A. B. Plowman — Electromotive Force in Plants. 101 



the reaction at Y. Since the plant had been resting during 

 the hour before and the hour after sunset, it was thought that 

 its reversal of normal current might be due to a change in 

 physiological activity. Accordingly, a little later in the season 

 a plant was tested for daily variations in its normal current. 

 Fig. 13 shows its behavior in this respect for the first 54 hours 

 after it was removed from the greenhouse to the laboratory. 

 The sudden drop in temperature caused the plant-current to 

 turn downward for a few hours, after which it flowed upward 

 with increasing intensity until about 6 P. M. At 9 A. M. next 

 day the current was downward. At noon no current was pres- 

 ent, and then it was upward until 6.30 P. M. With minor 

 variations for temperature-change, this day's record was found 

 to be quite typical. Other experiments also point to the con- 

 clusion that when plants are physiologically active the normal 

 current is upward, while in the comparatively passive plant this 

 current is either wanting or downward. 



Figs. 14& and 14 show the results of a series of experiments 

 to test the effects upon the normal E.M.F. of mechanical in- 

 jury and heat. The plant showed the usual upward current at 

 2 P. M. When the stem was heated at the lower contact to 35° 

 C. the intensity of the upward current was increased. Upon 

 cooling this terminal to the normal temperature and heating the 

 upper one the current was reversed, as shown at I. On cooling 

 this terminal the current came back almost to its original condi- 

 tion. Next, light pressure was applied to the stem at the point of 

 contact, with resulting current toward the other contact through 

 the stem, as shown at II in the figure. Piercing the stem near 

 the contacts with a platinum needle produced no perceptible 

 effects on the E.M.F. At III, the terminal points were alter- 

 nately heated to 40° C. for a very short time. After this test 

 the normal current was reduced to zero. The stem was then 

 crushed midway between contacts with no indication of current. 

 At IY the stem was crushed at the contact, showing a momen- 

 tary E.M.F. through the stem, away from the point of injury. 

 Heat to 40° C. gave the reaction at Y, the current always flow- 

 ing through the stem from the point of high temperature. 



The stem was now completely crushed between the contacts, 

 but no E.M.F. was shown while this was being done. Heat to 

 40° C. gave YI. At YII the usual battery current was applied. 

 The reaction indicates a higher conductivity than in a normal 

 stem. This is readily explained by the fact that in the crushed 

 stem the intercellular spaces are filled with the cell-sap, which 

 thus forms a conductor of less specific resistance than is met with 

 in the normal condition of ihe plant, where the electric current 

 must pass through cell walls and films of living protoplasm. At 

 YIII the forced current was repeated, and the reaction indi- 



