356 



Mr. J. Small. Changes of Electrical 



paper and with a cloth arranged over several nails to form a chamber which 

 was kept moist by running water. In this way most of the roots were 

 obtained straight and vertical and the preparatory manipulation was carried 

 out with the root as nearly as possible in the position of rest. 



The decrement of resistance furnishes a standard for the measurement of 

 the degree of excitation, instead of the time-ratio used in fig. 1. The 

 resistance was measured with the root vertical at intervals of 15 min. until 

 two similar readings were obtained as explained above, then after another 

 interval of 15 min. the root was tilted at an angle of 20° and a reading taken 

 as quickly as possible. The time taken to get the silence point varied from 

 15 to 20 sec. As the ratio of the relaxation time to the time of excitation in 

 Vicia Faba is 5:1,* the root was returned to the vertical position 

 immediately after the reading had been obtained and an interval of 

 10-15 min. allowed to elapse between each reading. Other angles were taken 

 in succession. The advantage of the present method over previous methods 

 lies in the fact that the behaviour of the same bean root towards different 

 stimuli can be studied ; this eliminates to a certain extent that individual 

 variation (c/. Trondle (8) ) which complicates the interpretation of most 

 results in geotropism. 



AC 



The resistance is calculated thus : — P : E : : AC : CB, .". P = t-?^ x 



AB 



JExperiment 1 : Fixed resistance or K = 6400 ohms. 



Angle of root to vertical ... 0° -20' -30" -45° -60° -90° 

 Total resistance in ohms ... 9600 8660 8145 7667 9600 8839 

 Decrement of resistance ... — 940 1455 1933 761 



It will be seen that the first part of the curve, fig. 8, closely approximates 



I 2000 



o 



^ 1500 



u 

 z 



in 1000 



1^ 500 



o 



10 20 30 AO 50 $0 



ANGLE WITH VERTICAL 



70 



80 



90 



Fig. 8. 



* Fitting (3), Tell II, p. 340. 



