CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 367 



demonstrate the storage phenomenon and other after-effects of irradi- 

 ation with seeds. If the ESR methods enable the detection of the 

 same kinds of events that initiate detectable biological injury, it 

 seems apparent that the biological system has a greater resolving 

 power than the physical system by several orders of magnitude. In 

 this connection, because the production of some reactive entities may 

 be dose-dependent, there is a distinct need for the physicist and the 

 biologist to maintain the closest liaison in all of their work on pre- 

 and post-irradiation effects. Only in this way can it be certain that 

 the biological significance of a particular event observed with elec- 

 tron spin resonance methods will be placed in proper perspective. 



Material and Methods 



In the entire series of experiments reported, dormant seeds of 

 Himalaya barley were used as the test material. In all instances they 

 were of uniform size from the same harvest year and had an embryo 

 water content of about 4 per cent before being used experimentally. 



The X-rays used in the studies were unfiltered and were gen- 

 erated with a constant potential machine operated at 100 KV and 7 

 ma. For irradiation, the seeds were placed on a turntable 9.7 inches 

 from the target, with the embryo oriented toward the beam. Under 

 these conditions the seeds received a dose of approximately 3,000 r 

 per minute. 



In different experiments the seeds were subjected to a variety of 

 pre- and post-irradiation treatments before germination. To elimi- 

 nate confusion, these particular conditions are indicated in the experi- 

 mental results. However, in all experiments, hydration immediately 

 before the initiation of germination was accomplished by steeping 

 the seeds for 45 minutes in boiled distilled water through which 

 either oxygen or nitrogen was continuously flushed. This was done 

 to eliminate a variable that had previously been demonstrated to be 

 of profound significance (5, 10). Furthermore, on all occasions where 

 seedling height determinations were made, the seeds were grown in 

 petri dishes in a controlled environment room. Measurements were 

 made to the nearest millimeter after either 6 or 7 days of growth, 

 depending on the objectives of the particular experiment. 



To determine the frequency of interchanges in the irradiated 

 generation, cytological analyses were made at the first meiotic meta- 



