CHAPTER V 



THE ELECTRICAL INDICATIONS OF POSITIVE AND 

 NEGATIVE TURGIDITY VARIATIONS 



Motile responses of opposite signs, characteristic of positive and negative 

 turgidity -variations Indirect hydrostatic effect of stimulus causes expansion 

 and erection of leaf Positive and negative work Wave of increased hydro- 

 static tension transmitted with relatively greater velocity than wave of true 

 excitation - Method of separating hydro-positive and excitatory effects In- 

 direct effect of stimulus, causing positive turgidity-variation induces galvano- 

 metric positivity Antagonistic elements in the electrical response Separation 

 of hydro-positive from true excitatory effect by means of physiological block. 



HAVING now described that fundamental electrical response 

 of galvanometric negativity which is characteristic of excita- 

 tion, I shall next proceed to deal with an opposite type 

 of response namely, that of galvanometric positivity. The 

 combination of these two factors, in varying degrees of each, 

 in the electrical response of plants, has been a source in the 

 past of the greatest perplexity, leading investigators to con- 

 tradictory results. And it can only be by disentangling them, 

 and by ascertaining the conditions under which each invari- 

 ably occurs, that precision will be arrived at in the field of 

 electrical response. 



We have seen that excitation of the pulvinus of Mimosa 

 induces negative turgidity-variation, with fall of the leaf, and 

 galvanometric negativity. What, then, would be the out- 

 ward expression of an increase of turgidity that is to say, of 

 the positive turgidity-variation ? 



With regard, first, to the mechanical expression, we may 

 subject the question to an experimental test. The cut end 

 of a branch of Mimosa, bearing leaves, is fixed watertight 

 in one end of a U-tube, filled with water, and the other 



