254 RADIATION BIOLOGY 



Illations. In both maize and Antirrhinum, mutation data may bo 

 ()btaiiu>d throuj^h tho detection of sefi;re}j;atin}i; characters in F-j popula- 

 tions, (instrria has been used only tor the study of induced chromosomal 

 changes appearing in the cells of I*\ embryos. 



Compared with the spermatozoids of Hverworts, the pollen grains of 

 angiosperms are relatively large, those of maize, for example, being almost 

 100 n in diameter. AVhen comparisons are being made of the relative 

 effects of dilTerent wave lengths or of different doses of the same wave 

 length, it becomes necessary to take into account the factor of internal 

 filtration, since the energy incident at the surface of the pollen grain is 

 very greatly reduced by absorption in the extranuclear material. Inter- 

 nal filtration varies greatly with the wave length of radiation employed 

 (ll)er, 1939; Stadler and Uber, 1942), and failure to correct for these 

 ditYerences of penetration may lead to gross error in wave-length com- 

 parisons. The filtration factor can be roughly calculated, as Stadler and 

 Uber have shown, but the difficulties involved stress the need for better 

 genetic materials in this area of investigation. 



Cytological studies of ultraviolet-irradiated chromosomes have been 

 carried out in the Fi progeny of maize, Gasteria, and Drosophila. The 

 chromosomes of surviving Fi individuals represent a selected group from 

 which all inviable aberrations have been screened. Through genetic 

 technicjues, the types of chromosomal rearrangements induced by ultra- 

 violet may be inferred without cytological examination, but there remains 

 the possibility that certain aberrations may be eliminated after the pas- 

 sage of several cell generations. The pollen-tube technique overcomes 

 this difficulty in that it permits a direct examination of irradiated chromo- 

 somes before the elimination of inviable changes can take place. The 

 technique involves the culturing of pollen tubes on an agar-coated slide, 

 with sucrose or lactose added to the agar as a carbon source. The 

 generative cell, after passing from the pollen grain into the tube, is 

 covered only by a thin cytoplasmic layer and a thin tube wall. Since 

 the pollen tube is narrow (approximately 5 ^ in Tradescantia) , the amount 

 of radiation absorbed before it reaches the nucleus is not great. The 

 chromosomes, undergoing mitotic division in the tube, may therefore be 

 readily exposed to ultraviolet, and an analysis of structural changes may 

 be made at metaphase by blocking the division with colchicine (Swanson, 

 1940, 1942). The method, as first employed with Tra(l(t<can(ia, has been 

 materially improved by Bishop (1949). Certain limitations in the tech- 

 ni<iue must be recognized, however, if the derived data are to be logically 

 compared with those obtained from other organisms. In the first place, 

 cytological analysis is made on the heavily condensed metaphase chromo- 

 somes; small aberrations such as interstitial deficiencies, if present, are 

 quite likely to pass unnoticed. Second, the chromosomes cannot be 

 maintained and studied beyond the metaphase stage. Any aberrations, 



