variation among plants due to the influence of 

 X rays and the variety of directions that the 

 changes took as well as the profundity of the 

 transformations. This permitted him to select 

 a large amount of material for experiments in 

 the creation of new forms. The new varieties 

 of wheat which he obtained by means of X rays 

 are already being tested. His experiments are 

 proof, par excellence , of the inaccuracy of the 

 concept that X rays produce only defective 

 changes, since completely viable new forms 

 appear that are in no way inferior, and some- 

 times are superior, to the forms from which 

 they are derived. 



Horlacher and Killough (1931) observed the 

 effect of X rays on somatic changes of cotton. 

 They observed mostly the appearance of dwarfs, 

 then branching, and fused cotyledons. In addi- 

 tion, they noted the appearance of wrinkled 

 leaves, whose form indicated that their various 

 parts developed at different rates of speed. All 

 the plants that received high dosages of radiation 

 (100 kv, 5 ma, with an exposure of 50 minutes, 

 and a distance of 17 cm) showed variegation of 

 color in some of their leaves. But plants re- 

 ceiving a smaller dosage also had variegated 

 leaves. It would be approximately true to say 

 that the number of variegated leaves was pro- 

 portional to the dosage used. There were two 

 types of variegated leaves: spotted ones and 

 those with yellow tissue at the corners. The 

 first type were of three colors: green, yellow, 

 and white. This condition of the tissues can 

 result, according to the authors, either as a 

 result of cytoplasmic changes, or of abnormal 

 conduct of the chromosomes due to the effect of 

 X rays, or of both. They consider the variega- 

 tion which shows up in the corners, or the 

 chimera type, most interesting. In their 

 opinion it indicates changes within the nucleus. 

 One plant from the yellow line produced only 

 green leaves; it was depressed in its develop- 

 ment and lay prostrate on the ground. The 

 authors consider it quite possible that all the 

 growing buds were killed except one, which 

 gave rise to the green shoot. Horlacher and 

 Killough assume that their experiments indicate 

 the possibility of progressive changes due to 

 X rays. They regard the appearance of green 

 leaves on a yellow line or changes from forked 

 leaves to normal ones as examples of such 

 progressive changes. These facts also indicate 

 that gene mutations took place in the somatic 

 tissues of the plants. 



De Mol (1933) irradiated flower bulbs of 

 hyacinth, variety "Moreno, " at the time when 

 the flower clusters were forming. Having 

 divided the bulbs into four groups he gave them 

 doses of 50, 100, 200, and 300 r respectively. 

 Among the plants that matured 49 different 

 anomalies were observed. Meiosis took place 

 normally, but the greater part of the pollen be- 

 came sterile. Sterility increased proportion- 

 ately with the dosage. 



Similar dosages were used on tulip bulbs, and 

 in the mature plants 40 anomalies were observed. 

 The anomalies, however, were chiefly in the 

 flowers themselves, which were irradiated 

 while in the flower [-forming stage]. Their 

 cells were slowed down in the process of cell 

 division, from which stems the disharmony of 

 their development. Summarizing the color 

 changes of the descendants, de Mol came to the 

 following conclusions: 1) changes in color are 

 most pronounced in those tulips which were 

 irradiated at a time when the process of cell 

 division was at its maximum, 2) apparently, 

 the intensity of irradiation was causally related 

 not to the qualitative changes in color, but to 

 the quantitative ones. 



With various doses of X rays and depending 

 on the time of irradiation (nearer or further 

 from the time of reduction -division) it was pos- 

 sible to find sometimes completely sterile 

 pollen, sometimes all the pollen seeds had 

 diploid and tetraploid numbers of chromosomes, 

 and sometimes the cells were double or quad- 

 ruple. 



MacArthur (1934) set up experiments with 

 seeds of tomatoes and spinach (also with hens' 

 eggs), but only the tomatoes gave interesting 

 results. Seeds (variety Vaughen Meerlina) 

 were divided into seven samples, 1000 seeds per 

 sample, one to be used as control and six for 

 irradiation (110 r, 7 ma, 25 cm from the tube, 

 without a filter). The irradiation lasted 9, 18, 

 27, 36, 46 1/2 and 60 minutes, the maximum 

 dose being equal to 14 1/2 erythema doses. 

 About 600 control and 1200 irradiated seeds 

 were planted. Among the latter were 8 yellow - 

 white plants with chlorophyll deficiencies which 

 died without forming leaves. From 346 descend- 

 ants 43 had clear mutational changes (12.4%). 

 The number of mutations was proportional to 

 the dosage: those irradiated for 60 minutes 

 gave 15.9% mutants, those irradiated for 46 1/2 

 minutes gave 8. 3%. It is possible that higher 

 dosages would have had still greater effect 

 since the lethal boundary had not yet been found. 

 Changes showed up in the coloring of leaves and 

 stem, leaf form, height, branching, and rate 

 of growth. The majority of mutations consisted 

 of changes in the plastid mechanism: the coty- 

 ledons and first leaves had white or brown 

 splotches or streaks, or they were dark reddish 

 purple and shiny. Some underwent changes in 

 the shape of the cotyledons; 25% of their de- 

 scendants had polycotyledonous sprouts. The 

 shape of one X-ray mutant leaf resembled a 

 potato leaf, another mutant had curly leaves. 

 There were also two ultradwarfs, but they had 

 very different leaves. Several X-ray mutants 

 were noticeable only by their strongly retarded 

 and squat size. The most interesting new forms 

 were those whose top growth was partially 

 affected. Further investigations showed that all 

 mutations were recessive and single -factored, 

 more than half of the mutations were lethal, 



39 



