X-KAY EFFECTS ON GREEN PLANTS 971 



to elapse between irradiation and germination lowered the percentage of 

 germination. 



Many investigators (1, 13, 15, 17, 20, 26, 30, 68) have noted the 

 increased susceptibility of seeds to X-rays when they have been soaked 

 or have started to germinate. Stadler (63) claims that dormant barley 

 seeds will withstand 15 to 20 times as heavy doses as germinating seeds. 

 Water absorption by both seeds and seedlings tends to increase sus- 

 ceptibility to X-rays, according to Petry (48). 



Russ (55) states that a single large dose of X-rays has a greater effect 

 than the same amount of radiation given by repeated small doses. He 

 believes that the process of repair can more easily cope with feeble radia- 

 tion acting for a long time than with intense radiation acting for a short 

 time. Arntzen and Krebs (6) find that a single full exposure produces a 

 stronger biological effect on peas than the same dose administered in a 

 series of "divided doses." 



Lessening the effect of subsequent doses by giving a preceding lighter 

 one is called " radiophylaxis " by Ancel and Lallemand (5) and Lallemand 

 (35, 36). A light dose followed by a medium one was found to produce 

 less injury to root tips of bean than the application of one dose of medium 

 intensity. These investigators, after eight repetitions of experiments 

 involving 40 seeds each, concluded that the cell reaction to the first dose 

 had rendered it less sensitive to the action of doses subsequently given. 

 An interrupted dose had less effect than the same dose given all at one 

 time, owing to the existence of a radiophylactic reaction. 



Effects of radiation are thought by some to be influenced to a certain 

 extent by temperature. Wemhart (68) states that the reaction of the 

 plant varies with the temperature and humidity. Promsy and Drevon 

 (50) find that with moderately high temperature irradiation favors 

 germination and accelerates the development of plants. Less harm is 

 manifested in irradiated material when germination takes place at 

 20° to 25°C. than when at 10° to 14°C., according to Ancel (26). 



Weber (66) reports that forcing resting buds of Syringa vulgaris may 

 be accomplished by using doses ranging from 26 to 150 H. The former 

 dose will cause forcing after a resting period. The use of higher doses will 

 produce a necrosis in the basal region of the bud causing the bud scales to 

 fall. Reiss (52) agrees with Weber (66) that Syringa buds are forced 

 if the irradiation is not too strong. Shoots are injured if the dose is 

 beyond the optimum. In view of the controversy concerning the 

 stimulatory effects of X-rays, studies of this nature should be repeated 

 and the exact dosages determined which produce the so-called stimulation. 



A review of the conditions affecting the ray-sensitivity of plants 

 indicates that there is considerable resistance to the rays, for even when 

 very heavy doses are given seeds, the embryo develops to some extent. 

 Not only is there variation among different species of plants, but members 



