232 THE POPULAR SCIENCE MONTHLY 



The above experiments were all confirmed by repetitions, and 

 clearly indicate that radium rays act as a stimulus to the varioiis 

 physiological processes of plants. If the strength of the radium, the 

 duration of exposure, and other conditions are suitable, the response 

 is an excitation of function, but if the method of treatment is other- 

 wise, the radium too strong, the exposure too prolonged, the result is 

 a retardation, or complete inhibition of function, or the death of the 

 plant. There are not only differences in sensitiveness between indi- 

 viduals, but also between different species and different tissues. As in 

 the case of animals, embryonic and younger tissues are more sensitive 

 than those that are older and more mature. 



