362 



Mr. J. Small. Changes of Electrical 



He also carried out experiments which proved that the responsive peculiari- 

 ties of the root are shared by the shoot. 



The present investigation proves that the mechanism of geotropic response 

 by the root depends upon permeability changes. These changes in perme- 

 ability explain the general curvature, the stronger curvature at 90° to the 

 vertical, and the retardation of the elongation of the axis during curvature 

 (Sachs, 7). The perception of gravity by plants is thus brought into line 

 with the response in animals to the same stimulus. It is suggested that it 

 is a general property of protoplasm to react to stimuli according to the 

 sigmoid relation, and that the perception of gravity by protoplasm in general 

 attains an accuracy of between 3 and 4 per cent, of the total stimulus [vide 

 fig. 2 and "Weber (10) on the perception of gravity by man with an accuracy 

 of 3*3 per cent.]. 



Summary. 



1. That the perception of gravity by the root is a protoplasmic phenomenon 

 is proved by the record of the changes in the electrical resistance of the 

 second millimetre of one side of the root-tip of Vicia Faba at various angles 

 to the vertical. These changes show the same sigmoid curve as is shown by 

 animal tissue in response to stimuli. 



2. The permeability of the cortical cells of both the upper and' the under 

 sides of the root-tip increases when the root is placed at an angle with the 

 vertical, but that of the underside does so to a greater extent. The con- 

 sequent relatively greater turgidity of the cells of the upper side explains the 

 curvature. The increase in permeability, i.e., decrease in turgor and electrical 

 resistance, of the upper and under sides increases with the angle to the 

 vertical, but the permeability of the upper side increases at a slower rate than 

 that of the under side ; this explains the stronger curvature when the plant 

 is horizontal {vide fig. 13). 



3. The increased permeability, giving decreased turgor, on both sides of the 

 root explains the retardation of the elongation of the axis shown by Sachs (7) 

 to take place during curvature. 



In conclusion, I would record my indebtedness to my colleagues of the 



Chemical, Physiological, Physical and Psychological Departments at Bedford 



College, especially to Dr. J. S. Edkins and Dr. J. F. Spencer, for valuable 



assistance and advice.* 



* It is obviously desirable to confirm the results obtained with the telephone method 

 by the galvanometer, for two reasons : (1) the induction currents used in the telephone 

 method produce alterations of resistance ; (2) the subjective detei'mination of the 

 minimum-sound point is sometimes dubious and cannot be demonstrated to an audience. 

 Experiments with the galvanometer are in progress but are not yet completed. 



