154 RALPH S. MILLIE. 



filaments are formed only in the immediate neighborhood of the 

 copper and are entirely absent near the zinc. Copper and zinc 

 influence the reaction of the iron wire with the solution in oppo- 

 site manners, the former promoting the formation of iron fila- 

 ments, the latter inhibiting this process. The zinc and the 

 copper, although separated by the length of iron wire, can be 

 shown to modify each other's action; thus filaments grow out 

 from the zinc under the influence of the iron, but more slowly 

 than in the absence of the copper; similarly the growth of 

 filaments from the iron near the copper is much slower than in 

 the absence of the zinc. The effect is as if the formation of 

 filaments from the zinc inhibits or holds in check the formation 

 of filaments from the iron, and vice versa. This mutual inter- 

 ference is obviously due to the intersection of the electrical 

 currents from the two circuits, and resulting compensation- 

 effects. Severing the iron wire between the copper and the zinc 

 removes this influence, with the result that then filaments are 

 put forth rapidly from both iron and zinc; i. e., the process at 

 either filament-forming metal is released from a certain con- 

 straint, due to the polarizing influence of the electrical current 

 from the other circuit; it then proceeds more rapidly than 

 before. Physiological analogies to this type of effect readily 

 suggest themselves; the development of shoots in one region 

 of a plant stem inhibits their development in adjoining regions; 

 removal of a rapidly growing shoot may thus cause buds to 

 develop from the axils and elsewhere, which otherwise would 

 have remained dormant. 1 Similar phenomena are seen in hy- 

 droids and other animals, and will be discussed more fully below. 

 Physiological "dominance" of this type is often simulated in a 

 suggestive manner by experiments of the above kind. For 

 example, a thin strip of zinc 3 cm. long was in contact at one 

 end w T ith a piece of pure iron wire and at the other end with a 

 piece of magnesium. A well-marked gradient was shown; near 

 the iron filaments were rapidly formed from the zinc, but near 

 the magnesium slightly and slowly. It was noteworthy that 

 the piece of iron wire gradually formed filaments in spite of the 

 contact with the zinc; i. e., the presence of the magnesium 



1 For numerous examples cf. McCallum, "Regeneration in Plants," Botanical 

 Gazette, 1905, Vol. 40, pp. 97, 241. 



