THE ELECTRO-MOTIVE RESPONSE OF PLANTS 13 



points of the midrib of the leaf. On stimulation of the 

 lamina, this current was found to undergo a diminution, or 

 negative variation. But the current in the stalk, or petiole 

 was found to undergo the opposite change that is to say, a 

 positive variation. Kunkel, in working with the pulvinus of 

 Mimosa, found that on excitation a series of opposite or 

 oscillatory electro-motive variations was induced. He also 

 found electro-motive differences to be induced as the result 

 of the flexure or injury of ordinary stems He believed all 

 these electrical phenomena to be consequent on hydrostatic 

 disturbance or water-movement. 



By means of this ' migration of water' Kunkel attempted 

 to explain all the electrical phenomena in vegetable organs. 

 The electro-motive difference between different parts of an 

 organ was due, according to him, to their different powers of 

 absorption of water. The greater absorptiveness of one 

 point, with its consequent greater movement of water, would 

 render that point relatively positive. As against this, how- 

 ever, Haake pointed out that electrical differences between 

 different points were also to be found, even in submerged 

 plants, like Valisnaria and Nitella, in which there could not 

 possibly be any differences of absorptiveness. This difference, 

 therefore, he suggested, must be ascribed to some vital 



\process, inasmuch as the P.D. is seen to undergo a change 

 whenever the respiratory process is interfered with, as, say, 



I by the substitution of hydrogen for oxygen. 



We shall study later in greater detail the conditions on 

 which this ' current of rest,' so called, actually depends 

 (Chapter X.). But more important is the excitatory variation 

 induced by stimulus. It has already been stated that 

 Kunkel found oscillatory variations of the current to occur 

 on stimulation of the leaf of Mimosa. These alternating 

 variations were difficult to reconcile with his theory of the 

 active, single-phased displacement of water, as he did not 

 fail to see, and he suggested that the first negative swing 

 observed might be due to the disturbance of the diffusion- 

 process through alterations of the protoplasm. 



