466 BIOLOGICAL EFFECTS OF RADIATION 



an open ionization chamber, the results are found to agree within 2 per 

 cent. By this use of the survival curve the relative effectiveness of equal 

 doses of different wave-lengths can be determined with great precision. 



The first investigators who employed small test objects in the study 

 of the wave-length problem were handicapped by the lack of a standard 

 unit by which to express the quantity of radiation which they used, and 

 by the lack of accurate dosimeters. It is not surprising, therefore, that 

 there was little agreement in their results. But as the means of measure- 

 ment have improved the diversity of opinion has grown less, until at the 

 present time nearly all are agreed that equal doses of all wave-lengths 

 within a very wide range produce equal quantitative results. 



First to demonstrate this was Wood (40, 41) who used mouse tumor 

 tissue as a test object. The finely minced tumor particles were exposed 

 to homogeneous beams having wave-lengths of 0.21 and 0.71 A and then 

 inoculated into healthy animals. The criterion of effect was the failure 

 of the particles to grow in the host animal. Since the tumor strain which 

 he used always "takes," any failure to grow^ was due to the lethal action 

 of the rays. The doses were measured with an open ionization chamber. 

 Although the degree of variability in this material is fairly large, the 

 results of numerous tests, involving thousands of animals, clearly showed 

 that there is no constant difference in effectiveness between these two 

 qualities of radiation. 



Evidence on this question derived from the study of the reaction of 

 various seedlings is still somewhat confused. The earlier literature which 

 is extensive may be passed over because the experiments lacked precision 

 both in dosage and in biological technique. The usual procedure is to 

 radiate germinated specimens whose roots are of about the same length. 

 The criterion of effect is the increase in the length of the irradiated roots 

 as compared with that of the controls, or else, the proportion of seedlings 

 which cease to grow after a definite time. The variability in this material 

 is partly due to methods of handling. Specimens that have been soaked 

 in water for some hours to start germination are more sensitive than those 

 soaked for only a short time. Then also, sensitivity varies with the 

 length of the root at the time of radiation. Injuries to the root tip are 

 not uncommon. This leads Henshaw (16) to reject from each sample 

 one-third of the irradiated seedlings whose root length appears to depart 

 most widely from the average. Recently Glocker and his collaborators 

 (13, 14) have made experiments on a variety of seedlings, using appro- 

 priate culture methods and carefully measured doses. They find that the 

 dose needed to kill 50 per cent of the samples of the horse bean, Vicia Faha, 

 and of mustard and sunflower seedlings is the same whether the radiation 

 is hard, soft, or medium (0.18, 0.56, 1.54 A). But Lachmann and 

 Stubbe (24) who also used Vicia conclude that hard rays are much more 

 effective than soft. However, they failed to take into account the fact 



