CHAPTER XVI 



HISTOLOGICAL EFFECTS OF THE RAYS OF RADIUM* 



The discovery of the fact that, following certain conditions of 

 exposure to the rays of radium, growth is retarded, raises the question 

 as to how the effect is produced. Several alternatives are suggested, 

 involving the various factors concerned in the normal growth of an 

 organism. If we disregard the numerous attempts at a rigid defini- 

 tion of growth, at least four such factors are to be considered: (i) 

 increase in the size of the cells ; (2) increase in mass ; (3) cell- 

 division;! (4) cell-differentiation. 



Therefore when the growth of a plant or of a plant organ is 

 diminished by any agency, the result may theoretically be accom- 

 plished by a retardation of any or of all of these four factors. 



As to the effect of radium rays on constructive metabolism and 

 resulting increase in mass we have no definite information as yet. 

 My own experiments on the subject have so far yielded negative 

 results. The same must be said in regard to the effect of these rays 

 on osmosis, turgidity, and consequent cell-enlargement. 



With reference to cell-division, however, we have more to say. 

 It is known, for example, that karyokinesis may not only be inter- 

 fered with, but completely inhibited by exposure to radium rays. 

 The experiments demonstrating this are described in Chapters II 

 and XVII. The presumption, then, is in favor of the theory that 

 retardation or cessation of growth may be due in part to either 

 partial or complete inhibition of cell-division. A histological exami- 

 nation of the tissues of plants that have suffered retardation of growth 



*The substance of this chapter was given before the bi-weekly Botanical Conven- 

 tion of the New York Botanical Garden, April i, 1908. 



t Of course cell-division and cell-differentiation are not a part of growth proper, 

 but the discussion of this question need not be taken up here. Growth I would define 

 as increase in size, or increase in mass, one or both, with or without an accompanying 

 change of form. Cell-division is a factor because the number of the cells present modi- 

 fies the amount of growth possible for the given tissue or organ. Cell-differentiation 

 is a factor because it may modify the rate of growth. If a tissue rapidly matures (cell- 

 differentiation), growth will rapidly cease. The longer the growth period of any given 

 tissue or organ the slower the process of cell-differentiation in that tissue. 



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