Vernalization and Photoperiodism — 42 — A Symposium 



sume a somewhat hopeless trend, but after much deliberation it was concluded that 

 the only remaining seasonal phenomenon that could be a factor was change in relative 

 length of day and night. The importance of this conclusion, of course, lies in the fact 

 that length of day was dissociated from the factor of amount of solar radiation. The 

 decisive test was then made in the simplest possible way but without great hope of 

 success, mainly for the reason that there appeared to be no accepted basis in plant 

 physiology for consideration of day length as other than a purely quantitative factor. 

 Failure on the part of earlier workers to distinguish between duration and amount of 

 daily illumination perhaps explains why photoperiodism was not discovered much 

 earlier, for usually in reducing the hours of light the mistake was made of darkening 

 the experimental plants in such a way as to subject them to two or more daily light 

 periods. Ineffectiveness of mid-day darkening in inducing photoperiodic responses is 

 brought out in the original paper (p. 574) and more fully developed in later articles." 



While tobacco and soybeans were used largely in the preliminary trials, 

 subsequently a wide variety of field, garden and ornamental plants were 

 studied by Garner and Allard (1920, 1923, 1930, 1940) as to their re- 

 sponses to photoperiods of various lengths. The effects on sexual repro- 

 duction were taken primarily into consideration, but much evidence was 

 secured also on the influence of length of day on vegetative growth, such 

 as increase in size of the plants, formation of bulbs and tubers, character 

 and extent of branching, root growth, pubescence, pigment formation, ab- 

 scission and leaf fall, dormancy and death. To this have been added by 

 other investigators observations on the effects of the photoperiod on sex 

 reversal, duration of the growth period and other phenomena (Schaffner, 

 1923, 1930, 1935 ; Krajevoj and Kiricenko, 1935 ; Kramer, 1937 ; Mann, 

 1942; Danielson, 1945). Garner and Allard, of course, noted that 

 other environmental factors, such as light intensity, soil moisture and nu- 

 trients, etc., may and usually do modify photoperiodism in plants. It is 

 significant that temperature was found most important in relation to the 

 action of the light period on plant growth and development. 



By the simple means of exposing only parts of a plant to definite photo- 

 periods, Garner and Allard (1923, 1925) were able to demonstrate with 

 Cosmos that the response is largely localized. But the fact is pointed out 

 that the determinative influence of the light period can be transmitted, in 

 some cases at least, to other portions of the plant, as, for example, from 

 the aerial parts to tubers of the potato. This was subsequently verified 

 with several other tuber-bearing plants by Rasumov (1931, 1935). 



In his extensive work on photoperiodism in spinach Knott (1934) was 

 led to the belief that the leaves appear to function in some way to hasten 

 the photoperiodic effect in plants. Soon thereafter it was demonstrated by 

 MosHKOv (1935) and by Cajlachjan (1936) and later by many others 

 that leaves are the organs of perception of the length of day stimulus, 

 whence it travels to the meristems and other parts of the plant. More 

 recent evidence has shown that the youngest fully developed leaves are 

 most sensitive to the photoperiod (Moshkov, 1937; Psarev, 1937; Ul- 

 RicH, 1939; Navlor, 1941). 



Classification and Adaptation: — As regards their photoperiodism, 

 plants have been classified by Garner and Allard (1920, 1923) into short- 

 day, long-day and day-neutral types. While this grouping is not fully ac- 

 ceptable in the light of our present evidence on plant development as af- 



