A. B. Plowman — Electrotropism of Boots. 235 



less perfectly normal way. As suggested in a former report,* 

 this condition of the root is most satisfactorily accounted for by 

 attributing the effects upon the protoplasm to the direct action 

 of the positive electrons. That the effects are not of an ordi- 

 nary chemical nature is evidenced by the fact that, so far as is 

 known, the nature and rapidity of the response is conditioned 

 solely upon the density of the current, regardless of the chemi- 

 cal composition of the electrolyte. This conclusion is based 

 upon the results of large numbers of trials with a great variety 

 of acids, bases, and salts, used always, of course, in extremely 

 dilute solution, but giving, nevertheless, a wide variation in the 

 chemical nature of the positive ions. But so long as the cur- 

 rent density is kept constant the roots behave in precisely the 

 same manner, regardless of the chemical composition of the 

 electrolyte, provided always that the chemicals are so dilute as 

 not to be directly and immediately harmful to the plants. 



Upon the basis of the electron theory all the phenomena of elec- 

 trotropism are readily and naturally explained. Whenever an 

 electric current flows through an electrolyte, there is a stream 

 of positive electrons flowing from the positive pole to the nega- 

 tive pole, while an equivalent stream of negative electrons 

 flows toward the positive pole. Any object, such as the root 

 of a seedling, dipping into the electrolyte parallel to the elec- 

 trodes, will have that side toward the positive pole exposed to 

 the stream of positive electrons, while the other side is equally 

 exposed to the negative electrons. Consequently the cells on 

 the side of the root toward the positive pole are sooner or later 

 killed by the positive electrons, while the other side of the root 

 continues to grow more or less vigorously, pushing the tip of 

 the root around in the direction of the positive pole. "W hen 

 the growing end of the root has reached a horizontal position, 

 or has become parallel to the streams of electrons, it is kept in 

 that position, since any deviation would expose the more 

 rapidly growing side to the positive electrons, with a conse- 

 quent checking of the growth and a return to the horizontal 

 position. The same explanation holds good in the case of roots 

 which are placed originally in the horizontal position with their 

 tips pointing toward the positive pole. That the delicate grow- 

 ing point is not at once killed when thus directed against the 

 destructive stream of electrons is no doubt to be accounted for 

 by the presence of the root-cap, which more or less effectually 

 shields the meristematic tissue for a time at least. 



Similarly we may explain the behavior of the seedlings used 

 as electrodes. The positive one is injured only at the point of 

 contact of the platinum wire, where the positive charge enters 

 it. The part dipping into the water is receiving a stream of 

 negative electrons, and consequently is uninjured. The nega- 

 tive seedling, on the other hand, is receiving a stream of posi- 

 *See this Journal, vol. xiv, p. 131, 1902. 



