' PLANTS UNDER MECHANICAL STIMULUS. 281 
represents the intensity of variation of current, and the abscissa 
the time. The curves are obtained (1) directly, by tracing the 
excursion of the galvanometer spot of light on a revolving drum. 
The drum, on which is wrapped the paper for receiving the 
record, is driven by clockwork (fig. 4). Different speeds of 
revolution can be given to it by adjustment of the clock-governor, 
or by changing the size of the driving-wheel. The galvanometer 
spot is thrown down on the drum by an inclined mirror. A 
stylographic pen attached to a carrier rests on the writing 
surface. The carrier slides over a rod parallel to the drum. 
On stimulation, the resulting excursion of the spot of light is 
followed by moving the carrier which holds the pen; the rising 
portion of the response-curve is thus obtained. On the cessation 
of stimulus the excitatory effect will gradually disappear, and 
the galvanometer spot will then return more or less gradually 
to its original position, and that part of the curve which is 
traced during this process constitutes the recovery. As said 
before, the ordinate in these curves represents the E. M. variation, 
and abscissa the time. We can calibrate the value of the 
deflection by applying a known E.M.Force to the circuit from a 
compensator, and noting the deflection which results. The 
speed of the clock is previously adjusted so that the recording 
surface moves exactly through, say, one inch a minute. Of 
course this speed can be increased to suit the particular 
experiment, and in some it is as high as 6 inches a minute. 
In this simple manner very accurate records may be made. 
It has the additional advantage that it can at once be seen 
whether the specimen is suitable for the purpose of investigation. 
A large number of records might be taken by this means in a 
comparatively short time. 
Photographic Recorder.—Or the records may be made photo- 
graphically. ‘A clockwork arrangement moves a photographic 
plate at a known uniform rate, and a curve is traced on the 
plate by the moving galvanometer spot of light. All the records 
that will be given are accurate reproductions of those obtained 
by one of those two methods. Photographic records are repro- 
duced in white against a black background. 
Means of Graduating the Intensity of Stimulus. 
One of the necessities in connection with quantitative measure- 
ments is to be certain (1) that the intensity of successive stimuli 
LINN, JOURN.—BOTANY, VOL. XXXV. Z 
