422 BREAKDOWN OF GENETIC SYSTEMS 



incompatibility groups in clonal cultivated forms of Prunus and 

 their non-occurrence in non-clonal populations shows that the 

 clonal forms are, in fact, inbred (D., 1928). Their functional 

 hybridity must therefore, in some measure, result from internal 

 change and not from hybridisation. This question suggests another 

 possibility of some interest in horticulture. Induced changes, of 

 the kind that will be considered next, likewise determine structural 

 hybridity and its concomitant sterility. Seed-sterility is an 

 advantage for many ornamental plants. Triploids are often selected 

 on this account. This property can now be produced artificially by 

 irradiation. 



(iii) Changes Induced by Irradiation. X-ray treatment was shown 

 to determine gene-mutation in Drosophila by Muller in 1927. The 

 natural rate of mutation was known accurately as a result of 

 Muller 's experiments and this rate could be increased 100 times or 

 more, by appropriate doses of X-rays. Experiment with gene- 

 mutation has now shown the aetiology of the changes induced with a 

 high degree of accuracy (cf. Stadler, 1930, 1931 ; Muller, 1934, 

 1935 ; Timofeeff-Ressovsky, 1934, 1935). Mutation can probably 

 be induced in all organisms, in all their tissues and at all stages of 

 development. It is known that the frequency of change is 

 independent of the wave-length used. It is merely proportionate 

 to the energy that can be made available in the nucleus. Thus any 

 short-wave radiation between ultra-violet of 2,500 A° and gamma 

 rays of o-ooi A° wiU determine mutation, but the long wave- 

 lengths are usually impracticable because most of the radiation is 

 absorbed before it penetrates to the nuclei and the absorption may 

 be fatal (Altenburg, 1934 ; Stadler and Sprague, 1936 ; Brittingham, 

 1936). 



The frequency of mutations induced in this way is, unlike that of 

 natural mutations, independent of the genotype or of the tempera- 

 ture. The rate of induced mutation, however, depends on the gene 

 concerned, although to a less extent than that of natural mutation. 

 Thus some of the " mutable genes " in Drosophila (Demerec, 1934) 

 and others of the least mutable genes in Zea according to Stadler 

 (1932) are scarcely affected by treatment. The rate of induced 

 mutation is influenced by anaesthesia and by the stage of develop- 



