228 HISTOLOGICAL EFFECTS 



a cessation of cell-division, an acceleration of tissue-differentiation, 

 a decrease in the size of the cells, a lack of coordination in histo- 

 genesis, either one or all in any given case. 



Decrease in the size of the cells may be due, either to diminished 

 turgor or to relatively* less vigorous constructive metabolism. It is 

 not possible at present definitely to say which, though, since no 

 partial loss of tissue-tension, nor any other evidence of a loss of tur- 

 gidity, has been detected, following exposure to the rays, a less vig- 

 orous constructive metabolism appears to be the more probable cause. 



The first two effects mentioned, viz., cessation of cell-division and 

 accelerated tissue-differentiation, have both the same significance, 

 that is, early senescence. In such instances as those shown in 

 FIGURE B^ PLATE I, and FIGURE D, PLATE 2, where the cambium 

 cells have persisted, and retained their characteristic appearance, 

 they have evidently ceased to multiply. 



It is true that, in each of these instances, the tissue-differentiation 

 has not been vigorous, nor normal in any other respect, but the im- 

 portant point to emphasize is that such differentiation has taken place. 

 Embryonic tissue has either entirely disappeared or its units have 

 lost the peculiar function of such cells, the power of reproduction. 

 Every protoplast passes normally from a period of youth through 

 maturity and old age to ultimate death. At each stage it manifests 

 certain morphological and physiological features peculiar to that 

 stage. In general, the period of youth is marked by both structural 

 and functional plasticity both of which features gradually diminish 

 and finally disappear as old age approaches and advances. 



Whatever picture we may try to form as to just what occurs in 

 the protoplast when it is exposed to the rays of radium, the foregoing 

 histological effects seem clearly to indicate that one of the ultimate 

 results is an acceleration of the period of senescence. If this accel- 

 eration proceeds gradually enough, the cells and cell-complexes 

 may assume during growth, the various morphological configurations 

 characteristic of the successive ages ; but if the acceleration is too 

 rapid, physiological senescence is reached quickly, without the 

 usually accompanying structural changes, while a sufficiently intense 

 over-stimulus by the rays may be quickly followed by complete loss 

 of vitality and death. 



In this connection it is interesting to recall the fact that old age 



* Relatively, that is, to either the destructive metabolism, or to normal construc- 

 tive metabolism. 



