X-RAY EFFECTS ON GREEN PLANTS 967 



increased chlorophyll content. A group of sunflower plants rayed 3 min. 

 l)lossomed before the controls, indicating a shortening of life history by 

 the treatment. The conditions which the authors believe necessary for 

 such stimulative action are: the use of metallic screens, high voltage and 

 low amperage, and brief exposures. The total dosage for stimulation 

 does not much exceed 100 r-units. Even with the 1-mm. aluminum 

 screen, sunflowers given 150 to 200 r-units were overtreated. Optimum 

 growth occurred with about 115 r-units (3 min.). 



Investigations will now be given in which there has been found no 

 case of stimulation but always definite retardation. Capizzaro (8) 

 writes of temporary retardation of development occurring in proportion 

 to the time of irradiation and the quality of rays produced. Maisin and 

 Masy (39) obtained marked inhibition of growth when seeds of Pisum 

 sativum were exposed to X-rays. No lethal effects occurred with the 

 dosage employed. Glocker, Hayer, and Jtingling (11) found retardation 

 in growth of bean seedlings, particularly with softer rays (those produced 

 with lower voltage). 



Johnson (20a) irradiated numerous seedlings of tomato, sunberry, 

 sunflower, and two species of vetch with light X-ray doses which have 

 been reported to cause stimulation. After the plants had grown for 

 periods of sufficient length for the detection of any stimulative action, 

 comparisons were made of green and dry weights of experimental and 

 control plants. In two groups, root growth was considered as well as 

 growth of tops, and in two groups stem height was also considered. 

 In all cases studied a considerable number of treated plants with a like 

 number of controls were grown under the same environmental conditions. 

 No increased growth of experimental plants over the controls was evi- 

 denced by measurements either of height or of green- and dry-weight 

 determinations. 



Horlacher and Killough (16) found that X-rays produced a dif- 

 ferential growth rate in cotton seedlings. When a few days old, the 

 rayed seedlings could be divided into three classes : normal, intermediate, 

 and dwarf. 



Schwarz, Czepa, and Schindler (58), working with wheat, horse bean, 

 and lentil, found no stimulation with weak doses. In their experiments 

 with wheat, 20 seedlings were used in each of 12 groups and lots were 

 exposed for 5, 10, 20, 30, and 45 sec, and for 1, 3, 5, 10, 20, 40, and 50 

 min., respectively. Nine groups of controls were measured. The 

 authors found that seedling length increased when the time of irradiation 

 was 5 min. or less, after which it decreased. But when the controls 

 were measured, it was found that the highest value for the control was 

 1 mm. higher than the greatest measurement for the irradiated. Essen- 

 tially the same results were obtained with the horse bean and lentil. 

 Their conclusions agree with those of Ancel (4) who, from numerous 



