56 MUTATION AND PLANT BREEDING 



of changes initiated when ionizing radiations impinge upon living 

 cells. Partly because of the space limitations and partly because of 

 the large numbers of more detailed considerations of this topic already 

 published elsewhere, this review will be brief and aimed primarily 

 at readers looking for a fairly short and elementary survey of the 

 subject. In the sections concerned with radiation effects, I shall 

 emphasize chromosome breakage and derived phenomena rather than 

 intragenic changes, partly because of my greater personal interest 

 in this topic and partly because induced mutation in the more 

 restricted sense is being covered in this Symposium by others who 

 are better qualified to discuss it than I am. 



I. Ionizing Radiations and Their Characteristics 4 



By definition, ionizing radiations have the ability to produce 

 ionization (ion pairs) when they interact with matter. With the 

 removal of each electron a positively charged atom or molecule is 

 left. In addition to the process of ionization, energy transfer also 

 occurs by a process known as excitation. The major effect of the 

 ionizing radiations is considered to result from their ability to ionize 

 and to rupture chemical bonds. In contrast to the ionizing radiations, 

 ultraviolet, except for the very shortest wave lengths, does not have 

 the capacity to ionize but transfers energy primarily through the 

 process of excitation. The fact that ultraviolet radiation does produce 

 genetic effects (both mutation and chromosome breakage) is adequate 

 evidence that excitation itself can produce a biological effect. 



Ionizing radiations include two different types: (a) electro- 

 magnetic radiations which include X and gamma rays, and (b) the 

 so-called particulate radiations (alpha, beta, protons, deuterons, etc.). 

 Neutrons are generally classed as ionizing radiations, but it should 

 be pointed out that they do not ionize directly but indirectly through 

 the nuclear reactions which occur following their absorption by 

 atomic nuclei. After absorption they may emit a, proton, electron, or 

 7 rays, or disintegrate. 



Cosmic radiation is not of any serious interest to the plant 

 breeder so far as artificially produced mutations are concerned but no 

 doubt plays a part in the production of so-called spontaneous muta- 



tor definitions of physical terms used, see Glossary of Terms in Nuclear Science (111). 



