862 BIOLOGICAL EFFECTS OF RADIATION 



is continually forming as the plant grows, the total injurious effect pro- 

 duced on a plant was much greater when it was exposed often than 

 otherwise. No marked beneficial effects were observed in any of these 

 experiments. Nor was any injury produced within the extreme limits of 

 wave-lengths present in solar radiation except under conditions which 

 never occur in nature, namely, irradiation through a concentrating lens 

 and a screen transmitting faintly to 2890 A for 163^^ hr. Injury did 

 result, however, from wave-lengths but slightly shorter than those occur- 

 ring in solar radiation. 



These carefully conducted experiments confirm once again, and more 

 accurately than previous experiments had, the results of earlier workers 

 who have obtained injurious effects with short-wave ultra-violet radiation. 



That the injurious effects of ultra-violet radiation are only temporary 

 has been emphasized by Sibilia (104), Jacobi (40), Popp and Brown (73), 

 Arthur and Newell (3), and Detwiler (22) who found that temporary 

 effects on general appearance, color, size, general vigor, and weight 

 disappeared when the injurious irradiations ceased, the length of time 

 necessary for this being dependent upon the degree of injury, unless the 

 injury was too severe, in which case the plants died. 



Recently Fuller (30) has concluded from an experiment with 12 

 tomato plants per culture, and three cultures that the injurious effects 

 of the open arc at short distances are due in considerable degree to infra- 

 red radiation from the arc. Unfortunately, the author has worked with 

 relatively uncontrolled conditions with regard to the radiations under 

 consideration. He assumed that interposing a 1.5-cm. quartz water 

 cell between the mercury arc and the plants caused no diminution in the 

 intensity of the ultra-violet reaching the plants as compared with the 

 unscreened lamp. That this is not true is indicated by an examination 

 of the coefficients of absorption of water in the ultra-violet as given in 

 the International Critical Tables. In addition, measurements made in 

 our laboratories with a Westinghouse P.E. ultra-violet meter indicate that 

 the ultra-violet intensity is cut down 25 per cent in passing through such 

 a cell. Furthermore, the shorter the wave-lengths the greater is the 

 absorption. Thus the destructive radiation is reduced much more than 

 the longer, less destructive radiation. Obviously the diminished injury 

 by irradiation through the water cell cannot be assumed to be due merely 

 to the elimination of infra-red. The possible injurious effect of infra-red 

 radiation of high intensity is by no means denied, but Fuller's experi- 

 ments do in no way disclose that the injurious effects indicated for 

 short-wave ultra-violet by the more accurately controlled experiments 

 of previous investigators are without foundation. If Fuller actually 

 wished to demonstrate injurious effects of infra-red, it is difficult to 

 understand why he did not expose some plants to the infra-red in the 

 absence of all ultra-violet. 



