PREVIOUS INVESTIGATIONS UPON PLANTS 69 



oxycellulose. The weakening effect became inappreciable at a 

 distance of i8 mm. Under the microscope the broken ends of the 

 fibers of exposed threads were straight at their ends, those not 

 exposed curled, indicating a loss of elasticity through exposure to the 

 rays. 



In July, 1907, Hussakof'^' published a review of the more impor- 

 tant papers bearing on the physiological effects of the rays of 

 radium. Previous announcements of my own results have appeared, 

 with one exception, as abstracts of papers presented before various 

 scientific societies ; citations to them will be found in the Bibliography 

 appended to this chapter, and further reference to them here is 

 unnecessary. 



The details of the investigations reviewed above are numerous, 

 but the results may be briefly summarized. There is very general 

 agreement on the following points : 



1. Radium rays have the power to modify the life-processes of 

 both plants and animals. 



2. Rontgen rays and radium rays produce similar physiological 

 results. 



3. Sensitiveness to these rays varies with the species of either 

 plant or animal. 



4. Younger, and especially embryonic tissues, are more sensitive 

 than those more mature. 



5. With only one or two exceptions, exposure to radium rays has 

 been found to either retard or completely inhibit all cell-activities. 

 The rays may cause irregularities in mitosis. 



6. Experimental evidence for or against the existence of a radio- 

 tropic response is conflicting. 



7. Whatever the immediate, internal change produced in the pro- 

 toplast may be, the result, with animals as well as with plants, 

 appears to be more or less profoundly modified by the presence of 

 chlorophyll in the cell. 



8. Radium rays appear to retard the activity of enzymes. 



