768 



THE AMERICAN NATURALIST 



[Vol. XLII 



and R became the control. At the end of five days after 

 this change the plants in CR (Fig. 5) were nearly as 

 tall as those in C and the exposure was photographed. 

 Eventually the plants in CR exceeded those in R, and 

 thus, by changing the radium tube from one pot to the 

 other, the growth of either culture could be accelerated 

 at will. 



In order to ascertain the effect on growth of exposing 

 seeds for the same length of time to radium of various 

 degrees of activity, three sets, A, B and C of six dry seeds 

 each of Lupinus albus were exposed to rays from radium 

 in sealed glass tubes by laying the tubes against the 

 hilum-edges of the seeds. The duration of exposure was 

 91.5 hours, and the strengths of the radium as follows: 

 A, 1,800,000 X ; B, 1,500,000 X ; C, 10,000 X ; D, control, 

 not exposed. At the close of the period of exposure the 

 seeds were planted in soil, each set in a different pot. 

 The experiment was photographed eleven days after the 

 seeds were planted (Fig. 6), and curves of growth of the 



four cultures are shown in Fig. 7. It is seen at a glance 

 that the effect of the rays on growth varies directly with 

 the degree of activity of the radium. The apparently 

 anomalous rise of the 1,800,000 curve during the first 

 days record (Fig. 7) is due either to poor exposure of 

 some one of the seeds of that culture, or represents an 

 individual variation in resistance to the rays that was 

 not compensated for because of the small number of 

 seeds necessarily employed. 10 



