liENTLEY GLASS 897 



sections growino in the light autotropliically, but are ol little cifcct 

 on green sections growing heterotrophic ally on sucrose in the dark. 

 Surprisingly, the groAvth of etiolalctl sections is markedly promoted 

 by the same urea herbicides, and this growth too can be inhibited by 

 red light. The photoperiod ol the donor plant has an effect on the 

 growth of excised sections. If a green ])lant is grown under continuous 

 illumination, sections from its stem will grow better on sucrose in 

 the light and more poorly in the dark than will segments from plants 

 exposed to 8- or 16-hour photoperiods. Conversely, under a short 

 photoperiod such as 8 hours, the excised sections show better growth 

 in the dark and poorer growth in the light than do segments from 

 16- and 24-hour photoperiod plants. One may therefore consider the 

 plants grown under continuous illumination to be "light-adapted" 

 and those grown under a short photoperiod to be partially "dark- 

 adapted." Explanations for these phenomena remain to be sought. 



The relation of the daily photoperiod to the induction of flowering 

 in plants has been studied for many years, but the problem has been 

 refractory because of the laborious character of the experiments. 

 William S. Hillman has found a nearly ideal solution that will re- 

 mind a geneticist of the happy choice by Thomas Hunt Morgan of 

 Drosophila melanogaster for genetic studies, Hillman's organism, 

 truly small and economical to grow, is the common duckweed, Lemna, 

 of which several species exist and permit comparisons. Most species 

 of the Lemnaceae flower infrequently in nature. Lemna gibhn may 

 be induced to flower, as Kandeler first showed, by exposure to a long- 

 day regime (12-14 hours) . Lemna perpusilla, on the contrary, is a 

 conditional short-day plant; that is, it is indifferent to the length of 

 the photoperiod under certain conditions but is induced to flower 

 by exposure to a short-day period when the medium is supplemented 

 with certain chelating agents such as ethylenediamine tetraacetic acid 

 (EDTA) , citrate, or tartrate. Exposure to red light even briefly and 

 at low intensity during the dark period is sufficient to inhibit this 

 short-day flowering. Alternatively, one might look on the case as one 

 of specific long-day inhibition of flowering. 



When Lemna is grown aseptically on modified Hoagland's medium, 

 the photoperiod fails to affect either species until EDTA is added. 

 At a concentration of EDTA above 10-^ M, and under a long-day 

 regimen, the chelating agent stippresses the flowering of L. perpusilla, 

 which has previously been flowering; and it brings about the flowering 

 of L. gibba, which has previously not been flowering. A short photo- 

 period (8-hour) does not change the original condition in either 



