The Effect of Ionizing Radiations on the Broad Bean Root—Part X (concluded)

Experiments are described from which the following conclusions are drawn. The results are of a statistical nature, so that they do not constitute proof, but probability is in their favour. (1) The decrease in sensitivity of the broad bean root to X rays is the same whether anærobiosis be produced by bubbling nitrogen, argon, or hydrogen, through the water in which the roots are immersed. (2) Oxygen supplied immediately after X irradiation under anærobic conditions has no effect. (3) When the oxygen concentration in the water in which the roots are immersed is rapidly changed, the sensitivity to X rays of the roots follows with a time lag, certainly not greater than one minute, if any. (4) The oxygen concentration acts as a simple multiplying factor on the dose. (5) When the multiplying factors are plotted against the concentration of oxygen in the water, a sigmoid curve results, which has its maximum slope at about 5 c.c./litre, and reaches a constant maximum of 3 in the range 15 to 20 c.c./litre, i.e., a three times greater dose must be given with no oxygen present than is necessary with 20 c.c./litre to produce the same effect on the roots. (6) The oxygen consumption of the root has been measured. When the concentration gradient necessary to supply this amount is calculated it appears that the oxygen concentration in the root must be considerably less than that in the water. (7) The histological appearances of root tips irradiated with and without oxygen seem qualitatively the same, but roots irradiated with oxygen are affected to a much greater degree. In particular the inhibition of mitosis is greater. (8) It is considered probable that these conclusions have close parallels in the case of X irradiation of animal tissues in general, including human neoplasms as a most important example.