2 24 HISTOLOGICAL EFFECTS 



by the rays confirms the correctness of this presumption, and, at the 

 same time, discloses the important part played by cell-differentiation. 



In PLATE I are given reproductions of photomicrographs of 

 cross-sections of the hypocotyl and roots of seedling lupines {Lupintis 

 albus). Figure D shows the normal appearance of a fibro-vascular 

 bundle and the adjacent tissues in the hypocotyl ; A and B the 

 corresponding region in the hypocotyl of plants grown from seeds 

 that were exposed for 72 hours, while dry, to rays from radium 

 bromide of 1,800,000 activity. After this exposure the seeds were 

 planted in soil. The seedlings were those of Experiment 27 (p. 121). 

 On the same plate, C illustrates the appearance of the corresponding 

 region of the hypocotyl of one of the exposed plants of Experiment 

 29 (p. 127). This plant was grown from seeds exposed for 91.5 

 hours, while dry, to rays from radium bromide of 1,800,000 activity. 

 The section was taken fourteen days after planting in the soil. 



In D the fascicular and interfascicular cambium is well devel- 

 oped, as are also the phloem and xylem regions of the bundle. 

 Comparison of A with D discloses profound modifications of these 

 tissues. In A the cambium is entirely absent, all of its cells apparently 

 having been differentiated into either phloem or xylem. The com- 

 plete disappearance of the cambium clearly indicates a total inhibition 

 of cell-division in that tissue. No new cells have been formed to 

 replace those that have been transformed. 



This effect was to be expected, being in harmony with the results 

 of other investigators which show that the tissue most susceptible to 

 radium rays is the embryonic. Thus Danysz * found that if i eg. 

 of radium (activity not given), in a glass tube, was placed above the 

 backbone and part of the cranium of a mouse one month old, phe- 

 nomena of paresis and ataxia were produced in about three hours. 

 Under a similar exposure a mouse three to four months old died with 

 the same symptoms in three to four days, while a one year old mouse 

 survived for from six to ten days.f The less mature tissue was 

 the most quickly affected. The same author f found the epithelial 

 tissues of young animals more sensitive than those of adults. The 



* Danjsz attributed this result to the fact that, in the joung mouse, the rajs had 

 to pass through cartilaginous tissue before reaching the cerebellum, whereas, in the 

 older specimen, this tissue had become transformed into bone. It is possible also that 

 the greater resistance of the older mice was due, in part at least, to the greater maturity 

 of the nerve cells. 



t See Chapter II. and the bibliography there given. 



