HISTOLOGICAL EFFECTS 2 25 



experiments of Bohn* on various animals, of Schaper * on the frog, 

 of Zuelzer* on malign tumors, and of Hewlet * on cancer all indi- 

 cate that embryonic tissues are more readily affected by radium rays 

 than are mature tissues. 



Now tissue-differentiation is distingruished from cell-division 

 (tissue-formation) on the one hand, and from growth on the other. 

 It is a function of maturity. The greater the degree of tissue-dif- 

 ferentiation in any given species, the greater the maturity of the 

 organ. The condition of the tissues in A, therefore, suggests that, 

 as it were, the organ had become aged more rapidly following exposure 

 to the radium rays. The youthful power of reproduction (cell-divi- 

 sion) has been lost earlier than normally. It is also evident that, fol- 

 lowing exposure to the radium rays, the cells in the regions of the 

 xylem and the phloem are smaller, and in every way less perfectly 

 developed than normally. 



In B is shown a cross-section of the hypocotyl of another plant 

 of the same Experiment (No. 27, p. 121), exposed precisely as was 

 A, and also of the same age as I?. Here the cambium is still 

 present, though within the bundle it is being differentiated. Xylem 

 and phloem are less perfectly developed than normally. 



Still a third variation in result is seen in C. Here again the cam- 

 bium has disappeared, being entirely transformed into xylem and 

 phloem. In the phloem-region of the bundle there has been more 

 differentiation of tissue than in either A or B. 



Figures ^ and /^illustrate sections of the roots from plants of 

 Experiment 29. ^is from a control (normal) plant. In ^ the ex- 

 posure of the seed was for 72 hours to rays from radium bromide of 

 1,800,000 activity. As in the case of the hypocotyl, the cambium 

 has disappeared, the cells are smaller, and the tissues appear in every 

 way abnormal. -f 



In PLATE 2, FIGURE A represents a cross-section of the hypo- 

 cotyl of a Lufinus albus seedling grown from a seed exposed (Ex- 

 periment 16, p. 99) to rays from a Lieber's radium-coated rod of 25,000 

 activity during imbibition of water in moist sphagnum. The material 

 was collected six days after the seeds were placed to germinate in the 

 sphagnum, and the exposure to the rays was continuous during this 

 period. The hypocotyls of the control plants {B) were nearly 7 mm. 

 longer than those of the exposed specimens. 



* See Chapter II. and the bibliography there given. 

 fThe distortion of the cells of the cortex is an artifact. 



