7.78 



THE AMERICAN NATURALIST [Vol. XLII 



than any other. After an exposure of seeds under cer- 

 tain conditions, the cambium is frequently entirely lack- 

 ing, all of the cells in the given organ having passed over 

 into the mature state. The treatment appears to ac- 

 celerate the approach of senescence. 



28 

















































riment 53 























Exposure to the rays also induces marked irregu- 

 larities in mitosis. This is shown, among other ways, 

 by the failure of some of the chromosomes to take part 

 in the organization of the daughter nuclei. Usually such 

 chromosomes organize smaller, nuclear-like structures 

 within the daughter-cells. In one instance they were ob- 

 served to be undergoing an independent karyokinesis at 

 one side of the main mitotic figure. Interesting possi- 

 bilities are here suggested, along the line of experimental 

 mutation. 



Experiments like those described in this paper have 

 been many times repeated with confirmatory results, and 

 seem amply to justify the general conclusion, earlier 

 stated, that radium rays are a stimulus to plant activities. 

 The reaction to a stimulus between the minimum and 

 optimum points is an excitation, or acceleration of the 

 given process; the reaction to an over (superoptimal) 

 stimulus is a depression, or retardation of function, and, 

 if the stimulus is sufficiently intense, complete inhibition 

 or ultimate death. 



