258 PLANT RESPONSE 



I overcame the difficulty regarding extraneous dis- 

 turbance by using, instead of a loose, a steady pressure- 

 contact, capable of the finest adjustment, by means of a 

 micrometer screw. The next problem lay in choosing 

 contact material of great sensitiveness. For this purpose 

 I used different materials, the sensitiveness of some being 

 very great, while that of others was moderate. Carbon 

 contacts belonged to the latter class, but had the advantage 

 of being easily adjustable. Various metallic powders, how- 

 ever, such as that of bronze, were considerably more sen- 

 sitive, but at the same time required greater care for 

 adjustment. 



The most important factor in the arrangements, by which 

 the sensitiveness of the contact may be exalted to a very 

 high degree, lies in the proper adjustment of the electro- 

 motive force acting on the contact circuit. The sensitiveness 

 to pressure-variation increases with increasing electromotive 

 force, being greatest when this is just short of a certain 

 critical value, after which the electric contact is observed to 

 become unstable, and give rise to spontaneous oscillatory 

 variations. For the purpose of easy adjustment of the 

 electromotive force I use a sliding potentiometer. The 

 tissue, say a stem of Mimosa, is placed between points, 

 A and B, and by means of the micrometer screw, S, it is 

 pressed against the spring B. The electric contact, say of 

 carbon points, is between B and C, which are in circuit with 

 a galvanometer, and the potentiometer. The pressure is 

 so adjusted that a feeble current flows through the gal- 

 vanometer. In order to increase the sensitiveness of the 

 detector, the electromotive force may be gradually increased, 

 by proper adjustment of the potentiometer, short of the 

 critical point (fig. 106). The deflected spot of light from 

 the galvanometer will remain steady, provided the adjust- 

 ments have been properly made. 



The tissue is now stimulated at a distant point, care 

 being taken that it is not in any way jarred mechanically. 

 Stimulation without jar is effected, however, without difficulty 



