THE ELECTRIC PROBE 329 



the action of gravitational stimulus ; it will disappear 

 when the organ is held in a vertical position and thus freed 

 from geotropic excitation. 



Actual Observations 



The degree of the induced galvanometric negativity of 

 the upper side of the horizontal stem was found to exhibit 

 variation at different depths, and to attain a maximum 

 value at a definite layer, beyond which there is a decline. 

 The geo-perceptive layer can thus be experimentally localised 

 by measuring the depth of the intrusion of the probe at 

 which the maximum galvanometric negativity is detected. 



The electric response of the lower side of the organ to 

 gravitational stimulus is of an opposite sign to that of the 

 upper side, galvanometric positivity, indicative of expansion 

 and increase of turgor. The electric indications on the 

 lower side also exhibit variation in the different layers, the 

 maximum positivity occurring at the perceptive layer. 



In animals it has been found that the weight of heavy 

 particles acting on sensitive protoplasm causes the percep- 

 tion of the direction of gravity. From histological con- 

 siderations Haberlandt and Nemec came to the conclusion 

 that heavy particles, such as starch-grains, perform a similar 

 function in many plants. The electro-physiological investi- 

 gation which I undertook had as its objects the exact 

 locahsation of the sensory cells in a living condition, and, 

 as already stated, the recording of the changes accompany- 

 ing their functional activity under geotropic stimulation. 

 The electric results obtained fully confirm the statolith- 

 theory in the conclusion that it is the ' starch-sheath,' con- 

 taining a number of large-sized starch-grains, which is the 

 geo-perceptive organ. 



The Method of Experiment 



In the practical apphcation of the probe the difiiculty 

 anticipated was the after-effect of the wound inducing 



