A wide variation of the wave length (40, 56, 

 81, 98, and ll6kv) did not alter the frequency of 

 mutations. The application of ultrasoft or grenz 

 rays (10 or even 7 kv) also induced mutations. 



And, finally, it was Stadler who made the 

 interesting observation that when oats and wheat 

 are irradiated, their frequency of mutations is 

 considerably lower than that of barley. The 

 probable reason for the low mutability of oats 

 and wheat lies in the reduplication of their genes, 

 and in the large number of chromosomes. If 

 the dominant gene A is present in triplicate, 

 then a recessive mutation will not show up. The 

 coincidence of three homologous genes under- 

 going mutation at the same time is so rare that 

 such occurrences can be overlooked. 



As for Goodspeed's works, they will be de- 

 scribed in large part in the third chapter. Let 

 us examine here only those experiments in 

 which he describes the morphological effect of 

 X rays. Irradiated tobacco seeds and sprouts 

 revealed in their development a whole series of 

 morphological changes in plant size and leaf 

 size and shape. It should be noted that these 

 variants were not always recessive in character. 

 Crossing one of the offspring of the irradiated 

 plant with normal plants, Goodspeed established 

 that in Xg five plants out of 50 revealed identical 

 and significant changes in the form and coloring 

 of the flowers; at the same time one of the plants 

 reestablished the parental type. From the 

 descendants of these plants a stable type ("127") 

 developed. Cytological investigations have 

 shown that by reorganization of the chromosome 

 elements, or of the chromosomes themselves, 

 a true selective type can arise that is completely 

 different from the original type. Thus, Good- 

 speed was one of the first investigators to show 

 that X rays constitute a factor by means of 

 which we can create new types of higher plants. 

 For lower plants this had been demonstrated 

 previously, specifically by Nadson's experi- 

 ments with Saccharomyces cereviseae .* 



The experiments of Stubbe were set up on an 

 even greater scale. Beginning with 1927, he 

 has been publishing his interesting works on the 

 effects of X rays on snapdragons. His experi- 

 ments constitute the continuation of Baur's 

 investigations of 1927*, 1930, and 1932, which 

 had been started for the express purpose of 

 obtaining mutations experimentally. The initial 

 material in Baur's experiments was strain 50 

 (del), which was derived from a plant obtained 

 in 1907. From that time this strain had been 

 allowed to multiply by means of continued self- 

 pollination. Consequently, this was a homozy- 

 gous strain. Investigators watched carefully 

 for natural mutations to arise during this whole 

 period. During the 20 years, 30, 000 individual 

 plants were obtained, including only 16 indi- 

 vidual cases of observed mutations, which 

 corresponds to a mutation frequency of 0. 5%. 

 As irritants [mutagens] Baur used various 



chemicals with and without the aid of centrifu- 

 gation. He succeeded in raising the number of 

 mutations to 2. 1%. Stubbe' s experiments were 

 much more successful. He used short-wave 

 radiation [ionizing and ultraviolet]. His works 

 will be described later on since they were con- 

 tinued until 1940. 



The experiments of MacKay and Goodspeed 

 in 1930 were performed with cotton, the flowers 

 of which were emasculated the day before 

 blooming, and the unopened anthers placed in 

 gelatine capsules. On the following morning the 

 anthers were subjected to radiation after which 

 the pollen was placed on the stigma of the emas- 

 culated flowers. The irradiation was performed 

 for periods of 4, 8, 12, 16, and 25 minutes (50 

 kv, 5 ma, 10 cm distance from the tube, with an 

 aluminum filter). From 17 ripe fruit, 311 seeds 

 were obtained, from which only 21 plants germi- 

 nated. The authors point out that the greater the 

 dosage, the smaller the number of seeds per 

 fruit. The external morphology of the seeds was 

 changed by comparison with the control plants. 

 The authors observed a rounding and a deforma- 

 tion of the stigmas, anastomosis of the veins of 

 the leaves, special leaf forms, fasciation and 

 thickening of the stems, imperfect flowers, and, 

 finally, dwarf size. From their experiment the 

 authors came to the conclusion that X rays could 

 be used to induce quantitative and qualitative 

 changes in the hereditary matter of cotton. 



Sapegin, in 1930, presents in his article the 

 results of experiments with wheat. The maxi- 

 mum effect was obtained from a dose at 130 

 kilovolts, 5 milliamperes, 30 centimeters from 

 the tube, with 1 millimeter aluminum filter and 

 exposures from 20 to 30 minutes (i. e. , around 

 2500 r). Four hundred and eighty plants from 

 several pure lines of winter and spring wheat 

 were subjected to irradiation. Asa consequence, 

 thousands of changed descendants were obtained. 

 A large part of the descendants were chromo- 

 somal aberrants. Many of the X-ray mutants 

 were defective, but strong fertile specimens, 

 which were of practical interest, were also ob- 

 served. Eventually (in 1934*) Sapegin traced the 

 offspring of the first generation of the X-ray 

 mutants. He pointed out the variation of this 

 generation and the relative homogeneity of the 

 second generation. The author established spe- 

 cial characteristics of mutation for each bio- 

 logical type. Karyological analysis carried out 

 for wheat, tobacco, and Jimsonweed [ Datura 

 stramonium ] showed that the X-ray mutants, for 

 the most part, are chromosomal aberrations. 

 Marked anomalies of the chromosomes lead to 

 significant changes of the phenotype, while small 

 changes in the nucleus are conducive to small 

 changes in the phenotype. The relationship be- 

 tween the phenotype and fertility is even closer. 

 Sapegin explains the homogeneity of the second 

 generation by low fertility. The X-ray mutations 

 of winter varieties are very peculiar; they pro- 

 duce a large number of speltoids: "Zemka" — 



37 



