232 A. B. Plowman — Electrotrojpism of Roots. 



from the action of the current, bend downward again near the 

 tip, forming a double curve, and continuing to grow in an alto- 

 gether normal way again. In such cases there is always devel- 

 oped a conspicuous enlargement of the root at the second, or 

 lower, curve, and a constriction at the first, or upper, one. 

 This constriction is always in the form of a flattening of the 

 concave side of the curve. 



The location of the greatest curvature in the root is, as Elf- 

 ving observed, at the point of most rapid normal elongation. It 

 should be added, however, that the sharpest part of the curve 

 is at the point of most rapid elongation at the time when the 

 current is first turned on. This fact is most strikingly evident 

 in the case of very vigorous roots exposed to a rather weak cur- 

 rent. In long roots which are exposed for their whole length to 

 the current, there is but little curvature in the piliferous zone. 

 Hence it appears that the curvature is dependent upon active 

 growth of the cells. 



When the seedlings are suspended so that the root-tips just 

 touch the water in the jar, the curvatures are usually different 

 from those developed when the roots are wholly submerged, 

 and the reaction varies more noticeably with change in amount 

 of current. For example, if seedlings of Lupinus alhus are 

 arranged with roots dipping l mm or less into the water, and a 

 very strong current, say 10 milli-amperes, is passed for a few 

 minutes, the root-tips soon curl to such an extent as to be lifted 

 out of the water. In a few instances the roots have continued 

 to grow horizontally for several millimeters just above the sur- 

 face of the water, and toward the positive pole, but they are 

 usually killed at once by a current of this density. With a 

 very weak current the roots do not commonly begin to curve 

 perceptibly until the zone of most rapid elongation reaches the 

 surface of the water, when a prompt response occurs, just as 

 in the case of wholly submerged roots. 



When the seedlings are placed with their roots horizontally, 

 or parallel to the path of the current, those which point toward 

 the positive pole continue to grow in that direction, contrary 

 to the influence of the force of gravity, so long as the current 

 is comparatively weak. A stronger current quickly kills these 

 roots, though without producing any curvature in them. The 

 roots pointing toward the negative pole usually bend downward 

 in a perfectly normal manner for a little time, then the curva- 

 ture becomes very abrupt, and the tips are turned back toward 

 the positive pole. If the current is turned off at this stage, the 

 roots sometimes continue to bend, forming a complete coil, 

 and then growing downward again. It usually happens, how- 

 ever, that the seedlings are killed by this treatment. 



Elfving has shown that when an electric current is passed 

 upward through a root the cells are quickly killed, while other 

 roots transmitting a similar current downward are not seriously 



