GEO-BLECTRIC RESPONSE OB^ ROOT 



469 



size of the tip this is by no means an easy operation. 

 Two phitinum points tipped with kaolin paste are very 

 carefully adjusted so as to make good electric contacts at 

 the two opposite sitles, without exerting undue pressure. 

 For geotropic stimulation the root has to be laid horizontal, 

 and as the root of the bean plant is somewhat long and 

 limp, displacement from the vertical position is apt to 

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

 supporting the root from the t<tp and also from the 

 sides ; for the latter purpose, I use paddings 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 of the root 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 galvanometrically negative ; when the root is 

 brought back to the vertical, the responsive current dis- 

 appears ; rotation through - 90° makes the point h occupy 



Fi(4. 172. — Diagrammatic representation of geo-electric response of root-tip. 

 Tiie middle ligure shovvs root in vertical position. Rotation through + !tO^ places 

 a above, which becomes galvanometrically negative. Rotation through - '.tO^, 

 places 6 above and mai<e3 it negative. 



the upper position, and the responsive current is from a to 

 h ; the upper side thus exhibits in every case, an excita- 

 tory electric change of galvanometric negativity (Fig. 172). 



