770 



THE AMERICAN NATURALIST [Vol. XLII 



empty of starch, was placed a Lieber's radium-coated rod 

 of about 25,000 activity. The rod was placed under the 

 leaf in order not to shade the latter. After an exposure of 

 twenty-four hours the leaf was decolorized and tested 

 with iodine. Abundant starch was found on the edge 

 of the leaf farthest from the rod, but only very slight 









SO hrs^ 





















I ft 



/// 







Expert 

 feci of dura 



ment 27. 

 tion of expos 

 ofLupinusal 



... 







E 



1 2 3 4 



6 



Days 



traces elsewhere, especially in the region that was directly 

 over the rod. A print of the leaf was made by laying it 

 over a sheet of velox paper in a printing frame and ex- 

 posing it to light. In this way the portions having less 

 starch, and, therefore, more transparent, showed darkest 

 on the print (Fig. 9). 



All attempts to demonstrate a direct tropistic response 

 to the rays, by either roots or shoots, were unsuccessful. 

 When, however, a sealed glass tube containing about 50 

 mg. of radium bromide of 10,000 activity was suspended 

 horizontally in a culture solution, at a distance of from 

 2-10 mm. from the root-tips of L. albus, the roots curved 

 toward the tube (Fig. 10). In an experiment in which 



