ULTRA-VIOLET AND SEED PLANTS 859 



from the lamp at such short distances, especially under periods of irradia- 

 tion lasting several hours. In his lots H and K the seeds, in Petri dishes, 

 were covered with G586AW glass and then brought to a distance of 

 8.5 cm. from the arc. Under these conditions heating effects must have 

 been pronounced. 



In view of all these considerations, it is obviously impossible to accept 

 the author's conclusion that "ultra-violet radiation of X3650 A, in the 

 time and intensity employed and in the absence of all other radiations 

 throughout the experiment, exerts a stimulative action on the subsequent 

 rate of growth of the hypocotyl of pea seeds irradiated in the air-dry 



state." 



As opposed to these papers reporting beneficial or "stimulating" 

 effects, there have been since 1927 several papers in which only harmful 

 or indifferent effects of ultra-violet on seeds and seedlings are indicated. 

 Unscreened-arc experiments in general have yielded such results and have 

 failed to show beneficial or "stimulating" effects. 



Popp and Brown (72, 73, 74) have over a period of years carried out 

 experiments on seed germination and early growth of seedlings using 

 chiefly turnip but also radish, cucumber, pigweed, and curled dock. 

 Many thousands of seeds have been used in these experiments. No less 

 than 50 seeds per culture were ever used in any test. A number of differ- 

 ent series of experiments have been completed. The seeds were usually 

 germinated on moist filter paper and cotton and kept, except for short 

 daily exposures to the mercury- vapor arc, some in the dark, some in 

 diffused light, and some in diffused light minus all ultra-violet. Irradia- 

 tions were given with the unscreened arc and with the arc screened with 

 various Corning filters. A special effort was made to provide adequate 

 controls. The irradiations were usually given for 10 days when rapidly 

 germinating seeds were used. 



The outstanding results of these experiments have been (a) the marked 

 and invariably injurious effect on seedlings of the radiation of the 

 unscreened arc even for exposures as brief as 3^ min. per day, (6) the 

 lessened injurious effect of the radiation through a Corex filter, and (c) the 

 failure of any region of the ultra-violet studied to influence significantly 

 the rate or the percentage of germination or to stimulate the growth of 

 seedlings. It should be emphasized that no significant stimulation was 

 ever obtained when adequate controls were used. The effects were recorded 

 by general appearance of cultures, hypocotyl lengths, leaf measurements, 

 and dry weights. 



Cluzet and Kofman (11), Mezzadroli and Vareton (62), and Fires de 

 Lima (67), also report unfavorable or harmful effects on seeds and seed- 

 lings of ultra-violet radiation from unscreened merciiry-vapor lamps. 

 Detwiler (22) found that the region 2700 to 3200 A caused delayed 

 germination and pronounced stunting of seedlings of Rihes rotundifolium. 



