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THE AMEBIC AN NATURALIST [Vol. XLII 



The growth of roots was retarded in water exposed 

 for twenty-four hours to the rays. The experiment was 

 made as follows: Into 100 c.c. of tap-water, in which 

 sealed glass tubes of radium bromide had lain for twenty- 





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four hours, the radicles of four germinated lupines were 

 suspended up to an ink mark, placed 10 mm. back from 

 the root-tip. Three cultures were arranged: A, with 

 radium of 1,800,000 activity; B, with radium of 1,500,000 

 activity ; and C, with no radium, serving as a control. At 

 the end of five days the average lengths of the hypocotyls 

 were, for A, 79.62 mm. ; for B, 85.50 mm. ; for C, 117.75 

 mm. The result, then, was a retardation of growth, in 

 direct proportion to the degree of activity of the radium 

 to which the water was exposed (Fig. 16). 



Following up the suggestion in the discovery that 

 freshly fallen rain is radioactive, several experiments 

 were made with a view of ascertaining the effect of this 

 radioactivity on plant growth. Rain-water was caught 

 in the open, in chemically clean glass dishes, after about 

 four hours of rain, so that the air was well washed. This 

 water was kept carefully covered, for one month, when an- 

 other opportunity presented itself of collecting another 

 lot of rain under similar favorable circumstances. The 



