270 RADIATION HIOLOGY 



lion may Ix' raised concerning the relali\c resistance of the chromosomes 

 to breakage at the 0- and 1 -In- periods. At these time periods the genera- 

 tive nuclei have not, as a rule, passed from the pollen grains into the pollen 

 tubes, and some uncertainty exists as to the degree of absorption of ultra- 

 \iolet by the heavily pigmented pollen wall. At the 2-hr period, however, 

 the nuclei are in the pollen tube, where absorption by overlying materials 

 is at a minimum. After the 10-hr period the fretiuency of induced 

 aberrations does not exceed that found in untreated nuclei. This period 

 corresponds roughly to late prophase. 



In addition to the terminal deficiencies induced by ultraviolet in the 

 pollen tube chromosomes of Tradescantia, there is also found a type of 

 aberration which, for want of a more definite term, has been called an 

 " achromatic lesion " (Swanson, 19-40). This type of aberration is induced 

 by X rays also. The lesions extend completely or partially across the 

 chromatid in the form of a nonstainable gap. Their fre(iuency increases 

 with increasing dosage. Whether they represent incompletely separated 

 deficiencies, interstitial losses of chromatin, or merely separated coils 

 within the chromosome is not known. Since many of them extend only a 

 part of the way across the diameter of the chromatid, a large subjective 

 error would be involved in any determination of frequency, and for this 

 reason they have been omitted in the tabulated data. 



The nature of the ultraviolet-induced deficiencies in the pollen tube 

 chromosomes of Tradescantia suggests that, structurally at least, they are 

 comparable to the fractional endosperm deficiencies in maize even though 

 the changes are induced in dissimilar types of nuclei. Each involves the 

 loss of a portion of a chromatid. No aberrations were found in Trades- 

 cantia, however, which involved both chromatids, and which would corre- 

 spond to the entire endosperm deficiencies. Whether this difference can 

 be ascribed to differences in the nuclei studied, to their different states of 

 chromosome condensation, or to some unknown factor cannot be answered 

 at present. ]Muller (1941) suggests that the preponderance of fractional 

 endosperm deficiencies in maize treated with ultraviolet may result from 

 a mutational process initiated in a single-stranded chromosome but 

 delayed in completion until the chromosome has doubled, the effect being 

 restricted ordinarily to only one of the two chromatids. This hypothesis 

 appears unnecessary in light of the Tradescantia data. Since half- 

 chromatid aberrations are found occasionally in treated pollen tube 

 chromosomes, the chromosomes must have at least four strands, and 

 chromatid deficiencies must therefore involve the fracture of t\vo half- 

 chromatids at the same locus. If the chromosomes of the sperm cells of 

 maize pollen have only two strands, the loss of both chromatids by simul- 

 taneous breakage to give rise to entire endosperm deficiencies becomes 

 understandable. 



P'igure 7-3 illustrates the effectiveness of X rays (370 r) in inducing 



