase our assurance that the facts reported in this 

 paper concerning such data are common to all such plants. These addi- 

 tional data however, will probably not lead us to an identification of the 

 compound that is active in the photoperiodic process. This step must ap- 

 parently await the development of a biological assay, an achievement that 

 at the moment seems beset with great difficulties. 



The hypothesis proposed in this paper to explain the photoperiodic 

 reaction has been useful in designing the action spectrum experiments and 

 interpreting the results of various photoperiodic investigations reported in 

 the literature. If it should stimulate work that may either lend it further 

 support or disprove it, the hypothesis will have served an additional useful 

 purpose. 



References : — 



1. Allakd, H. a., Jour. Agr. Res. 57: 775-789, 1938. — 2. Blum, H. F., Photodynamic Ac- 

 tion and Diseases Caused by Light. A. C. S. Monograph no. 85, New York, 1941. — 3. Cajlachjan, 

 M. H., Compt. Rend. Acad. Sci. U.R.S.S. 4: 79-83, 1936. — 4. Cholodny, N. G., Herb. Revs. 7: 

 223-247, 1939. — 5. Eguchi, T., Proc. Imp. Acad. Japan 13: 332-333, 1937. — 6. Funke, G. L., 

 Rec. des Trav. Bot. Neerland. 40: 393-412, 1943. — 7. Katunskij, V. M., Compt. Rend. Acad. 

 Sci. U.R.S.S. 15: 509-512, 1937. — 8. Kleshnin, A. F„ Compt. Rend. Acad. Sci. U.R.S.S. 40: 

 208-211, 1943. — 9. Lang, A. and Melchers, G., Naturwiss. 29: 82-83, 1941. — 10. Melchers, 

 G., Umschau 44: 244-250, 1940. — 11. Melchers, G. and Lang, A., Biol. Zentrbl. 61: 16-39, 

 1941. — 12. Melchers, G. and Lang. A., Naturwiss. 30: 589-590, 1942. — 13. Murneek, A. E., 

 p. 39, this volume. — 14. Olmsted, C. E., Bot. Gaz. 106: 46-74, 1944. — 15. Olmsted, C. E., 

 Bot. Gaz. 106: 382-401, 1945. — 16. Parker, M. W., Soil Science 62: 109-119, 1946. — 17. Parker, 

 M. W., Hendricks, S. B., Borthwick, H. A. and Scully, N. J., Science 102: 152-155, 1945.— 

 18. Parker, M. W.. Hendricks, S. B.. Borthwick. H. A., and Scully, N. J.. Bot. Gaz. 108; 

 1-26. 1946. — 19. Rasumov, V. I.. Bull. Appl. Bot., Genet., and Pit. Breed. 3d: 217-251, 

 1933. — 20. Seybold, A., Planta 18: 479-508, 1932. — 21. Seybold, A., Planta 21: 251-265, 

 1933. — 22. Stout, M., Bot. Gaz. 107: 86-95, 1945. — 23. Withrow, R. B., and Benedict, H. M., 

 Pit. Physiol. 11: 225-249, 1936. — 24. Withrow, R. B., and Biebel, J. P., Pit. Physiol. 11: 807- 

 819, 1936. — 25. Withrow, R. B., and Withrow, A. P.. Pit. Physiol. 15: 609-624, 1940. 



Note Added in Page Proof.— Important experimental advances have 

 been made recently, after the preceding article was written. The action 

 spectrum for spike formation in Winter barley has been completely worked 

 out by H. A. Borthwick, S. B. Hendricks, and M. W. Parker, who 

 will report shortly about this in the Botanical Gazette, comparing it with 

 curves for cocklebur and soybean. Essential similarity of the various 

 curves is clearcut requiring the same pigment or closely related pigments 

 as the initial absorber of the radiant energy in these long- and short-day 

 plants. In barley, moreover, the action curve for stem elongation is 

 identical with that for floral initiation. This indicates that the two 

 processes are integrally associated. 



