upper strip of a differential thermometer be made of the more expansible brass and the lower of iron, the compound strip bends down with the rise of temperature. Similarly the movement of the tree might be due to the upper half being physically more expansible. It would have been possible to discriminate the physical from the physiological action by causing the death of the tree; in that case physical movement would have persisted, while the physiological action would have disappeared. As this test was not practicable, I tried the effect of physiological depression on the periodic movement of the leaf of Arenga saccharifera.
Effect of Drought: Experiment 3—In Fig. 6 is given a series of records of movement of the leaf-stalk of Arenga, first under normal condition, afterwards under increasing drought, brought about by withholding water. Tim uppermost is the thermographic record which remained practically the same for successive days. Below this are records of movement of the leaf (a) under normal condition, (b) after withholding water for three days, and (c) after deprivation for seven days. It will be noticed how the extent of movement is diminished under increasing physiological depression brought on by drought. On the seventh day, the responsive movement disappeared, there being now a mere fall of the leaf, which was slow and continuous. After this I supplied the plant with water and the periodic movement was in consequence nearly restored to its original vigour.
Effect of poison: Experiment 4.—In another experiment the normal diurnal record with the leaf was taken and the plant was afterwards killed by application of poisonous solution of potassium cyanide. The diurnal movement
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