Historical Note • II 



nightlength or daily duration of darkness. These definitions all 

 convey the general idea, but they may be misleading. A more 

 general definition is that photoperiodism is a response to the dura- 

 tion and timing of the light and dark conditions. Total light 

 quantity, even light intensity above a certain threshold level, is 

 of secondary importance in photoperiodism, although it may be 

 a modifying factor. The relative length, or ratio of the lengths of 

 dark and light exposures, is also secondary. It is the time relations 

 in which light and darkness succeed each other that appear to 

 be crucial. 



Under natural conditions of a 24-hour day-night cycle, of 

 course, the duration and timing of light exposure cannot be 

 changed without a complementary change in the dark exposure, 

 but cycle lengths totaling more or less than 24 hours have been 

 used to study photoperiodism experimentally, as have brief light 

 (or dark) interruptions of extended dark (or light) periods. Results 

 from this sort of work have led to the definition given above. In 

 nature, however, the lengths of day and night change seasonally 

 except on the equator, and it is evident that photoperiodism might 

 be expected to have some relation to the seasonal changes in 

 biological events. In fact, it was observations on the relation 

 between seasonal daylengths and flowering that led to the discovery 

 of photoperiodism. 



HISTORICAL NOTE 



Like many important phenomena, photoperiodism was observed 

 frequently before being finally "discovered." References to early 

 observations by workers such as Tournois, Klebs, and others can 

 be found in Murneek and Whyte (1948), a volume recommended 

 to those interested in the history and early development of flower- 

 ing physiology. Such observations suggested that flowering in 

 plants such as hops (Humulus) or houseleek (Sempervivum) could 

 be brought about by artificially shortening or lengthening their 

 daily exposure to light. It remained, however, for Garner and 

 Allard, plant physiologists in the U.S. Department of Agriculture, 

 to show that such effects were not isolated curiosities. It was their 

 early papers (1920, 1923) that attracted other workers to the field 



