474 LECTURE XVIII. 



Constant Galvanic Ctirrent. Elfving, in his researches on 

 the effect of the passage of electric currents through growing 

 organs (p. 410), found that when a root is placed in a vertical 

 position between two electrodes, it usually curves towards the 

 positive electrode, that is, against the direction of the current. 

 He found that a similar curvature takes place when the root 

 is traversed longitudinally by a descending current ; only in 

 the case of Brassica oleracea was the curvature towards the 

 negative pole. These phenomena may be conveniently 

 designated by the term Galvanotropism. 



Miiller-Hettlingen, in his investigation of the same subject, 

 found that the curvature of the roots of seedlings was always 

 such as to tend to place their long axes in the direction of the 

 current, the curvature being towards the negative pole. In 

 his experiments, in which the mode of experimentation was 

 somewhat different from Elfving's, the roots did not, except 

 in a few cases, die. Miiller concludes that the curvature 

 observed by him is the true galvanotropism, whereas that 

 observed by Elfving is a pathological phenomenon, not de- 

 pendent necessarily on growth, for it could be induced after 

 cutting off the tip of the root as Elfving himself states. It 

 may be pointed out that Elfving used 2, 4, or 6 Leclanche 

 cells, and Miiller 4 Grove's cells. Miiller found, further, that 

 the curvature was induced when the current traversed only 

 the tip of the root, an observation which confirms Darwin's 

 view, to which allusion has already been made, that irritability 

 is localised in the root-tip. 



It is clear that the passage of a current through a root 

 exercises a directive influence, that it induces heterauxesis and 

 so alters the direction of growth. If we accept Elfving's 

 results, we must regard the majority of roots as being negatively 

 galvanotropic, and some as being positively galvanotropic ; if 

 we accept Miiller's results, we must regard roots as being all 

 positively galvanotropic. 



The Substratum. Dutrochet observed that the hypocotyl 

 of the Mistletoe, in whatever position the seed may have been 

 placed, assumes such a direction of growth that its long axis 

 is perpendicular to the surface of the branch or other boo!y 



