XIX 

 PHOTOPERIODISM 



W. W. Garner 

 U. S. Department of Agriculture, Bureau of Plant Industry, Washington 



Introduction. Discovery of the length-of-day effect. Long-day and short-day plants. 

 Flowering and fruiting responses. Growth relations. Formation of tubers, bulbs, and 

 thickened roots. Senescence, dormancy, and related phenomena. Morphological and 

 anatomical effects. The photoperiodic aftereffect {photoperiodic induction). Supple- 

 mentary artificial illumination and continuous light. Interrelationship of other environ- 

 mental factors. Effects of abnormal light periods. The photoperiodic response and 

 heredity. Photoperiodism in the lower green plants. Internal conditions of the plant 

 in relation to photoperiodism. Length of day as an ecological factor. References. 



INTRODUCTION 



Nearly all of the early work relating to the influence on plants of the 

 daily duration of light was directed toward ascertaining the extent to 

 which both the general growth or increase in mass and the development 

 of the plant may be stimulated by lengthening the normal light period. 

 The question as to whether, on the one hand, plants generally require a 

 daily rest period for normal growth and development or, on the other 

 hand, are capable of thriving under continuous illumination was given 

 considerable prominence in these investigations. The two methods of 

 approach consisted essentially in obtaining observations on plant growth 

 within the Arctic circle, where continuous sunlight prevails during the 

 summer months, and exposing plant cultures in lower latitudes to arti- 

 ficial light for a portion or all of the night as a supplement to natural 

 daylight. 



According to Smith (69) apparently the first mention in literature of 

 the influence of length of day on plants is found in Carl von Linne's 

 "Ron om vaxters plantering grundat pa naturen" (41), published in 

 1739. However, Linne ascribes the rapid growth and early maturity 

 attained by plants in polar regions to additional heat supplied by the 

 continuous sunlight rather than the additional light as such. Schiibeler, 

 in 1879 (66), advanced the idea that the cereals and other species of plants 

 when gradually transferred from lower to higher latitudes undergo 

 definite change in growth characteristics. They were believed to acquire 

 ultimately the capacity to shorten the vegetative period, increase the 

 size of leaf, produce larger seeds, and increase their content of aromatic 

 and coloring matters. However, the data on which these conclusions 

 were based are meager and inadequate. Schiibeler ascribed the observed 

 effects to direct or indirect action of the additional sunUght. In 1880 



677 



