EFFECTS ON TROPISTIC RESPONSE 2l7 



King " corn {Zea Mays) were exposed to the rays from RaBr2 of 

 1,500,000 activity for 45 hours, and to the rays from RaBrg of 

 1,800,000 activity for periods of 24^ hours and 27 hours, by being 

 placed in contact with the sealed glass tube containing the radium salt. 

 The grains thus exposed, together with a control set of five 

 grains, were planted without soaking in similar pots of soil, and kept 

 under similar conditions. The average heights of the seedlings were 

 recorded as follows : 



May 29 (4 days after planting). 



4§ Hours 2J Hours 24.^ Hours Control 



3.75 mm. 2.00 mm. S-oo mm. 8.00 mm. 



May 30. 



10.30 mm. 10.00 mm. 9'5o mm. 15*25 "i 



m. 



June 2. 



47.60 mm. 33-00 mm. 18.20 mm. 72.80 mm. 



On this date three of the seedlings from the seeds exposed for 

 45 hours (1,500,000 x) had partly ceased to respond normally to 

 the stimulus of gravity, and four of the seedlings in each of the cul- 

 tures exposed to the radium of 1,800,000 activity had wholly or partly 

 ceased to grow erect. On June 7 observations were recorded as 

 follows : 



45 Hours 2^ Hours 24. § Hours Cofitrol 



165.00 mm. 133.00 mm. All have ceased 273.00 mm. 



243.00 12.00 (ceased growth) growth since about 2 244.00 



139.00 147.00 (etiolating) days ago, except one; 205.00 



20.90 25.00 (ceased growth) heightofthatoneequals 217.00 



188.00 60.00* 162.00 mm., with par- 163.00 



7^."^ mm. 3^7:^ mm. ^'^^ ^^^^ «^ P«^^^' ^^ 1102.00 mm. 



151.00 mm. 75.40 mm. make chlorophyll. 220.40 mm. 



The radium of 1,500,000 activity, though applied for nearl}' twice 

 as long as that of 1,800,000 activity, has retarded germination and 

 growth much less than the stronger. In the case of the radium of 

 1,800,000 activity, a difference in the length of exposure of 2.5 hours 

 was at first followed by no marked difference in results, though sub- 

 sequently a decided difference was noticeable. 



* Has ceased to grow erect. 



