EFFECTIVENESS OF X-RAY WAVE-LENGTHS 467 



that the latter radiation was largely absorbed in the upper layer of the 

 root tips, the lower portion receiving only a fraction of the total dose. 

 When a correction is made for absorption, the effects of both radiations 

 are found to be about equal. 



A few experiments by Glocker and others have been made on certain 

 algae (26) and on yeast (12). In each case undivided cells were irradi- 

 ated, and, after incubation, the proportion of individuals which had 

 divided only once or not at all was determined. These were reckoned as 

 injured. The results indicate that both types of cells are much more 

 affected by soft rays (1.54 A) than by rays of medium hardness (0.56 A). 

 Very large doses were given because it is difficult to check cell division 

 completely in unicellular organisms and in fertihzed eggs. For example, 

 fertilized but uncleft frog eggs may be given a dose of 72,000 r and yet 

 divide at least once. But 400 r, given at the same stage, suffice to cause 

 death when gastrulation begins. Because of the somewhat indefinite 

 criterion of effect, and because both yeast and algal cells are very sensitive 

 to external conditions, these results are not wholly convincing. 



On bacteria very little careful quantitative work has been done. 

 The necessity of using only those types which can be spread uniformly 

 on the plates, and of preventing multiplication during the period of 

 exposure has not been realized (43). Wyckoff (42), who used Bacillus 

 coli, finds that the biological action of radiations having a wave-length of 

 1.54 A or less is proportional to their measured ionization. But very soft 

 rays are less effective. 



Braun and Holthusen (2) and others have carried on extensive 

 studies on the wave-length problem, using Ascaris eggs as a test material. 

 Unsegmented eggs from a single worm are irradiated and, after four days 

 of incubation, are examined to determine the proportion of injured 

 embryos. When eggs are exposed to soft and hard rays of the same 

 intensity and for the same length of time the percentage of injuries in 

 both cases is alike within the hmits of experimental error. But Zuppin- 

 ger (44) holds that hard rays are in general more effective than soft rays. 

 He finds, however, that the effectiveness of the latter diminishes slightly 

 as the wave-lengths increase to 0.53 A, where it is at a minimum, and 

 then increases more rapidly with decreasing wave-length. Dognon (3) 

 on the contrary has stated that the soft rays are the more effective. But 

 he remarks (4) that if due allowance is made for absorption of the softest 

 radiations, the corrected doses indicate an equality of effect. 



In a very extensive series of experiments with Drosophila eggs, various 

 investigators have demonstrated the complete lack of any differential 

 action due to wave-length. From the softest rays, generated by 12 kv. 

 to very hard rays produced at 700 kv., the quantitative effect of equal 

 doses is always the same. The criterion of effect is the proportion of 

 eggs which live. These eggs are almost free from disturbing environ- 



