Page:Life Movements in Plants.djvu/152

122 That is to say, ''the passage of a feeble current modifies conductivity for excitation in a selective manner. Conductivity is enhanced against, and diminished with, the direction of the current''.

The minimum current which induces a perceptible change of conductivity varies somewhat in different specimens. The average value of this minimal current in autumn is 1.4 microampères. The effect of even a feebler current may be detected by employing a test stimulus which is barely effective.

Having demonstrated the effect of direction of current on the velocity of transmission, I shall next describe other methods by which induced variations of conductivity may be exhibited.

In this method we employ a minimal stimulus, the transmitted effect of which under normal conditions gives rise to a feeble response. If the passage of a current in a given direction enhances conductivity, then the