A. B. Plowman — Electrotropism of Roots. 231 



water in which these roots are growing, they almost without 

 exception turn, sooner or later, toward the positive pole. The 

 time required for this reaction depends upon the kind of plant, 

 as well as upon the density of the current. In a uniform cur- 

 rent, those plants which show most rapid normal growth of the 

 roots are the ones which show a typical curvature most quickly. 

 For any given kind of root there is a maximum current density 

 at which the curvature is produced most rapidly, while a weaker 

 current requires a longer time to bring about the same results. 

 On the other hand, if the current density be raised even 

 slightly above the specific maximum, the roots are killed before 

 curvature can take place. As is to be expected, this specific 

 maximum differs considerably in different plants, and also for 

 roots of different degrees of maturity in the same plant. Thus 

 the maximum effect is produced in a radicle 2 cm long of Lupinus 

 albus by a current density of about I milli-ampere, while a 

 similar root of Zea mais reacts most rapidly in a current of 1*5 

 milli-amperes. For roots which have reached a length of 12 cm 

 or 15 cm the maximum current density is less than for the shorter 

 roots, being about -8 milli-ampere for Lupinus and 1 milli- 

 ampere for Zea ; and the maximum curvature is less rapid than 

 in the case of young roots. 



The abruptness of the curvature is dependent chiefly upon 

 the kind of plant and the vigor of the root, but, other things 

 being equal, it appears that the sharpest curves are formed in a 

 current considerably below the specific maximum density. It 

 frequently happens that roots of Zea mais are curved through 

 an angle of 90° in a length of l cm or less, and very young 

 radicles of Lupinus albus have been bent through a right angle 

 in a length hardly exceeding twice the diameter of the organ 

 at the middle point of the curve. 



Fig. 2 represents a number of young seedlings of various size 

 of Zea mais, after being exposed for one hour to a current 

 density of 1*2 milli-amperes. The sharpest curvature in this 

 case is shown by the shortest of the radicles, while the longer 

 ones show a more gradual bending. If a weak current is kept 

 on such seedlings as these for several hours, it is found that the 

 majority of the roots continue to grow horizontally toward the 

 positive pole. The maximum current density invariably kills 

 the roots in a short time, as was observed by Elfving, but it 

 has been found possible to adjust the current to such a strength 

 as not to kill the roots and yet to hold them to their horizontal 

 course against their normal geotropic tendency. 



On the other hand, if the current is turned off after an hour, 

 and the seedlings are left in the water for a time, we often ob- 

 tain such results as those shown in fig. 3. Here most of the 

 roots have continued to curve, in some cases forming complete 

 coils. It has often happened that roots which have been less 

 seriously affected than those shown in fig. 2 will, when removed 



