330 CHAP. XXX. THE GEO-PERCEPTIVE LAYER 



variation of excitability in the different layers of tissue. 

 The wound-irritation is, however, reduced to a minimum 

 by making the probe exceedingly thin. A fine platinum 

 wire, o-o6 mm. in diameter, passes through a glass tube 

 drawn out into a fine capillary and fused round one end 

 of the platinum wire, which protrudes very slightly beyond 

 the point of fusion ; the exploring electrode is thus insulated 

 except at its protruding sharp point. The length of the 

 capillary is, generally speaking, about 6 mm., just long 

 enough to traverse the experimental plant-organ trans- 

 versely from one side to the other ; the average diameter of 

 the capillary is about 0-15 mm. The other end of the 

 platinum wire comes out of the end of the glass tube and 

 is led to one terminal of the galvanometer, the other being 

 connected with an indifferent point on the organ. The 

 probe can be gradually forced into the plant-organ by 

 rotation of a screw-head, one complete rotation causing a 

 forward movement through 0-2 mm. 



Prick-reaction. — A prick acts as a mechanical stimulus, 

 and in normal excitable tissues induces an excitatory 

 change of galvanometric negativity; this prick-reaction 

 increases with the extent of the wound, and the suddenness 

 with which it is inflicted. On account of the fineness of the 

 probe, it insinuates itself into the tissue without making any 

 marked rupture ; the probe is, moreover, introduced very 

 gradually ; with these precautions the reaction due to 

 prick is found to be greatly reduced. The immediate 

 effect of the prick is a negative deflection of the galvano- 

 meter, which declines and attains a steady value in the 

 course of a few minutes. The depressing effect of the 

 passage of the probe on geotropic excitability disappears, 

 I find, in the course of about 10 minutes. 



In the choice of experimental material it is necessary 

 to find specimens which are not merely geotropically 

 sensitive, but also give a large electric response under 

 stimulation. In these respects the petiole of Tropaeolum, 

 the peduncle of Nymphaea, and the shoot of BryophyUum, 



