It was while working with Maryland Mammoth tobacco prior to 

 1920 that W. W. Garner and H. A. Allard, two scientists of the United 

 States Bureau of Plant Industry, discovered that the plants came 

 into flower only when the period of illumination each day was rela- 

 tively short. When the light periods were long the plants remained 

 vegetative. Subsequent experiments on many kinds of plants showed 

 clearly that they differ greatly in their growth and flowering responses 

 with respect to the relative lengths of the periods of illumination and 

 darkness to which they are exposed. This discovery represented a 

 distinct departure from the then-prevalent idea that intensity of 

 illumination was the principal factor concerned in the determination 

 of the characteristic growth, flowering, and fruiting of plants. This 

 newer concept placed emphasis principally on the influence of succes- 

 sive, rhythmic occurrence of light and dark periods and the relative 

 lengths of such periods rather than on light intensity only. 



On the basis of these experiments Garner and Allard further deter- 

 mined that for many plants there is a certain critical length of the 

 light period at which they flower. If this light period is lengthened 

 some kinds will bloom more quickly and perhaps even more pro- 

 fusely. Such varieties are called long-day plants. On the contrary, 

 certain others will not bloom if the light period is lengthened but will 

 do so provided it is made shorter than the critical. Such plants are 

 classified as short-day plants. There is a third, day-neutral group, 

 so called because they flower whether the day length is relatively long 

 or short. 



In addition to these effects of day length upon the production of 

 flowers and fruits, Garner and Allard also reported in their early work 

 that other characteristics of plant development were influenced by 

 this factor. They found that formation of bulbs, tubers, and thick- 

 ened roots, the production of pigments, and the initiation of dormancy 

 were in a large measure controlled by the daily duration of light. 



Photoperiod 



Scientists studying the influence of the relative length of light and 

 dark periods upon plant growth have performed many experiments 

 in which the length of the combined dark and light periods has been 

 more or less than a 24-hour day. For convenience in discussing such 

 experiments, as well as those based on a natural 24-hour day, any 

 single combination of light and dark periods of whatever length has 

 come to be designated as a "cycle," and the light portion of such a 

 cycle has been designated as the "photoperiod" rather than the day 

 length. For brevity and ready reference the behavior of plants in 

 relation to the relative lengths of alternating periods of light and dark- 

 ness is now called photoperiodism. 



More recently the relation of other factors to the photoperiod 









