ALPHA PARTICLES 567 



from 20 to 100^. This advantage has been used by Zirkle (27) to compare 

 the effects of irradiation of a nucleated fraction of the cell with those of a 

 corresponding irradiation of an equal fraction lacking the nucleus. The 

 spore of the fern Pteris longifolia was especially favorable material for 

 this investigation because the nucleus (10/i in diameter) lies at the 

 periphery of the protoplast (38m in diameter) and its location can be 

 readily ascertained from its proximity to a Y-shaped set of sutures on the 

 spore wall. By interposition of a thin aluminum screen the penetration 

 of the alpha particles was reduced sufficiently to insure that only the 

 top third of each cell was irradiated. If the wall sutures of a given 

 spore were upward, the nucleus was in the irradiated volume; if the 

 sutures were downward, an equal total volume was irradiated but the 

 nucleus escaped. The qualitative effects of irradiation were the same 

 under both of these conditions. Three distinct activities of normal 

 germination — cracking of the spore wall, chlorophyll development, and 

 cell division — were inhibited. Division was more easily inhibited than 

 chlorophyll development and the latter more easily than cracking.^ 

 When the nucleus was in the irradiated volume, the dose of alpha particles 

 needed to inhibit any one of these processes in any given percentage of 

 individuals was far less than when the nucleus was not irradiated. An 

 exact numerical comparison is impracticable, but a lower limit of 20:1 

 may be set for the ratio of effectiveness of a given dose of alpha particles 

 under the two conditions. This result is, of course, not surprising as 

 far as cell division is concerned. The essential role of the nucleus in 

 mitosis is well known. It is of interest, however, to note that injury 

 to the nucleus is also especially effective in prevention of chlorophyll 

 formation and in prevention of absorption of enough water to crack 

 the spore wall. 



Henshaw and Henshaw (9) also describe results which demonstrate 

 that alpha particles are particularly effective when they strike nuclei. 

 These authors studied the change in susceptibility of eggs of Drosophila as 

 development progresses. The transverse diameter of the egg of Dros- 

 ophila is about 100/i, and under the particular experimental conditions 

 the alpha particles from polonium must not have penetrated more than 

 15m. Hence only a relatively thin peripheral layer of each developing 

 egg was irradiated. In the early stages of development the nuclei are 

 located well within the egg. As they multiply, they migrate toward 

 the periphery and form a blastoderm. Later, as gastrulation begins, 

 many of the nuclei move inward, out of range of the alpha particles. 

 Henshaw and Henshaw found that this migration of nuclei was definitely 

 correlated with sensitivity, the eggs being most susceptible when the 

 nuclei were located peripherally. This was true in spite of the fact that, 



* These effects are qualitatively the same as those of soft X-rays (unpublished 

 data, obtained with Dr. A. L. Patterson). 



