588 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



would be more profoundly influenced by the fixed course of the 

 annual cycle in day-length than by other climatic factors which are 

 so variable and uncertain in their annual cycle. The wonder is that 

 we have been so slow in recognizing the real significance of the rela- 

 tive length of day and night as a factor in plant development. 



We may conclude that flowering and fruiting of many kinds of 

 plants is induced by exposure to a specifically favorable length of 

 day which varies widely with the species, some plants requiring 

 relatively long days, others* requiring comparatively short days. 

 There are also definite day-lengths which are particularly favorable 

 to purely vegetative development. Exposure to a daily illumination 

 period which is favorable to vegetative development but unfavorable 

 to flowering and fruiting tends to cause more or less indefinite con- 

 tinuation of growth, a phenomenon which is designated as gigantism. 

 Exposure to daylight of a duration which is favorable to flowering 

 and fruiting and relatively unfavorable to growth tends to produce 

 heavy flowering and fruiting but dwarfed or restricted growth. A 

 daylight period favorable alike to growth and to flowering and 

 fruiting may induce the everblooming or everbearing type of de- 

 velopment, the plant thus dividing its energies between growth and 

 reproduction. It is a comparatively simple matter to hasten or to 

 delay almost at will the flowering and fruiting of most plants by 

 properly shortening or lengthening the daylight period. The natural 

 daylight period may be shortened by use of dark houses and may be 

 lengthened by use of artificial light. 



The term photoperiodism has been adopted to designate the re- 

 sponse of the organism to the relative length of day and night. 1 



1 For a more technical account of the work which has been discussed in this paper the 

 reader is referred to an article by the authors which appeared in the Journal of Agri- 

 cultural Research, Vol. XVIII, pp. 555-G06, Mar. 1, 1920, under the title " Effect of the 

 Relative Length of Day and Night and Other Factors of the Environment on Growth and 

 Reproduction in Plants." 



