CONDUCTIVITY AND PERMEABILITY 235 



easy to see that reactions may occur which may produce 

 the responses characteristic of mechanical stimulation. 

 The occurrence of such reactions seems probable, since 

 many cases are known where substances in close juxta- 

 position are prevented from reacting by the presence of 

 semipermeable layers: when these layers are destroyed 

 (by crushing the cells) the reaction at once takes place. 



If these processes occur it is evident that purely 

 physical alterations in the protoplasm can give rise to 

 chemical changes. Responses to contact and mechanical 

 stimuli may thus be explained; and since gravitational 

 stimuli involve deformation of the protoplasm we may 

 extend this conception to geotropism. 66 



In this conception of mechanical stimulation the 

 essential things are (1) substances which are more or 

 less completely prevented from reacting by semipermea- 

 ble surfaces, (2) a deformation of the protoplasm suffi- 

 cient to produce in some of these surfaces a rupture which 

 is not at once repaired, (3) a resulting reaction which 

 produces the characteristic response to the stimulus. 



Some authors (particularly Lillie and Hober) 67 as- 

 sume that stimulation is always associated with a change 

 in permeability, whereby the cell surface (which is 



60 Small (1918) has made experiments on geotropism by inserting 

 electrodes (a short distance apart) near the tip of the root and measuring 

 the electrical conductivity before and during stimulation. He states 

 that when the root is placed at angle to the vertical the resistance of 

 both the upper and lower sides decreases, but the decrease is less in the 

 upper side. He interprets this to mean that the permeability of both 

 sides increases, but the increase is less in the upper side: in consequence 

 it is more turgid and thus causes the downward bending of the tip. 



Owing to the source of error in this method it is difficult to judge of 

 the value of the results. It seems highly probable that upon changing 

 position of the root there would be a movement of liquids and gases in 

 the intercellular spaces which would change the conductivity. 



"Lillie (1911, 1913, 1914, A, B] ; Hober (1914). 



