428 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
eight times more sensitive to an electrical current than a human 
being. With regard to the stimulus of induction shock, Mimosa 
is ten times as sensitive. As with the animal so also with the plant, 
the effect of stimulus is additive; that is to say, effective stimulation 
is determined not only by the intensity, but also by the duration 
of application. In fact, I have been able to establish in plants a 
strictly quantitative relation as regards the additive effect of sub- 
minimal stimulus, which is, that the effective excitation is equal to 
individual intensity of stimulus multiplied by the number of repeti- 
tions. In order that successive stimulations may be uniform, we 
have to assure ourselves that the duration of the tetanizing shock 
Fic. 5.—Diagrammatic representation of automatic plant-recorder. Petiole of Mimosa, 
attached by thread to one arm of lever L; writing index W traces on smoked 
- glass plate G the responsive fall and recovery ofleaf. P, primary,and S, secondary, of 
induction coil. Exciting induction shock passes through the plant by electrodes E, E’. 
A,accumulator. C, clockwork for regulating duration of tetanizing shock. Primary 
circuit of coil completed by plunging rod R dipping into cup of mercury M. 
is maintained absolutely constant. This I am able to secure by 
means of the special device of automatic stimulator. The results 
of experiments to be presently described appeared so astonishing 
that for many reasons it became highly desirable to remove com- 
pletely all elements of personal equation. In fulfilment of this, I 
spent several years in perfecting various instruments by which 
the plant attached to the recording apparatus is automatically 
excited by successive stimuli which are absolutely constant. In 
answer to this it makes its own responsive records, goes through its 
period of recovery, and embarks on the same cycle over again, 
without assistance at any point from the observer. (Fig. 5.) In 
this way the effect of changed external condition is seen recorded 
in the script made by the plant itself. 
