412 EFFECT OF ELECTKICITT [Ch. XVI 



The location of the sensitive region has been. demonstrated 

 by various methods. Mxjllee-Hettlingen placed the tip only 

 of a radicle in contact with moist flannel through which the 

 current was passing ; the radicle turned from the anode. Also 

 Beunchoest ('84) found that Avhen merely the apex was in 

 contact with the electrified water there was a marked turning 

 from the anode, even when the current was so powerful as to 

 cause the submerged root to turn towards the anode. The tip 

 is sensitive. Next Betjnchokst cut off the tip and found that 

 no response occurred when the current traversed the radicle. 

 Hence the tip is necessary to response, and it may be con- 

 cluded provisionally that as in geotropism so in galvanotropism 

 the root tip is the only sensitive part of the radicle. 



The critical point at which the electrotropic effect passes 

 into the mechanical one is indicated by a sigmoid turning of 

 the radicle. According to Betjnchoest's observations this 

 critical point lies, at a temperature of 20°, for Phaseolus seed- 

 lings near 1.2 S; for Helianthus near 1.3 S; for Lupinus, 

 2.5 S; for Brassica, 38; for Lepidium, 3.5 8. These results 

 indicate that the different species have a diverse sensitiveness 

 to the electric current, so that an intensity which causes the 

 radicle of one species to turn from the anode will cause the 

 •other to turn towards it (false electrotropism).* 



3. Electrotropism in Other Organisms. — While the stolons 

 of hydroids, Bryozoa, and tunicates offer an excellent oppor- 

 tunity for experiments on electrotropism in animals, results 

 have been obtained, so far as I know, outside the group of 

 phanerogams only in the mold Phycomyces. Heglee ('92) 

 has subjected this organism not to the ordinary electric current, 

 but to Hbetz' electric waves.f The result was that, after 

 exposing to the radiant electricity for from three to six hours, 

 the sporangiferous hyphse curved markedly from the source of 



* We can now easily understand why Elfving obtained, with approximately 

 the same current, a positive curvature in many species, hut a negative one with 

 Brassica. As the list just given shows, the critical point in Brassica lies high. 



t These were obtained by reflecting, by means of a parabolic tin reflector, 

 the radiant energy upon the stems of the fungi. The latter were covered with 

 a pasteboard box, to keep out light, and the whole experiment was performed 

 in it darkened room. The fungi were experimented with when about 8 to 10 

 cm. long, a period when they are most sensitive to light. 



