CHAPTER 2 



CHANGES OF MORPHOLOGICAL CHARACTERISTICS 



From a theoretical as well as from a practical 

 point of view the morphological changes caused 

 by X rays are no less important than the physio- 

 logical ones. 



Of particular interest are the hereditary 

 changes, i.e., mutations. However, we also 

 should not overlook such changes as show up 

 for only one or two generations, i. e. , varia- 

 tions or modifications, since in agricultural 

 engineering the application of X rays is quite 

 conceivable as a technique by means of which 

 definite changes can be obtained. We shall say 

 a few words about terminology in order that we 

 will not have to return to this subject later. 



The process which causes hereditary changes 

 to arise as a result of the action of X rays we 

 are going to call X-ray mutation (roentgeno- 

 mutation); the mutated individual we shall call 

 the X-ray mutant (roentgeno-mutant); the indi- 

 vidual whose changes are not of a hereditary 

 nature we shall call the X-ray variant (roent- 

 geno-variant). Besides, it should be pointed out 

 that contemporary genetics distinguishes be- 

 tween two types of mutations: I) chromosome 

 aberrations and 2) gene mutations. 



The study of mutations in objects that have 

 been most thoroughly investigated (corn and 

 snapdragons) has shown that all plant organs, 

 as well as their properties, can be changed 

 under the influence of X rays. 



The scope and great range of the changes 

 can be seen from the survey of literature that 

 follows. These changes range from great 

 interruptions of development and even the 

 appearance or disappearance of structures to 

 insignificant changes invisible to the naked eye. 

 And, of course, there are gradations. The 

 greatest changes produced by irradiations are 

 lethal ones, i. e. , the kind where death sets in 

 during one of the stages of ontogeny. These 

 are followed by sublethal mutations. These two 

 groups of mutations are deviations from the 

 norm, and since they are very obvious, they 

 have given rise to the popular notion that irradi- 

 ation produces monstrous, defective, anomalous, 

 or pathological changes. This popular opinion 



points to weakening of the general constitution, 

 which shows up in the disharmony of the devel- 

 opment of the organs, shortening of the life 

 span, decrease of fertility, and loss of resist- 

 ance to the environment. However, by now 

 enough facts have accumulated which indicate 

 the existence of mutations that increase the 

 viability of plants on the one hand, and which 

 create new, and more valuable, forms of plants, 

 on the other. In general, as the methodology 

 of induction of mutations has improved, it has 

 turned out that often such mutations appear, in 

 our case X-ray mutations, which cannot by any 

 stretch of the imagination be considered to be 

 anomalies or monsters. It is a different mat- 

 ter, however, with the establishment of the 

 mutation changes, since the smaller the devia- 

 tion from the norm, the more difficult it is to 

 detect. Starting with a definite boundary, we 

 can speak of small mutants (Baur, 1933*), which 

 occur much more frequently than large mutants, 

 which (just as the former do) change the mor- 

 phological characteristics and physiological 

 properties of plants. The occurrence of these 

 changes often leads even to an increase in the 

 viability of the mutants. 



From what has been described, the tendency 

 of contemporary experimenters to look for 

 factors ensuring the appearance of mutants is 

 understandable. With this aim in mind, various 

 chemical and physical factors were tested. By 

 far the most brilliant achievement in this direc- 

 tion was Muller's experiments, the results of 

 which were reported at the International Con- 

 gress of Genetics in 1927. Muller showed that 

 the frequency of mutations in Drosophila melano - 

 gaster is greatly enhanced when they are ex- 

 posed to X rays. The year 1927 can be consid- 

 ered to mark the beginning of radio-genetics 

 and of exact investigation of mutations. It is 

 rare in the history of science to see any branch 

 of the natural sciences seized upon by so many 

 investigators and developed so systematically 

 as was the area of experimental investigations 

 of mutations induced by short-wave [ionizing?] 

 radiation. In his first experiments, Muller 

 showed the existence of a definite relationship 

 between the dose and the frequency of mutations 

 and was able to draw general conclusions 



33 



