Murneek — 41 — Research in Photoperlodism 



Hamner, K. C. Cold Spring Harbor Symp. Quant. Biol. 10 : 49-59, 1942. 

 Phasic development of plants: — 



Whyte, R. O. Biol. Revs. Cambr. Phil. See. 14: 51-87, 1939. 

 Anonymous. Herbage Revs. 7: 27-32, 94-95, 265-274, 1939; 8: 83-94, 1940. 



Discovery of Photoperiodism : — The formative effects of diurnal 

 length of exposure to natural light or artificial illumination have been ob- 

 served, previous to the discovery of photoperiodism, by several investiga- 

 tors of plant growth and development (Schubeler, 1880; Kjellman, 

 1885; Bonnier, 1895; Turnois, 1912). According to H. A. Allard 

 (1944) an interesting reference to the photoperiod as affecting plants is 

 given by A. Henfrey in his book "The vegetation of Europe," 1852, 

 wherein a theorem is proposed that the length of day is a factor in the 

 natural distribution of plants. Klebs also, as early as 1913, seems to have 

 had a fairly definite idea that the time of flowering of some of his experi- 

 mental plants, Sempervivmn funkii, was determined by the length of day. 

 Not being able to secure flower development in winter by changes in nutri- 

 tion, temperature, etc., Klebs exposed them for a few days to continuous 

 electric illumination. They produced flowers in the same greenhouse in 

 which other non-lighted plants remained vegetative. Concluded Klebs 

 (1913): 



"In der freien Natur vi'ird sehr wahrscheinlich die Bliitezeit dadurch bestimmt dass 

 von der Tag- und Nachtgleiche (21 Marz) ab die Lange des Tages zunimmt, die von 

 einer gewissen Dauer ab die Anlagen der Blijte veranlasst. Das Licht wirkt wohl 

 nicht als ernahrender Factor, sondern mehr katalytisch."* 



To W. W. Garner and H. A. Allard (1920), however, is due all the 

 credit for the disclosure and demonstration of the phenomenon of photo- 

 periodism. It was a real surprise to many investigators of plant life that 

 so "dilute" an environmental factor as length of day has so potent an effect 

 on many plants. 



The early experimentation leading up to the crucial tests with tobacco 

 and soybean plants are described by Garner as follows :f 



"The two cardinal observations with which we started were that, in contrast with 

 other tobaccos, the new Maryland Mammoth variety always continued a purely vegeta- 

 tive type of growth through the open growing season at Washington, and that succes- 

 sive plantings of certain varieties of soybeans made at short intervals through the 

 spring and early summer all tended to flower at the same date. In the case of tobacco, 

 it was not at first realized that a seasonal effect was definitely involved in the very 

 unusual behavior of the new variety. It was thought that perhaps the "shock" of 

 transplanting to the greenhouse was a major factor, possibly aided by increasing age 

 of the plants. We were temporarily misled by the observation that seedlings grown in 

 small pots in winter and early spring flowered and fruited freely, thus suggesting a 

 nutritional angle. The big lead came after several years with the observation that 

 with the advance of spring new shoots developing on stumps that had been producing 

 flowering shoots suddenly swung over to the indeterminate vegetative type of growth. 

 It was then perfectly clear that a seasonal factor was involved. Since the plants were 

 growing in a warm greenhouse, it appeared that temperature could be excluded. With 

 respect to light, we naturally reasoned that intensity and composition might be in- 

 volved. However, fairly extensive investigations were being conducted on other prob- 

 lems with both tobacco and soybeans that seemed to exclude intensity and spectral 

 composition of light as important factors. At this stage the problem appeared to as- 



* I wish to thank Dr. F. W. Went of the California Institute of Technology for 

 calling my attention to this reference. 



t Private communication from Dr. W. W. Garner, Aug. 22, 1944. 



