350 CHAP. XXXI. INCLINATION AND EXCITATION 



variation thus obtained is of considerable intensity, often 

 exceeding 15 millivolts. 



Excitatory reaction at 45° and 90°. — The special difficulty 

 encountered is that of the accurate determination of the angle 

 of inclination. An index is attached to the plant, and a 

 stationary circular scale permits the determination of the 

 angle at which the plant is inclined to the vertical.* But 



Fig. 206. Alternate geo-electric reactions at + 45° and — 45°, 

 also at -1- 90° and — 90° (Method of Vertical Rotation). 



the vertical or zero-reading itself is subject to an error 

 of a few degrees, which is accentuated by the fact that the 

 perceptive layer inside the plant may not be exactly parallel 

 to the surface of the stem or the petiole. The only means 

 of eliminating the error is by taking two responses, say 

 at + 45° and — 45°, by the Method of Reversal. The 

 mean of the two responses obtained through successive 

 positive and negative rotations of the specimen will 

 reduce or eliminate all errors. 



