EFFECTS ON TROPISTIC RESPONSE 



221 



the experiments were arranged on the same plan, except for the 

 nature of the Hquid, and the distance of the radium from the roots. 



Experiment 93 



Roots of Lu^inus albus were immersed in a glass dish of tap- 

 water to a measured length of 10 mm. On account of the size of the 

 seeds and the method of suspending them it was not practicable to 

 employ more than two or three seeds at a time. A sealed glass tube 

 containing 52 mg. of radium bromide of 10,000 activity was then 



Fig. 63. Fig. 64. 



Experiment 93. Curvatures of Root-Tips of Lupinus albus toward a sealed Glass 

 Tube of Radium Bromide in Water (figure 64), and in a nutrient Solution 

 (figure 63). 



suspended by threads so as to lie horizontally in the water with 

 the salt distributed evenly over the bottom of the tube. The latter 

 was placed in numerous trials at distances of from 2-25 mm. from the 

 tips of the roots. At a distance of 25 mm. no effects were observed, 

 but at distances of from 10 to 2 mm. curvatures took place toward 

 the tube. Photographs of two of these trials are shown in figures 

 (iT^ and 64. The results in figure 63 are quite similar to the electro- 

 tropic curvatures figured by Plowman. In this instance the tap- 

 water was replaced by a nutrient solution made according to direc- 

 tions given on page 85 of MacDougal's Elementary Plant Physiology. 

 No constant differences were observed between the curvatures in 

 tap-water and culture solution. 



ever, in the following year, stated that the electrical conductivity of water was not 

 altered by exposure for a short period, but was slightly increased by exposure for as 

 long as two days. The question is referred to further in Chapter XIX. 



