No. 504] PHYSIOLOGICAL EFFECTS OF RADIUM RAYS 769 



To show the effect of duration of exposure to radium 

 of the same activity, four sets of six dry seeds each of 

 L. albus were taken. Three of them A, B and C, were 

 exposed as follows to the rays from 10 mg. of radium 













Effect 



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Days 



bromide of 1,800,000 activity, in a sealed glass tube: A, 

 72 hrs.; B, 50 hrs.; C, 26 hrs.; D, control, not exposed. 

 The seeds' were then planted, without soaking, in sepa- 

 rate pots of soil. At the end of 5 days the average 

 heights of the seedlings above the surface of the soil were 

 as follows: A, 88 mm.; B, 95 mm.; C, 134.50 mm.; D, 145 

 mm. Inspection of the curves of growth for the four 

 cultures (Fig. 8) shows that, for a given activity of 

 radium, the effect on growth varies directly with the 

 duration of the exposure. 



The retardation of starch formation by a green leaf 

 in the light was shown as follows. A nasturtium 

 (Tropaolum) plant was removed to the light after being 

 in darkness for 18 hours. Under one of the leaves, 



» The length of the tube did not permit of satisfactorily exposing a 



