THE INFLUENCE OF THE LENGTH OF THE PHOTOPERIOD ON THE VEGETATIVE AND REPRODUCTIVE DEVELOPMENT OF RUDBECKIA BICOLOR SUPERBA, DELPHINIUM AJACIS, COSMOS SULPHUREUS, AND IMPATIENS BALSAMINA

Abstract

IN THE twenty years since Garner and Allard (1920) focused attention on the importance of the length of the daily light period as a factor influencing the development of plants a great volume of work on this problem, has been published. It is not surprising that in the rapid development of the study of photoperiodism some of the fundamental details were neglected. It was with the purpose of elucidating certain of these points that the experiments reported here were undertaken.2 EXPERIMENTAL METHODS.-The experiments were conducted in the greenhouse of the Natural Science Building at the University of Michigan. The temperature during the winter months was 65 0 to 70 OF., but in the spring and summer it was frequently as high as 90?F. during the middle of the day. The plants were subjected to three types of photoperiod: (1) the prevailing (normal) photoperiod, the length of which was reckoned as the number of hours between sunrise and sunset (fig. 1), calculated from tables in the World Almanac; (2) photoperiods shorter than the normal, produced by keeping the plants in a ventilated darkroom at approximately the temperature of the greenhouse during a part of the day; (3) photoperiods longer than the normal, produced by augmenting the normal light period with supplementary light of 100 to 150 foot candles intensity at the apices of the plants; the artificial illumination was provided by 200-watt tungsten-filament Mazda bulbs 21/2 to 3 feet above the tops of the plants. All the groups of plants used in each experiment were grown at the same time under various photo' Received for publication December 1, 1940. A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, in the University of Michigan. 2 The research reported in this paper was carried on while the writer was an F. C. and Susan Eastman Newcombe Fellow in Plant Physiology under Dr. F. G. Gustafson. periods, so that the environmental conditions, with the exception of the length of the photoperiod, were approximately the same for all the plants. Therefore, the data obtained from the various groups of plants in a given experiment are properly comparable. The data recorded included the date when each seedling appeared above the soil surface, the date when the main axis of each plant began to elongate (in the case of the rosette-forming species), the dates when the first flower bud appeared and the first flower opened on each plant, the number of nodes and the height of the main axis (from soil surface to apical bud) at intervals, and in the case of Impatiens balsamin.a measurements of leaf area and determinations of the green and dry weight of the tops and roots. The date when the first noticeably lengthening internode was detected in the rosette was taken as the date of the beginning of stem elongation. The date when the first flower bud became visible and when the first flower, or head of flowers (in the composite species), opened on each plant were taken as the dates of budding and flowering, respectively. From the data the age of each plant at each of the stages of development mentioned was calculated as the number of days from the time the seedling first appeared above ground. Thus, the age represents the actual period of exposure to the photoperiod in question. ANALYSIS OF DATA.-In analyzing the data impressions gained from daily observation of the plants were not relied upon. The mean age at which the plants of a given group attained each stage of development and the mean height at various times were calculated from the data on the individual plants in the group. The standard deviation was determined using the formula: