EFFECTS OF RADIOACTIVE AIR ON GROWTH I57 



tion of growth, approximated the optimum stimulation for the chloro- 

 phylless root, causing acceleration of growth. 



No appreciable effect resulted from exposing germinating timothy- 

 seeds in a closed bell-jar to the emanation that diffused from an open 

 ■ glass tube of about lo mg. of radium bromide of 7,000 activity. 



These experiments with the radioactive atmosphere all point to 

 the same general truth, namely, that the rays of radium act as a 

 stimulus to protoplasm. Retardation of growth following exposure 

 to the rays is an expression of over-stimulation ; acceleration of 

 growth indicates stimulation between a minimwn and an optimum 

 point. 



