W. D. JACKSON 



showed'^ that germination and growth was decreased by storage after 

 irradiation. Nichols-" showed that aged seed had the same symptoms of 

 damage as irradiated seed and that aged seed was strongly affected by 

 post-irradiation storage. Jackson-^ reported similar damage in aged seed 

 treated with oxygen and in irradiated seed. Nilan-'* showed that low doses 

 of irradiation producing undetectable effects when the seeds were germinated 

 immediately, caused greatly increased effects when stored for four months 

 especially when stored in oxygen. Yagyu and Morris^" have studied the 

 post-storage effect in air after irradiation with X-rays and thermal neutrons, 

 both producing qualitatively similar results. Sire and Niian^^ found that 

 pre- and post-treatment with oxygen increased X-ray damage in Crepis 

 seeds, the frequency of micro-acentrics being increased disproportionately 

 by post-storage. A non-random distribution of breaks between chromosome 

 arms was found. This was not influenced by pre- or post-storage. 



The following experiments were aimed at partitioning the total effect of 

 irradiation on the basis of mechanism. 



It was thought that by comparing the effect of the irradiation of dry seed 

 in nitrogen with that in oxygen-nitrogen mixtures, a partition of the total 

 effect into direct and indirect action of irradiation could be obtained. By 

 comparison with the breakage due to oxygen alone, that part of the total 

 breakage due to the direct effect of oxygen as distinct from irradiation- 

 oxygen effects could be determined. The remaining fraction is the total 

 indirect effect or oxygen-irradiation interaction. This effect of oxygen 

 could be divided further into that portion due to the increase in the primary 

 yield at ionization centres and that portion resulting from a longer effective 

 life-span of the mutagen. The latter partition could be obtained by com- 

 paring effects after post-storage in various oxygen concentrations with those 

 after post-storage in nitrogen or those with no post-storage. 



Attempts have also been made to extract mutagenic substances from 

 dormant seeds. 



MATERIALS AND METHODS 



Various stocks of seeds of Allium cepa var. Br. Spanish, of different ages, 

 were examined for chromosome breakage. Stocks were examined when fresh 

 and re-examined after two and four years. This seed was used for comparison 

 with fresh seed of the same strain in irradiation experiments using soft 

 X-rays. The irradiation was produced by a Coolidge type tube operating 

 at a peak of 22 -4 kV at 55 mA, giving an intensity of 50 r sec~^ at 10 cm 

 from the target. The seed was irradiated in thin-glass phials in various 

 mixtures of oxygen and nitrogen. The dose was measured by a thimble-type 

 Victoreen meter placed in the phials. Target distance was kept constant 

 and dose was varied by changing the duration of the exposure. To prevent 

 overheating of the tube the dose was given in fractions of eight seconds \vith 

 10 seconds between fractions. Various doses from 0-8 kr to 6-4 kr were 

 used, most of the experiments being done at low doses of 1 -2 and 2-4 kr. 

 The temperature at the time of irradiation and during storage was 18 + 1°G. 

 The dry seed was stored before, during and after irradiation in various 

 oxygen-nitrogen mixtures (0:100, 20:80, 100:0), in sealed thin-glass phials. 

 The period of storage before irradiation was two weeks. Its purpose was to 



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