1266 BIOLOGICAL EFFECTS OF RADIATION 



Another solanaceous form which has been extensively used as material 

 for genetical investigations, and in which genetic effects of irradiation 

 have been studied, is the tomato (Lycopersicum esculentum). Lindstrom 

 (28) has reported a study of variations appearing in the tomato following 

 radium treatment of seeds, growing tips, and young fruits. Five mutant 

 types inherited as simple monogenic recessives are described, three 

 affecting chlorophyll characters and two morphological characters. 

 Three of these arose in a pure line derived from a doubled haploid. In 

 addition, a dwarfed and sterile type, which appeared, apparently as a 

 recessive mutant, in the progeny of an irradiated plant of Lycopersicum 

 pimpinellifolium, is described. 



The genetic effects of X-rays on cotton (Gossypium hirsutum) have 

 been investigated by Horlacher and Killough (21, 21o, 22, 23). In a 

 preliminary account McKay and Goodspeed (30) have also reported 

 the occurrence of variations following irradiation in cotton, but the 

 genetic analysis of these variants has not yet been reported. Horlacher 

 and Killough treated dry seeds with heavy doses of X-rays. In the 

 following generation several mutations were found, including variations 

 of leaf color, leaf form, and type of plant. These authors have recently 

 submitted strong evidence for the occurrence of dominant "progressive" 

 mutations as a result of irradiation. In a family characterized by 

 "forked" leaf shape, a character inherited as a simple recessive (nn), 

 and in families heterozygous for forked, irradiation has repeatedly 

 induced the mutation of n to the normal allelomorph. Similarly in 

 the case of virescent yellow-plant color {vv), known to be a simple 

 recessive to the normal green, the treatment has induced mutation 

 to the normal allelomorph V. 



Mutations induced by X-rays in Antirrhinum majus have been 

 reported by Stubbe (51, 51a, 6, c). Spontaneous mutation in this species 

 had been intensively studied by Baur (2) who found in one line a mutation 

 rate of approximately 5 per cent, that is, 5 per cent of the plants tested 

 proved to be heterozygous for a gene not present in the ancestry of the 

 plant. The line used in Stubbe's experiments was apparently much 

 less mutable, the frequency of mutation in the untreated control being 

 about 1 per cent. About one-half of the spontaneous mutations affect 

 flower-shape characters, and it was found that mutations of this class 

 were not increased in frequency by the treatment. The mutations 

 observed in the treated series were largely mutations affecting vegetative 

 organs of the plant. Stubbe used X-rays of varying wave-length, 

 including the so-called grenz rays, and obtained mutation by treatment 

 of seeds, seedlings, and pollen, as well as young flower buds. 



The investigations by Stein (48, 48a, 49, 50) of the effects of radium 

 treatment in Antirrhinum majus also should be mentioned, although 

 these were concerned chiefly with the analysis of the direct effects of 

 irradiation on the plants treated, rather than with the permanent modi- 



