2 28 THE POPULAR SCIENCE MONTHLY 



example, the total growth in 24 hours was, for the exposed radicles, 

 28.66 mm., and for the control radicles only 16.08 mm. 



Thus it is seen that exposure to radium rays, though followed in 

 some cases by a retardation or inhibition of function may, under cer- 

 tain suitable conditions of exposure and with certain tissues, be fol- 

 lowed by an acceleration. 



Excitation of function is further illustrated by the following ex- 

 periment: In a flower pot of soil unsoaked seeds of oat were sown in 

 three concentric circles, distant, respectively, 7 mm., 22 mm. and 45 

 mm. from the center of the pot. Into the soil at the center was in- 

 serted the sealed glass tube of radium bromide of 1,500,000 activity. 

 The end containing the radium was about 15 mm. below the soil sur- 

 face. A second pot was arranged in a like manner except for the 

 substitution of an ■empty glass tube for the radium tube. At the end 

 of 106 hours the seedlings from the exposed seeds were much taller 



Fig. 7. 



than those in the control pot (Fig. 7), the amount of stimulation 

 being greatest in the outer circle of plants and least in the inner circle. 

 At the end of the 106-hour period the radium tube was placed in the 

 control pot and the empty glass tube in the pot R. Following this 

 change the seedlings in CR grew faster than those in R, now serving 

 as a control. Thus it was possible to accelerate the growth of the 

 seedlings in either pot at will by transferring the radium tube from 

 one culture to the other. 



The fact that incandescent gas mantles contain a large percentage 

 of thorium, a radioactive substance, suggested the following experi- 

 ment. On the surface of soil in a pot was sown a row of timothy grass 

 seed, and over this row and at right angles to it, was suspended a fresh, 

 ^^nburned mantle at a distance of three or four millimeters above the 

 seeds (Fig. 8). Germination and subsequent growth were both re- 



