The employment of the large Palm was obviously impracticable in these in­ves­ti­ga­tions. I, therefore, searched for other plant-organs in which the movement under variation of temperature was similar to that of the Date Palm. I found that the horizontally spread leaves of vigorous specimens of Arenga saccharifera growing in a flower pot executed movements which were practically the same as that of the Faridpur tree. The leaf moved downwards with rise of temperature and vice versâ.

There are many practical advantages in working with a small specimen. It can easily be placed under glass cover or taken to a glass house, thus completely eliminating the troublesome disturbance caused by the wind.

Diurnal movement in continued darkness: Experiment 2.—The plant was placed in a dark room and records taken continuously for three days. These did not differ in any way from the normal records taken in a glass house under daily variation of light and darkness. Exposure of plant to darkness for the very prolonged period of a week or more, undoubtedly interferes with the healthy photo-tonic condition of the plant. But such unhealthy condition did not make its appearance in the first few days.

PHYSIOLOGICAL CHARACTER OF THE MOVEMENT.

There may be a misgiving that the movement of the tree might be due to physical effect of temperature. If the 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.

Fig. 6. Effect of physio­logic­al depression on diurnal movement of the petiole of Arenga saccharifera. The uppermost curve exhibits variation of temperature, (a), normal diurnal curve, (b), modi­fi­ca­tion after 3 days’ and (c) after 7 days’ withholding of water.

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. The 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 was found permanently abolished at the death of the plant.