_Barium Chloride: Experiment 28._--The action of this agent is very characteristic, inducing great sluggishness in recovery. The preparation had been kept in 1-per cent. solution of this substance for two hours. After this the first response to a given test-stimulus was taken; the response was only moderate, and the recovery incomplete. The sluggishness was so great that the next stimulation, represented by a thick dot (Fig. 36), was ineffective. Tetanising electric shock at T, not only brought about response, but removed for the time being the induced sluggishness. This is seen in the next two records, which were taken under the old test-stimulus. There is now an enhanced response and a complete recovery. Beneficial effect of tetanisation disappeared, however, on the cessation of stimulus. This is seen in the next two records which were taken after two hours. The amplitude of response was not only diminished, but the recovery also was incomplete. [Illustration: FIG. 37.--Antagonistic action of alkali and acid. Arrest of response in contraction under NaOH (↑), restoration and final arrest in expansion under lactic acid. (↑)] _Antagonistic actions of Alkali and Acid: Experiment 29._--Alkali and acid are known to exert antagonistic actions on the spontaneous beat of the heart; dilute solution of NaOH arrests the beat of the heart in systolic contraction, while dilute lactic acid arrests the beat in diastolic expansion. I have found identical antagonistic reactions in the pulsating tissue of _Desmodium gyrans_, the telegraph plant. It is very interesting to find that these agents also exert their characteristic effects on the response of _Mimosa_ in a manner which is precisely the same. This is seen illustrated in Fig. 37, where the application of NaOH arrested the response in a contracted state; after this, the antagonistic effect of dilute lactic acid is seen first, in its power of restoring the excitability; its continued application, however, causes a second arrest, but this time in a state of relaxed expansion. _CuSO_4 Solution._--This agent acts as a poison, causing a gradual diminution of amplitude of response, culminating in actual arrest at death. Certain poisons, again, exhibit another striking symptom at the moment of death, an account of which will be given in a separate paper. EFFECT OF “FATIGUE” ON RESPONSE. [Illustration: FIG. 38.--“Fatigue” induced by shortening intervening period of rest.] With _Mimosa_, after each excitation the recovery becomes complete after a resting period of about 15 min. With this interval of rest the successive responses for a given stimulus are equal, and are at their maximum.