[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 ELECTRIC RESPONSE IN PLANTS--BLOCK METHOD Method of block--Advantages of block method--Plant response a physiological phenomenon--Abolition of response by anaesthetics and poisons--Abolition of response when plant is killed by hot water. I shall now proceed to describe another and independent method which I devised for obtaining plant response. It has the advantage of offering us a complementary means of verifying the results found by the method of negative variation. As it is also, in itself, for reasons which will be shown later, a more perfect mode of inquiry, it enables us to investigate problems which would otherwise have been difficult to attempt.