484 LIFE MOVEMENTS IN PLANTS 



rotation of a screw head, one complete rotation causing a 

 forward movement through 0'2 mm. (Fig. 175). 



Wound-reaction. — I have shown that a prick acta as a 

 mechanical stimulus, and in normal excitable tissues induces 

 an excitatory change of galvanometric negativity. This 

 wound-reaction increases with the extent of the wound, 

 and the suddenness wilh which it is inflicted. On account 

 of the fineness of the probe, it insinuates itself into the 

 tissue rather than make any marked rupture ; the probe 

 again is introduced very gradually ; with these precautions 

 the wound-reaction is found to be greatly reduced. The 

 immediate effect of the prick is a negative deflection of 

 the galvanometer, which declines and attains * steady value 

 in the course of about 5 minutes. 



Effect of wound on excitabilitij. — I have shewn (p. 81) that 

 severe wound caused by transverse section induced a tempor- 

 ary abolition of irritability in Mimosa, but that the normal 

 excitability was restored in the course of an hour. A prick 

 from a thick pin was shown to depress temporarily the 

 rate of growth, the normal rate being restored after an 

 interval of 15 minutes (p. 202). In the case of geo-electric 

 excitability, the depressing effect of the passage of the 

 probe, I find, to disappear in the course of about 10 

 minutes. 



For a choice of experimental material we have to find 

 specimens which are not merely geotropically sensitive, but 

 also exhibit large electric response under stimulus. In both 

 these respects the shoot of Bryophyllum and the flower 

 stalk of Nympluea give good results. 



ELECTRIC EXPLORATION FOR GEO-PERCEPTIVE LAYER BY 

 MEANS OF THE PROBE. 



Experiment 185. — I shall now proceed to give a detailed 

 account of the experiments. The first specimen employed 



