98 PHOTOCONTROL OF GROWTH 



imbibition for the 24 hr preceding irradiation is at 5° and if the seeds 

 are placed at 25 ° after irradiation, however, germination is practically 

 complete (Fig. 6), 



The light exposures discussed thus far were of only one short dura- 

 tion (measured in seconds or minutes) after a given imbibition time. 

 However, not all seeds respond with full germination after a single 

 short exposure to red light. The seeds of Puya berteroniana Mez 

 (Bromeliaceae) respond to daily irradiations.^ A short (19-min) ex- 

 posure to red light each day promotes germination of only a portion 

 of the seeds. If the 19-min exposure is divided and 4 min are given at 

 8 A.M. and 15 min at 4 p.m., germination of the viable seeds is 

 practically complete (Table VI). 



Table VI. Germination of Seeds of Puya berteroniana with 

 Different Timing of Daily Irradiations" 



" Seeds irradiated in red for first 12 days and kept at 15°C in 

 darkness except during irradiations. 



REFERENCES 



Borthwick, H. A., S. B. Hendricks, M. W. Parker, E. H. Toole, and Vivian K. 



Toole. 1952. A reversible photoreaction controlling seed germination. Proc. 



Natl. Acad. Sci. U. S. 38, 662-66. 

 Borthwick, H. A., S. B. Hendricks, E. H. Toole, and V. K. Toole. 1954. Action 



of light on lettuce-seed germination. Botan. Gaz-, 115, 205-25. 

 Crocker, William. 1936. Effect of the visible spectrum upon the germination of 



seeds and fruits. Biological Effects of Radiation, B. M. Duggar, Editor. 



McGraw-Hill, New York, pp. 791-828. 

 Evenari, Michael. 1956. Seed germination. Radiation Biology, Vol. Ill, A. 



Hollaender, Editor. McGraw-Hill, New York, pp. 518-49. 

 Flint, Lewis H., and E. D. McAlister. 1937. Wave lengths of radiation in the 



1 R. J. Downs and Toole, V. K., unpublished data. 



