INDUCED CHROMOSOMAL ALTERATIONS 1289 



In ^•ariolls races derived from X-radiation of Tabacum var. purpurea 

 there is evidence of occurrence of a considerable number of triploid plants, 

 but only three of them have been checked cytologically. They doubtless 

 owe their origin to the chromosomally unbalanced condition of the 

 parent plants, such unbalance being known to favor the formation of 

 somatic gametes. Two of the recognized triploids appeared in progenies 

 of "deformed" (cf. Goodspeed and Avery, 19), in which it was found 

 that abnormal meiotic behavior resulting from induced chromosomal 

 alteration gave rise to such gametes. The third, apparently, came 

 from an induced trisomic (Goodspeed, 13a). A vigorous basal lateral 

 of a highly abnormal, small, weak Xi plant of Tabacum var. Maryland 

 Mammoth proved to be tetraploid (Goodspeed, 13a) and, thus, had 

 its origin in a tetraploid cell lineage in that sector of the stem from 

 which the shoot arose. Stein (45, et seq.) has shown that polyploid 

 nuclei are found in all tissues of Antirrhinum plants possessing the 

 induced, genotypically determined "phytocarcinome complex" and 

 also in certain cell layers of chimeral plants obtained by radium treatment 

 of seed. The tetraploid shoot, above referred to, doubtless was a by- 

 product of treatment through the effects on somatogenesis of the chro- 

 mosomal alterations initially induced, since such shoots have not been 

 reported as occurring on untreated plants. However, tetraploid cells 

 and sectors are to be found not infrequently in root tips of untreated 

 Nicotiana species and probably they occur in the stem also. Again, 

 a tetraploid branch on a normal plant would readily escape detection, 

 whereas in this case it was conspicuous because of its greater vigor as 

 compared with other branches of the weak, variant plant. A second 

 tetraploid occurred in an Xh culture of A^. sylvestris, from the cross of a 

 plant heterozygous for a recessive "bunchy" type with the dominant 

 control. That this tetraploid arose from a suspended early zygote 

 mitosis rather than through union of somatic gametes was shown by the 

 fact that the progeny exhibited the tetraploid segregation ratio for 

 "bunchy" — one tetraploid "bunchy" in 36 plants. The occurrence 

 of this sylvestris tetraploid is thus not to be related to primary or 

 secondary effects of irradiation. Stadler (43) reports no tetraploids in 

 1000 plants, or 600 selfed progenies, of maize. Randolph (36), on the 

 other hand, has secured a large number of polyploids by heat treatment 

 of zygotes and proembryos of maize. 



The available evidence, just described, indicates that irradiation in 

 its primary effects is not a source of the haploid or polyploid condition 

 in higher plants. On the other hand, secondary effects based upon initial 

 chromosomal alterations may be expected to induce production of 

 somatic gametes, tetraploidy, and possibly also the proliferation of the 

 egg to give haploidy. 



It is clear that high-frequency radiations, through their primary or 

 secondary effects, increase the normal incidence of both nonconjunction 



