230 A. B. Plowman — Electrotropism of Moots. 



density as the amount of current passing a unit cross-section of 

 the conductor in unit time. This, in any case, will evidently 

 depend upon the E.M.F. of the current, the conductivity of 

 the conductor, and upon its sectional area and the distance be- 

 tween electrodes. The unit of current density is the milli- 

 ampere, and the current is of unit density when the conditions 

 of the circuit are such that one milli-ampere of current passes 

 l cm2 of sectional area of the conductor in one second. 



The general method of experimentation has been similar to 

 that followed by Elfving. Fig. 1 (Plate IX) shows one of the ves- 

 sels used in subjecting roots of seedlings to the action of an elec- 

 tric current. Internally this vessel measures about 5 cm wide, 15 cm 

 long, and 18 cm deep. When in use the cubette is wrapped with 

 black paper in order to protect the roots from the influence of 

 light. In the majority of cases the electrolyte employed has 

 been ordinary tap-water, though as one phase of the study, 

 dilute solutions of a great variety of substances were used. 

 Since the products resulting from the electrolysis of common 

 metallic electrodes such as copper, zinc, tin, etc., are usually 

 highly injurious to plants, it was found necessary to use elec- 

 trodes of platinum or carbon. The latter material has proved 

 to be entirely satisfactory, and it is to be recommended for its 

 cheapness and convenience. In the small vessel shown in fig. 

 1, the electrodes are "Electra" arc-lamp carbons, f" x 7", 

 fitted with binding-posts for connecting the wires. In a larger 

 vessel plate- carbon electrodes were used. These were of high- 

 grade carbon, -6 cm x 4 cm x 15 cm , each with a binding-screw. It 

 has been shown by repeated experiments that the product of 

 electrolytic decomposition of these carbons is perfectly harmless 

 to plants, even when present in sufficient amount to render the 

 electrolyte quite black. 



The seedlings are suspended by glass hooks passing through a 

 sheet of cork which serves as a cover for the vessel. In this way 

 perfect insulation is secured, thereby preventing any passage of 

 current through the root in the direction of its longer axis. 



The seeds are first germinated in moist sphagnum, and when 

 the radicles have reached a length of 2 cm to 4 cm , the seedlings 

 are attached to the glass hooks with their roots pointing 

 directly downward and dipping a short distance into the water 

 in the jar. If left to grow normally in such a situation, some 

 roots will grow directly toward the bottom, while others bend 

 about in a more or less tortuous path. It is a well-known fact 

 that different species of plants show characteristic behavior in 

 this respect. Thus Pisurn sativum, many, varieties of Zea mais, 

 and various other plants develop quite crooked roots when 

 grown in water, while other varieties of Zea mais, Lupinus 

 albus, Hyacinthus orientalis, etc., grow long straight roots 

 under these circumstances. 



However, if a current of electricity be passed through the 



