INDUCED MUTATIONS IN PLANTS 1269 



The genetic effectiveness of radiations of lower frequency than the 

 grenz rays has not been determined in plants. Between the grenz rays 

 and the ultra-violet is a broad band of frequencies now being explored 

 by physicists but not readily available for biological investigation. 

 The ultra-violet is so highly absorbed by intervening tissue that it is 

 difficult to apply significant intensities to the germ cells. Stubbe found 

 no effect of ultra-violet treatment of buds on the frequency of mutation 

 in Antirrhinum, but it is not certain that the radiation penetrated to 

 the germ cells tested. Mutation induced by ultra-violet radiation 

 has been reported in Drosophila (see Paper XXXIX). 



Mutation is induced similarly by beta rays and cathode rays. Stadler 

 (45a) reports mutations in barley induced by beta rays of radiothorium 

 and by cathode rays, and Kopf, according to Lindstrom (28), found a 

 mutation in the tomato following treatment with cathode rays. Beta 

 rays and cathode rays are radiations of similar nature, fundamentally 

 different from X-rays. They consist of streams of electrons moving 

 at high velocity. However, the secondary radiation produced within 

 the tissues by the absorption of X-rays is analogous in nature to beta 

 rays, while the secondary radiation produced by the absorption of beta 

 rays is analogous to X-rays. It is probable that the effective agent 

 in both X-ray and beta-ray treatment is the same. 



The relative effectiveness of X-rays of different degrees of hardness 

 has been investigated by Stubbe (51c). Treatments were applied to 

 the pollen of Antirrhinum majus, and the effect on mutation frequency 

 determined by examination of F2 progenies. The results are summarized 

 in Table 1. 



The percentage of mutations in excess of the control is somewhat 

 lower in the series treated with soft X-rays, but the differences are 

 obviously insignificant. 



The relation of radiation intensity to frequency of induced mutation 

 has been determined in several experiments. In barley mutation 

 frequency was roughly proportional to dosage, in experiments in which 

 both dormant and germinating seeds were X-rayed (44). A similar 

 relation was reported by Stubbe (516) for X-ray treatment of buds in 

 Antirrhinum, but the experiments of the same author with pollen treat- 

 ment of Antirrhinum (summarized in Table 1), show a very different 

 result. Here the frequency of mutation increases with increasing dosage 

 to 400 r, drops sharply at 800 r and 1600 r, and increases again at 3200 r. 

 For each grade of radiation the course of the dosage curve is similar, 

 although the numbers involved in the separate trials are too small to be 

 convincing when considered individually. The percentage of mutants 

 observed in the progeny of plants treated at 400 r was higher than that 

 in the progeny of plants given eight times as large a dose. To account 

 for this unexpected result Stubbe suggests that the mutations or chromo- 



