272 THEORETICAL CONSIDERATIONS 



ionizing the atoms of all the various substances (biogens and others) 

 that are involved in the metabolism of the cell, the effects being pro- 

 duced both directly and indirectly. Thus we should expect, a prt07'i, 

 a retardation and finally a complete inhibition of cell-division in all 

 tissues exposed to rays of sufficient activity and for suitable periods 

 of time. And this is what has been observed to occur. Theoretically 

 we ought also to be able to accelerate the process by suitable condi- 

 tions of exposure, but such conditions have not yet been discovered. 



The irregularities produced by radium rays in karyokinesis do 

 do not seem to call for any special explanation in addition to that 

 suggested in discussing the abnormalities of tissues and organs in 

 Chapter XVI. Such irregularities are only a morphological ex- 

 pression of physiological disturbance, and it may be seriously ques- 

 tioned whether we are justified in expecting the morphological 

 appearance and behavior of chromosomes to explain things, any 

 more than do variations in leaf-margins, or other purely structural 

 facts. The problem of the causes of variation and inheritance lies 

 deeper than morphologv, and the behavior of the chromosomes, even 

 in sexual cells, instead of explaining other and grosser facts of struc- 

 ture, quite possibly only presents another feature to be explained. 



Recovery from Stimulus : It has been frequently observed in 

 the study of stimulation, not only by radium rays but by other stimuli 

 as well, that a depression of function, caused by certain conditions of 

 exposure, may be followed, after a period, by recovery of the normal 

 rate of functioning, provided that the stimulant is discontinued. In 

 like manner excitation is followed by a return to the normal condi- 

 tion. This recovery of tonus probably means that the stimulus has 

 not been injurious to the biogens, or that it has produced its effect 

 largely, if not wholly, by its influence upon the non-vital steps in 

 metabolism. Thus, if the rays of radium acting on a resting seed 

 destroyed a large percentage of some enzyme necessary for germina- 

 tion, but affected the biogens only slightly or not at all, or if they 

 destroyed only a relatively small number of the latter, germination 

 and early growth would be retarded, but recovery would follow in a 

 short time, due to a replenishing of the necessary enzyme by meta- 

 bolic processes. A similar course of reasoning would apply to re- 

 covery from excitation. 



Radium Stimulus and Plant-electricity: The literature 

 dealing with the existence of electric currents in plants is far too 



