Light 315 



lished the results of their pioneer observations which showed that flower- 

 ing is not determined by the intensity or the quality of light alone but by 

 the length of daily exposure to light, or the photoperiod. What essentially 

 is involved is the relative length of the alternating light and dark periods, 

 or, perhaps more accurately, the length of the dark period for some 

 plants and of the light period for others. This phenomenon of photo- 

 periodism has now been subjected to intensive study. A history of the 

 work on it until 1948 was written by Murneek (1948). Other surveys 

 of the subject or particular aspects of it are those by Garner (1937), 

 Burkholder (1936), Hamner (1944), Leopold (1951), and Naylor 



(1953). 



All plants do not respond alike to photoperiodic stimulation. In the so- 

 called short-day forms, flowering is induced by relatively short periods 

 of daily illumination ( and thus longer dark periods ) . Such plants flower 

 naturally in fall or early spring. Long-day plants require a longer period 

 of light and in nature are summer-flowering forms (Fig. 13-4). Many 

 plants, such as the tomato, are day-neutral and will flower under long 

 or short photoperiods or continuous illumination. 



This classification is not a very exact one, for different steps in the 

 reproductive process may each have their optimal photoperiod. Thus most 

 strawberries flower under a relatively short day but fruit under a long 

 one, and Phlox is a long-day plant for flowering but is day-neutral for 

 fruiting. The optimal period for the formation of flower primordia at a 

 growing point may be different from the one determining the later 

 growth and opening of the flowers. These relationships have been dis- 

 cussed by Roberts ( 1954 ) . 



There is a close relation between temperature and photoperiodism 

 which has been studied by a number of workers, among them Purvis 

 (1953) and Vlitos and Meudt (1955). High temperature will sometimes 

 induce flowering even when day-length is not favorable for it. In vernali- 

 zation (p. 339) it is necessary not only to expose the germinating seeds to 

 low temperature but to provide the proper photoperiod for later growth. 

 Schwabe (1951) concludes, from experiments with vernalized short-day 

 and long-day Chrysanthemum cuttings, that the effects of vernalization 

 and of day-length in this plant are operative at different stages in the 

 train of reactions leading to flower initiation. Sometimes a high level of 

 nutrition, especially of nitrogen, may be substituted for day-length. Short- 

 day soy plants have a higher concentration of nitrogen than do long-day 

 ones. In many cases an exposure for a few days to a photoperiod favorable 

 for flower production will result in flowering regardless of the one to 

 which the plant is later exposed. Plants vary in their sensitivity to this 

 photoperiodic induction. 



Plant parts affected by photoperiodic stimulation in most cases are the 



