ULTRA-VIOLET AND SEED PLANTS 861 



INVESTIGATIONS DEALING PRINCIPALLY WITH INJURIOUS 

 EFFECTS OF SHORT-WAVE ULTRA-VIOLET RADIATION 



Earlier works such as those of Siemens (105), Deh^rain (19), Bailey 

 (4, 5, 6), Bonnier (7), and Rowlee (82) with electric arc light, while not 

 primarily concerned with ultra-violet, did demonstrate its destructive 

 action. The works of Hertel (37, 38), Maquenne and Demoussy (51, 52, 

 53), Kluy\'er (43, 44), Stoklasa (108, 109, 110), Ursprung and Blum (118), 

 and Martin and Westbrook (54) appeared later. These investigators 

 all noted the superficial destructive action of the short-wave ultra-violet. 

 This destructive action manifested itself in the discoloration, collapse, 

 and death of outer layers of cells, the depth of the injury depending upon 

 the intensity, quality, and duration of the irradiation as well as on the 

 nature of the tissues themselves. Kluyver (45) emphasized for the first 

 time that the destructive rays were not present to any appreciable extent 

 in solar radiation. 



More recently Arthur and Newell (3) have performed experiments 

 to determine what region of the ultra-violet is most injurious and whether 

 that region near 2900 A, the extreme limit for solar radiation, is injurious 

 to plant tissue. Using a quartz mercury arc and a series of Corning 

 filters which absorbed progressive increments of the extreme ultra-violet 



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between wave lengths 2000 and 2900 A, they noted the time necessary to 

 produce marked injury on young tomato plants. Spectrograms and 

 transmission curves, after continued use of the filters, were given so that 

 the nature and amount of ultra-violet actually reaching the plants were 

 definitely known. In addition, the total radiant energy reaching the 

 plants through the various screens was measured by a Weather Bureau 

 type pyrheliometer. Not more than 6 per cent variation was found to 

 occur. Provisions were made for maintaining the constancy of the 

 radiation of the mercury arc. The filters used were found not to solarize. 

 Hence these experiments were performed under carefully controlled and 

 relatively definitely known conditions of radiation. 



The results showed that the time of exposure to the arc necessary 

 to cause marked injury increased rapidly as more and more of the extreme 

 ultra-violet component was cut off from the plants. By means of a 

 quartz concentrating lens which increased the intensity three-fold it was 

 also shown that apparently the time of exposure necessary to cause 

 marked injury with a given quality of radiation is approximately inversely 

 proportional to the incident energy. The injury produced was found 

 not to be cumulative. That is, a plant but slightly injured by a single 

 irradiation of a given duration through a certain filter received little 

 further injury when irradiated through that same filter each day for 

 several weeks for that same length of time. However, since new tissue 



