368 CHAP. XXXIII. THE RESPONSE OE THE ROOT 



to the very small size of the tip, this is by no means an easy 

 operation. Two platinum points tipped with kaolin paste 

 are very carefully adjusted so as to make good electric 

 contacts at the two opposite points, without exerting undue 

 pressure. The root has to be laid horizontal for geotropic 

 stimulation, and as the root of the Bean is somewhat long 

 and limp, displacement from the vertical position is apt to 

 cause a break of the electric contact. This is avoided by 



Fig. 212. Diagrammatic representation of geo -electric response 

 of root-tip. The middle figure shows root in vertical position. 

 Rotation through -f 90° places a above, which becomes 

 galvanometrically negative. Rotation through — 90° places 

 h above and makes it negative. 



supporting the root from the top and also from the sides 

 with padding of cotton-wool. 



After due observance of these precautions, the electric 

 response obtained is found to be very definite. When the 

 root is made horizontal by rotation through + 90°, the 

 point a is above, and the responsive current is found to flow 

 from h to a, the upper side of the tip becoming galvano- 

 metrically negative ; when the root is brought back to 

 the vertical, the responsive current disappears. Rotation 

 through — 90° now makes the point h occupy the upper 

 position, and the responsive current is from a to h. Hence 

 the upper side exhibits in every case an excitatory electric 

 change of galvanometric negativity (fig. 212). The root- 

 tip thus gives the characteristic response to direct stimu- 

 lation. Experiments carried out with twelve different 



