130 Applied Biophysics 



a rather large dose of radiation was needed to produce a given 

 mortality, but the measurements were carried out at a time 

 when some uncertainty on the physical side was attached to 

 measurements of gamma-ray dose.-*' Packard (1932) extended 

 the measurements in the other direction down to 8 kilovolt 

 X-rays, and conchides that between 8 kilovolts and 1,000 kilo- 

 volts, the mortahty is independent of X-ray quahty. The cor- 

 responding range of ion densities is from 150 ions per (.i to 

 15 ions per |.i. 



The effects of 200 kilovolt X-rays and neutrons (A ^ 400 ions 

 per |.i ) were compared by Zirkle and Lampe.^^ The mortality 

 curve, as a function of dose, for neutrons had the same shape 

 as that for X-rays, so that the relative effects of the two radia- 

 tions could be expressed by a single figure which was 0.8 for 

 eggs 1^ hours old, 1.2 for eggs Ah hours old, and 1.1 for eggs 

 6 hours old. It is doubtful whether the variation with age is 

 significant, and we conckide that neutrons and X-rays are 

 roughly equally efficient, i.e., that the effect is independent of 

 ion density up to 400 ions per \i. 



As was mentioned earlier, there is evidence that, under the 

 conditions prevailing in the sperm, the chromosomes of Droso- 

 phila may be broken by an ionizing particle which leaves only 

 one or two ion clusters within the chromosome thread. If the 

 same is true of the chromosomes in the egg, then the fact that 

 the mortality does not depend on ion density over the range 

 investigated would ont exclude chromosome structural changes 

 as a possible origin of the lethal effect of the radiation. It would 

 be of great interest to investigate the effect of a further tenfold 

 increase in ion density by the use of alpha radiation. 



Lethal and Sublethal Effects on Root Tips, 

 Particularly of \^icia faba 



The meristematic cells in the shoot and root tips of organisms 

 are very sensitive to radiation, and the damage caused by 200 

 to 1,000 rontgens of X-radiation will lead to the death of a 

 variety of roots. In passing from gamma radiation (A ^ 11 ions 



