On account of its small size, the petiole-pulvinus preparation offers great facilities for mounting in different ways suitable for special investigations. Ordinarily, the cut stem with its lower end enclosed in moist cloth is supported below. A very suitable form of stimulus is that of induction shock from a secondary coil, the intensity of which is capable of variation in the usual manner by adjusting the distance between the primary and the secondary coils. The motile pulvinus, P, may be excited directly. For investigations on velocity of transmission of excitation, stimulus is applied on the petiole at some distance from the pulvinus, by means of suitable electrodes. Excitation is now transmitted along the intervening length of petiole, the conducting power of which will be found appropriately modified under the action of chemical and other agents. In this normal method of mounting, the more excitable lower half of the pulvinus is below; excitatory reaction produces the fall of the petiole, gravity helping the movement. The preparation may, however, be mounted in the inverted position, with the more excitable lower half of the pulvinus facing upwards. The excitatory movement will now be the erection of the petiole, against gravity. Under natural conditions the stem is fixed, and it is the petiole which moves under excitation. But a very interesting case presents itself when the petiole is fixed and the stem free. Here is presented the unusual spectacle of the plant or the stem “wagging” in response to excitation. THE CHANGE OF EXCITABILITY AFTER IMMERSION IN WATER. The isolated specimen can be kept alive for several days immersed in water. The excitability of the pulvinus, however, undergoes great depression, or even abolition, by the sudden change of turgor brought on by excessive absorption of water. The plant gradually accommodates itself to the changed condition, and the excitability is restored in a staircase manner from zero to a maximum. In studying the action of a chemical solution on excitability, the solution may be applied through the cut end or directly on the pulvinus. The sudden variation of turgor, due to the liquid, always induces a depression, irrespective of the stimulating or the depressing action of the drug. The difficulty may be eliminated by previous long-continued application of water on the pulvinus and waiting till the attainment of uniform excitability which generally takes place in the course of about three hours. Subsequent application of a chemical solution gives rise to characteristic variation in the response. QUANTITATIVE DETERMINATION OF THE RATE OF DECAY OF EXCITABILITY IN AN ISOLATED PREPARATION.