Response of Certain Plants to Length of Day and Temperature under Controlled Conditions'

Since the appearance of the first paper of Gamer and Allard on photoperiodism {5Y an extensive hterature covering various phases of the subject has developed. The accumulated data as a whole are sufficiently comprehensive and consistent to remove any doubt as to the practical importance of length of day as a factor in plant growth and development. However, only in comparatively few instances have the experiments in this field been conducted with sufficiently accurate control of the environment as a whole to yield results of a quantitative nature that could be readily reproduced. Since in most instances natural illumination, with or without supplemental artificial light, has been employed, control of the illumination conditions other than daily duration has been out of the question, and usually only partial control of other environmental factors has been attempted. Inasmuch as growth and development within the limits imposed by heredity ordinarily are the net result of interaction of the many environmental factors inñuencing the plant, it is necessary to supplement the above-mentioned data with observations made under controlled conditions and with suitable modifications of factors other than light duration if accurate information is to be obtained on the relationship of other factors to the day-length effect. It is true that rigid control of the environment necessarily will introduce conditions differing more or less from those obtaining in nature, but insofar as normal growth and development of the plant can be obtained, this method appears to furnish the best available means for studying the interrelation of the light period and other factors. In the present paper results obtained in growing typical short-day and long-day plants under controlled conditions are presented. The primary objectives were (1) to compare plant response to the daily light period under rigidly fixed environments involving artificial illumination with results previously obtained tmder more or less natural conditions and (2) to make a direct study of the interrelation of day length and temperature as ecological factors. To culture the higher plants under adequately controlled conditions offers many experimental difficulties and at best is both laborious and expensive, so the data presented are necessarily limited. These data, however, appear to throw additional light on the photoperiodic responses of the plants studied and on the relation of temperature to these responses. With the artificial climates employed it has been possible to obtain photoperiodic responses closely agreeing with those observed under outdoor conditions.