GENETIC AND CYTOLOGICAL EFFECTS 253 



at shorter wave lengths (Durand el al., 1941). The method, while useful 

 in ascertaining the kinds of gene and chromosome changes which may be 

 induced by ultraviolet in the mature spermatozoa, is not suitable for the 

 accurate (juantitative determination of dosage-mutation frequency rela- 

 tions and wave-length dependence yields. The chitin is of uneven trans- 

 parency and the intervening tissues vary in thickness and position, with 

 the result that considerable differences in the amount of radiation reach- 

 ing the spermatozoa must be expected from one individual to another. 

 The variations in penetration of radiation can be lessened somewhat by 

 the use of light-colored mutant stocks, but there remains the difficulty 

 of accurately determining the amount of energy absorbed by the se.x cells 

 at the site of effective action. Also, the practical limits of dosage are 

 determined largely by the tolerance of the adults to physiological damage 

 leading to sterility. Demerec et al. (1942) have reported induced sex- 

 linked recessive lethals in frequencies as high as 50 per cent, but such high 

 proportions of mutations are a rare exception. The usual frequencies of 

 recessive lethals obtained from optimal abdominal exposures range from 

 0-5 per cent, with a wide variation among similarly treated flies. 



The higher plants offer somewhat better technical possibilities for 

 studies of the genetic effects of ultraviolet radiation. The male cells are 

 of minute size, readily accessible, and free from extraneous tissues. 

 Three types of cells have been used: (1) swimming spermatozoids, (2) 

 pollen grains, and (3) generative cells in developing pollen tubes. 



Swimming spermatozoids of the liverworts, for example those of 

 Sphaerocarpus donnelUi, are particularly favorable objects for ultraviolet 

 studies, since they consist essentially of a naked nucleus approximately 

 0.5 n in diameter (Knapp, 1938). Absorption of the radiant energy by 

 extranuclear materials is at a minimum, and treatment of the cells can 

 be carried out in water. Their use in the determination of dosage rela- 

 tions has not been exploited to the extent warranted by the excellence of 

 the material. 



Exposure of the male cells of angiosperms can be readily made by treat- 

 ing a monolayer of loose, dry pollen. Pollens of maize, Antirrhinum, and 

 Gasteria have been successfully employed in ultraviolet studies, the 

 irradiated grains being used to fertilize untreated plants. The maize 

 pollen grain has both sperm cells fully formed at the time of anthesis. 

 One sperm fertilizes the egg to produce the zygote while the other unites 

 with the fusion nucleus to give rise to the endosperm. Maize also pos- 

 sesses the added advantage of a goodly number of clear-cut endosperm 

 marker genes whose presence or absence (or mutated state) can be 

 directly determined by examination of the kernels produced on ears pol- 

 linated by the treated pollen. These markers facilitate the collection of 

 massed data and provide a convenient measure of genetic effectiveness 

 when comparative studies are being made of wave-length and dosage 



