968 BIOLOGICAL EFFECTS OF RADIATION 



experiments involving thousands of seeds, concluded that weak doses 

 did not cause acceleration of growth. When large numbers of irradiated 

 plants and controls were used, the plants irradiated with weak dosage 

 behaved like controls. 



Bersa (7) has pointed out that because of the markedly variable 

 material used in studying the biological effects of X-rays, statistical 

 methods are especially necessary. Bersa believes with others quoted 

 that a dose causing genuine roentgen stimulus for seedlings cannot be 

 given as yet. 



Taken as a whole, conclusions from the numerous experiments on 

 the effect of X-radiation on seed germination and early growth of seed- 

 lings indicate that medium and heavy doses are injurious. Some investi- 

 gators believe that they have demonstrated the stimulative effects of 

 light doses, while others have found only retardation or lack of change. 

 It appears that many who have reported stimulation with X-rays have 

 not taken into account the variability of individual plants. The majority 

 of those who have used large numbers of seedlings and have repeated 

 their experiments many times agree that weak doses, which some have 

 regarded as stimulative, do not regularly cause increased germination of 

 seeds or increase of vegetative parts which results in greater dry weight. 



EFFECT ON ROOT DEVELOPMENT 



In 1920, Jiingling (23) reported a retardation in the development of 

 lateral roots as a direct effect of radiating seedlings. Nakagawa (44) 

 also found that medium to strong irradiation of Vicia seedlings dis- 

 turbed the development and growth of root tips. Root hairs and rootlets 

 did not form and the tissues of the root took on a permanent aspect. 

 Microscopic examination showed the greatest changes in endodermis and 

 dermatogen. According to Patten and Wigoder (47), roots of V. Faba 

 were not only stunted by medium doses of irradiation, but they became 

 slightly bulbous at the tip. Lateral roots never developed in the stunted 

 X-rayed specimens. Roots of barley. Patten and Wigoder report, 

 seemed little affected by the rays. Since others (63) have found the 

 barley plant very ray-susceptible, this point should be investigated. 



Ancel (3), using 30 to 60 seeds in each culture, recorded, after a 

 certain exposure, the mean length of main root, rootlets, stem, and total 

 length of the rootlets. Rootlets which showed the greatest growth 

 depression seemed to afford the best test of the biological effects of 

 radiation. 



Cattell (9), who based results on 200,000 measurements of control 

 and irradiated wheat seedlings taken 48 hr. after irradiation, showed that 

 each of the 4 growing parts was affected to a different degree by the 

 same dose. If one takes the relative percentage reduction of the coleop- 

 tile (with reference to the growth of the control) as 1, and compares it 



