These experiments conclusively prove that the periodic movement of the leaf-stalk induced by variation of temperature is a physiological phenomenon, and from analogy we are justified in drawing the inference that the movement of the Faridpur tree is also physiological. The question, however, was finally settled by the unfortunate death of the tree which occurred the other day, nearly a year after I commenced my investigations. While presiding at my lecture on the subject, His Excellency Lord Ronaldshay, the Governor of Bengal, announced that a telegram had just reached him from his officer at Faridpur that “the palm tree was dead, and that its movements had ceased.”
Since my investigation with the Faridpur ‘Praying’ Palm, I have received information regarding other Palms, which exhibit movements equally striking. One of the trees is growing by the side of a tank, the trunk of the tree being inclined towards it. The up-lifted leaves of this tree are swung round in the afternoon and dipped into the water of the tank.
The movement of the tree has been shown to be brought about by the physiological action of temperature variation; in other words the diurnal movement of the ‘Praying’ Palm is a THERMONASTIC PHENOMENON. I have found various creeping stems, branches and leaves of many trees, exhibit this particular movement of fall with a rise of temperature, and vice versâ. Such movements, I shall, for the sake of convenience, distinguish as belonging to the negative type.
Having found that the temperature is the modifying cause, the next point of inquiry relates to the discovery of the force, whose varying effects under changing temperature induces the periodic movement. I shall, in this connection, first discuss the various tentative theories that may be advanced in explanation of the movement.
TRANSPIRATION AND DIURNAL MOVEMENT.
It may be thought that the fall of the tree during rise of temperature may be due to passive yielding of the tree to its weight, there being increased transpiration and general loss of turgor at high temperature. I shall, however, show that the diurnal movement persists in the absence of transpiration.
Diurnal movement in absence of transpiration: Experiment 5.—In the leaf of Arenga saccharifera, I found that the petiole was the organ of movement. I cut off the transpiring lamina and covered the cut end with collodion flexile. The plant was now placed in a chamber saturated with moisture. The petiole continued to give records of its diurnal movement in every way similar to the record of the intact leaf. In another experiment with the water plant, Ipoemia reptans, immersed in water, the normal diurnal movement was given by the plant, where there could be no question of variation of turgor due to transpiration. (See also [Expt. 7].)
In the diurnal movement of the ‘Praying’ Palm the concave curvature of the rigid neck in the morning, became flattened or slightly convex in the afternoon. The force necessary to cause this is enormously great, and could on no account result from the passive yielding to the weight of the upper part of the tree.
From the facts given above it will be seen that the diurnal movement is not brought about by variation in transpiration. I now turn to another phenomenon which appeared at first to have some connection with the movement of the tree. Kraus found that the tissue tensions of a shoot exhibit a daily periodicity. He, however, found that between 10°C. and 30°C., variation of temperature had no effect on the daily period. But as regards the diurnal movement of the tree, it is the temperature which is the principal factor. Kraus also found a daily variation of bulk in different plant-organs; this variation of bulk is connected with transpiration, for the removal of the transpiring leaves arrested this variation. But the periodic movement of the tree, as we have seen, is independent of transpiration.
Millardet observed a daily periodicity of tension in Mimosa pudica. He found that maximum tension occurs before dawn; the petiole becomes erected, the movement being upwards or towards the tip of the stem. Tension decreases during the day, and reaches a minimum early in the evening; in correspondence with this is the fall of the petiole, the movement being away from the tip of the stem.[C] If the plant were placed upside down the periodic movement of the petiole in relation to the stem will evidently remain the same, but become reversed in space. Maximum tension in the morning will make the petiole approach the tip of the stem, i.e., the movement will be downwards instead of upwards as in the normal position. The experiment described below will show that the diurnal movement induced by variation of temperature is not reversed by placing the plant in an inverted position.