320 CHAP. XXIX. GEO-ELECTRIC RESPONSE OF THE SHOOT 



connections are made in the two methods of Axial and 

 Vertical Rotation. 



Method of Axial Rotation. — The principle of this method 

 has already been explained {cf. fig. 190). It is diagram- 

 matically represented in fig. 194, H. 



Method of Vertical Rotation is diagrammatically repre- 

 sented in fig. 194, V. The specimen is held vertical and 

 electric contacts, A and B, made on two opposite sides ; 

 it is then rotated round a horizontal axis perpendicular to 

 the long axis of the specimen. For the purpose of simplicity 



Fig. 194. Diagrammatic representation of the Method of Axial 

 Rotation h, and of Vertical Rotation v {see text). 



I confine attention to the electric change induced at the upper 

 side of the organ. Rotation may be begun in a right- 

 handed direction, making an increasing angle with the 

 vertical, when the point A is subjected to increasing 

 geotropic stimulation and exhibits increasing electric 

 change of galvanometric negativity. Continuous decrease 

 of the angle of inclination to zero by rotation in the reverse 

 direction causes a disappearance of the induced electric 

 change. The rotation is next continued in the left-handed 

 direction, by which the point B is subjected to increasing 

 geotropic stimulation. B is now found to exhibit excita- 

 tory reaction, the current of response having undergone 

 a reversal. Rotation to the right and left may be dis- 

 tinguished by plus and minus signs. 



