SOME EFFECTS OF THE PHOTOPERIOD ON DEVELOPMENT OF IMPATIENS BALSAMEA

In 1920 GARNER and ALLARD (1) discussed the influence of length of the daily light period on flowering. A later paper by the same investigators (2) recorded further observations of its effect on flowering and other phases of plant development. GARNER and ALLARD (1) proposed two terms, photoperiodism, "to designate the response of an organism to the relative length of day and night," and photoperiod, "to designate the favorable length of day [for flowering] for each organism." Photoperiodism appears frequently in the literature on the subject. Shortly after the term photoperiod was proposed, however, it was observed that the length of the daily light period also influences phases of plant development other than reproduction. The term thus became unserviceable and apparently has never been used since. To make it useful and to give it a logical relation to the term photoperiodism, it is therefore suggested that photoperiod be used to designate the daily period of illumination under the influence of which an organism is developing. It is so used in this paper. The experimental work here reported was carried on from the middle of March to the first of August, 1932. Plants of Impatiens balsamea, variety Wrhite Perfection, were grown in pots in the greenhouse and were subjected to photoperiods of 8, 16, and 20 hours, and normal. The normal photoperiod (sunrise to sunset) during the experiment ranged from 12 hours at the beginning to approximately 15 hours at the end. The plants were all kept in one section of the greenhouse during the day to secure as nearly uniform conditions of light intensity and temperature as possible. Eight hours after sunrise one lot of plants was removed to a ventilated darkroom until after dark when they were returned to their original places. Late in the afternoon two lots were removed to other sections of the greenhouse where they received electric illumination from sunset until 8 hours and 4 hours before sunrise, giving 16and 20-hour photoperiods respectively. The intensity of the artificial light was approximately 125 foot-candles at the apices of the plants, as measured by the MacBeth illuminometer. The plants receiving artificial light were returned to their original places early in the morning. During the cooler weather at the beginning of the experiment the temperaPaper from the Department of Botany, University of Michigan, no. 467. 545