854 BIOLOGICAL EFFECTS OF RADIATION 



of plants. Raybaud (75, 76), on the other hand, had reported that the 

 rate of germination of certain seeds was increased by such radiation, but 

 that the seedhngs were injured and died soon after emerging from the 

 seed coats. Schanz (89) had found that seeds did not sprout so readily 

 in daylight containing ultra-violet radiation as in daylight from which 

 this radiation was screened out. 



In 1921 Popp (69, 70) carried out a more detailed investigation of 

 this subject. In a series of experiments upon various types of seeds, 

 including foxglove, tobacco, mustard, corn, Canada field peas, lupine, 

 and sunflower, he made numerous tests using the mercury arc in quartz as 

 the only source of radiation. In some cases it was used unscreened and 

 in others screened by various filters. The length of exposure ranged 

 from a total of 1 to 2 up to 6 to 10 hr. per day for several days. The 

 ranges of radiation reaching the experimental plants were indicated by 

 spectrograms. From 50 to 100 seeds were used in each test with a corre- 

 sponding number of controls, involving a total of about 5000 seeds. 

 Plants were compared which had received approximately only the region 

 4200 to 3200 A plus some infra-red, only the visible and infra-red plus the 

 ultra-violet down to about 3000 A and the visible and infra-red plus 

 the ultra-violet down to about 2000 A. Intensity differences under the 

 various experimental conditions were not recorded because instruments 

 for measuring these were not available. 



The experiments indicated that exposures of dry seeds to the entire 

 radiation of a quartz mercury arc for as long as 188 hr. had no effect on 

 later germination and growth and that exposures of less than 2 hr. 

 of soaked seeds that had not yet begun to sprout had no effect on later 

 germination and growth. This was explained as probably due to the 

 failure of the short injurious rays to penetrate sufficiently to be effective. 

 Longer exposures of soaked seeds were injurious, and wave-lengths 

 below 3000 A were particularly harmful. No differences in rate of 

 germination were noted in seeds grown in the dark, in the radiation of the 

 lamp from which the ultra-violet was screened off, or under the lamp 

 from which only the ultra-violet below about 3000 A was screened off. 

 If, however, the principal radiation (aside from infra-red) that the seeds 

 received was ultra-violet of the approximate region 4000 to 3000 A, 

 injurious effects were indicated, more upon the seedlings after germina- 

 tion than upon the rate of germination, but these effects were probably 

 chiefly caused by the absence of sufficient light for growth. Seedlings 

 grown with the unscreened lamp as the only source of radiation never 

 developed beyond the stage that would result from food stored in the 

 seeds. This, plus the fact that starch tests were negative on mature 

 geranium leaves which had been irradiated, was thought to be an indica- 

 tion that food synthesis is slight under the radiation of the unscreened 

 lamp. 



