922 



BIOLOGICAL EFFECTS OF RADIATION 



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detecting mitogenetic rays with onion roots is no longer used in his 

 laboratory, but since it is the most fundamental of his experiments and 

 since the most severe criticism has been leveled against it, we shall 

 give here a short review of this work. 



In these experiments (for detailed experimental technique see 97, 103) 

 Gurwitsch used roots of the common kitchen onion. The roots selected, 

 about 4 to 12 cm. long, were left attached to the onion bulb, or a part 

 of it. The details of the arrangement are shown in Fig. 1. Special 

 attention was given to the symmetry and straightness of the roots 

 selected. Each of the "detector" roots was in a fairly close fitting glass 

 tube interrupted near the lower end so that the growing region of the 



root would be exposed when the 

 whole was introduced into a metal 

 tube having a window corresponding 

 to the region of root exposure. The 

 "sender" root was also placed in a 

 glass tube, but the tip of the root was 

 not covered, so that when arranged 

 horizontally and opposite the exposure 

 window of the "detector," a definite 

 root region might be acted upon. In 

 short, he arranged these two roots so 



Arrangement for Gurwitsch's that the "projected axis" of the 



sender struck the detector exactly in 



The adjustments 

 were made with a microscope and the 

 entire set-up was kept in position by 

 strong clamps. The roots were kept moist. The time of exposure 

 differed in the individual experiments, but in general he reported 

 that it required from 1 to 3 hr. to get a distinct effect. The 

 detector was allowed to stand after exposure for a certain time, then 

 it was marked and prepared for sectioning. This short length of root, 

 a few millimeters only, in the region selected and exposed, was cut into 

 longitudinal sections and the number of mitotic figures in definite stages 

 to the right and left of the median line was counted. The experiment 

 revealed, according to Gurwitsch, "distinct circumscribed preponderance 

 of mitoses in the center of the 'induced side' of the root." The excess 

 of mitotic figures on the side of the root toward the sender was about 

 30 to 50 per cent (see Table 1). The results were not substantially 

 different if a quartz plate was placed between sender and detector. 



In his original articles (92 to 96, 98, 99) defining the method of the 

 mitogenetic-ray effect, Gurwitsch gives a very detailed discussion of 

 the type of cells counted as well as an estimate of the exactness of the 

 method. But since there are several stages in cell division, the judgment 



Fig. 1 

 onion-root experiment. A, detector root; 

 B, capillary glass tube; C, metal tube; 

 D, sending root; E, spot marked for the median line, 

 sectioning, as seen in observation micro 

 scope. {After Reiter and Gabor, 231.) 



