Moto-excitability in Mimosa . 761 
3. The diurnal variation of excitability and its relation to the changes 
of external conditions. 
I will first give a diagrammatic view of the different parts of the 
apparatus which I devised for this investigation. The leaf of Mimosa is 
attached to one arm of a light aluminium lever, L, by means of thread. At 
right angles to the lever is the writing index, W, which traces on a smoked 
glass plate, allowed to fall at a definite rate by clockwork, the responsive 
movement of the leaf. Under a definite stimulus of electric shock the leaf 
falls down, pulling the lever L, and moving the writer towards the left. 
The amplitude of this response-curve measures the intensity of excitation. 
The leaf re-erects itself after a time, the corresponding record exhibiting 
recovery. A second stimulus is applied after a definite interval, say an 
hour, and the corresponding response shows whether the excitability of the 
plant has remained constant or undergone any variation. 
I. ( a ) Uniform Periodic Stimulation. 
Electric mode of excitation. I find that one of the best methods of 
stimulating the plant is by means of tetanizing induction shock. The 
sensitiveness of Mimosa to electric stimulation is very great; the plant 
often responds to a shock which is quite imperceptible to a human subject. 
By the employment of a sliding induction coil, the intensity of the shock 
can be regulated with great accuracy ; the secondary is gradually brought 
nearer the primary till a stimulus is found which is minimally effective. The 
intensity of stimulus actually employed is slightly higher than this, but 
within the sub-maximal range. When the testing stimulus is maintained 
constant and of sub-maximal intensity, then any variation of excitability 
is attended by a corresponding variation in the amplitude of response. 
The exciting value of a tetanizing electric shock depends (1) on the 
intensity, (2) on the duration of shock. The intensity may be rendered 
uniform by placing the secondary at a fixed distance from the primary, and 
keeping the current in the primary circuit constant. The constancy of the 
primary circuit is secured by the employment of an accumulator or storage 
cell of definite electromotive force. It is far more difficult to secure the 
constant duration of the tetanizing shock in successive stimulations at 
intervals of, say, one hour during twenty-four hours. The duration of the 
induction shock given by the secondary coil depends on the length of time 
during which the primary circuit is completed in successive excitations. I 
have succeeded in overcoming the difficulty of securing uniformity of 
duration of shock by the employment of a special clockwork device. 
The clockwork plunger. The alarum clock can be so arranged that a 
wheel is suddenly released and allowed to complete one rapid revolution at 
intervals of, say, one hour. There is a fan-governor by which the speed of 
the revolution can be regulated and maintained constant. This will specially 
