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). Supplementary artificial illumination and continuous light. Interrelationship of other environmental factors. Effects of abnormal light periods. The pholoperiodic 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.
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 artificial 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 Linné's Rön om växters plantering grundat pȧ naturen (41), published in 1739. However, Linné 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. Schübeler, 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. Schübeler ascribed the observed effects to direct or indirect action of the additional sunlight. In 1880 . . .