HISTOLOGICAL EFFECTS 229 



finds histological expression in a diminished vitality and relative size 

 of the nucleus. Careful measurements with an eye-piece micrometer 

 of the size of cells and of their nuclei near root-tips of Zea Mays 

 showed, as I have pointed out elsewhere,^ that in roots not exposed to 

 radium rays, the cells at a given region near the tips averaged 8.25 

 scale divisions, and their nuclei 2.75 divisions in diameter. This is 

 the average of a number of cells taken from several different roots. 

 Similar measurements of the same number of cells of roots exposed 

 to radium rays, and at corresponding regions of the root, gave, for 

 the average diameter of the cells, 6.10 divisions of the scale, and of 

 the nuclei 2.17 divisions. In other words, the average diameter of 

 the nucleus of the normal cells measured was 35.5 per cent, of that 

 of the cell, while for roots of the same age, and similarly grown 

 except for exposure to radium rays, the diameter of the nucleus is 

 33-33 P*^'* cent, of the diameter of the cell. This is a cyto-morpho- 

 logical expression of the fact that in tissues of the same " age," 

 the period of senescence is reached sooner than normally after ex- 

 posure to radium rays. Metchnikoff's" view, expressed by his state- 

 ment, '-'■ 011 resume la senilite -par tin seul mot: atrophie,^'' though 

 probably too narrow for a generalization, is quite in harmony with 

 the observations just described. 



In discussing the problem of age, growth, and death, Minot^ has 

 recently said that, " the growth and differentiation of the protoplasm 

 are the cause of the loss of the power of growth," and that "The 

 older we are the longer it takes us to grow a definite proportional 

 amount." These statements, originall}'^ made with reference to the 

 human organism, apply with equal force in the realm of plant physi- 

 ology, and make it more readily seen how a retardation or even a 

 complete cessation of certain processes may really be an expression 

 of what is fundamentally a stimulation. The facts here reviewed 

 substantiate the conclusion, drawn from other results,* that radium 

 rays act as a stimulus to living -protoplasm. 



Bibliography 



1. Gager, C. S. Acceleration of the approach of senescence by radium 



rays. Torreya 8: 172. 1908, and Science, N. S. 28. 1908. 



2. Metchnikoff, E. L'Annee Biologique 3 : 256. 1897. 



3. Minot, C. S. The problem of age, growth and death. Pop. Sci. Mo. 



71 : 460. 1907. 

 * See p. 157. 



