eness of stimulus dependent on rapidity also.# In order that
successive stimuli may be equally effective another point has to be
borne in mind. In all cases of stimulation of living tissue it is found
that the effectiveness of a stimulus to arouse response depends on the
rapidity of the onset of the disturbance. It is thus found that the
stimulus of the 'break' induction shock, on a muscle for example, is
more effective, by reason of its greater rapidity, than the 'make'
shock. So also with the torsional vibrations of plants, I find response
depending on the quickness with which the vibration is effected. I give
below records of successive stimuli, given by vibrations through the
same amplitude, but delivered with increasing rapidity (fig. 13).

[Illustration: FIG. 13.--INFLUENCE OF SUDDENNESS ON THE EFFICIENCY OF
STIMULUS
The curves _a_, _b_, _c_, _d_, are responses to vibrations of the same
amplitude, 30 deg.. In _a_ the vibration was very slow; in _b_ it was
less slow; it was rapid in _c_, and very rapid in _d_.]

Thus if we wish to maintain the effective intensity of stimulus constant
we must meet two conditions: (1) The amplitude of vibration must be kept
the same. This is done by means of the graduated circle. (2) The
vibration period must be kept the same. With a little practice, this
requirement is easily fulfilled.

The uniformity of stimulation which is thus attained solves the great
difficulty of obtaining reliable quantitative values, by whose means
alone can rigorous demonstration of the phenomena we are studying become
possible.

FOOTNOTES:

[8] A preliminary account of Electric Response in Plants was given at
the end of my paper on 'Electric Response of Inorganic Substances' read
before the Royal Society on June 6, 1901; also at the Friday Evening
Discourse, Royal Institution, May 10, 1901. A more complete account is
given in my paper on 'Electric Response in Ordinary Plants under
Mechanical Stimulus' read before the Linnean Society March 20, 1902.

I thank the Royal Society and the Linnean Society for permission to
reproduce some of my diagrams published in their _Proceedings_.--J. C. B.

[9] By this is meant a rapid to-and-fro or complete vibration. In order
that successive responses should be uniform it is essential that there
should be no resultant twist, i.e. the plant at the end of vibration
should be in exactly the same condition as at the beginning.

CHAPTER IV

ELECTR