vi ELECTROMOTIVE ACTION IN VEGETABLE CELLS 19 



proposes to the exclusion of the first two alternatives, there may 

 be two different kinds of electromotive elements, which are 

 affected in opposite directions by excitation, the variation 

 reaching its maximum in one set later than in the other. 

 " In consequence of excitation," says Munk, " the cells of 

 the upper parenchymatous layer of the leaf and upper mid- 

 rib undergo a negative, those of the under layer and under 

 mid-rib a positive, variation ; i.e. the negativity of the middle 

 of the cells to their poles diminishes, in consequence of excitation, 

 in the first-named cells, and increases in the second. These 

 changes must be propagated from the seat of excitation through 

 the entire cell-mass with great rapidity, in a period that is small 

 in comparison with the duration of the process in the single cells, 

 since otherwise differences in electrical manifestations could not 

 fail to appear according to the seat of the stimulus." Munk 

 therefore believes the electrical process to be for practical purposes 

 simultaneous in all the cells affected, which in so far as the 

 transmission is plasmatic is, in view of the very low rate at 

 which excitation is propagated in vegetable protoplasm, in itself 

 highly improbable. But as a matter of fact, Munk's fundamental 

 theory of the peripolar activity of the cells of the leaf-parenchyma 

 hardly calls for contradiction, since it is modelled upon Du Bois's 

 molecular theory, applying it to visible elements, of which the 

 structure and function are a priori exclusive of any such con- 

 ception. It is a purely arbitrary presumption to regard the 

 centre of the cells involved as permanently negative to the two 

 ends, and is indeed impossible where the plasma exhibits any 



streaming movements. 



The later investigations of Burdon- Sanderson (17) have 

 rendered these phenomena more intelligible. 



In order, from the outset, to exclude the excitatory move- 

 ments of the leaf, the two lobes were fixed in plaster of Paris 

 attached to the ends of the mid-rib, while a strip of dry wood 

 was further fastened with gypsum between the two edges of 

 the lamina to the marginal bristles (Fig. 142). A favourable 

 temperature (32 35 C.) was maintained, and the plant kept in 

 a moist chamber. 



As regards electromotive action daring rest, the important 

 fact appears in contradiction to the earlier conclusions of Munk, 

 that in an overwhelming majority of cases the two opposite 



