Fig. 4. Record of the Sijbaria Palm from noon for 24 hours. Successive dots, at intervals of 15 minutes.
Experiment 1.—Believing the phenomenon to be universal I experimented with a different Date Palm that was growing at my research station at Sijbaria on the Ganges, situated at a distance of about 200 miles from Faridpur. The surrounding conditions were very different. The tree was much younger; it was 2 metres in height and inclined 20° to the vertical. The curve obtained with this tree (Fig. 4) was very similar to that of the Faridpur Palm, though this extent to the movement was much reduced. The tree attained the highest erect position at 7-15 A.M. and the lowest at 3-45 P.M. Hence the movement of the Faridpur Palm is not a solitary phenomenon.
THE CAUSE OF PERIODIC MOVEMENT.
The recurrent daily movement of the tree must be due to some diurnal changes in the environment,—either the recurrent changes of light and darkness, or the diurnal changes of temperature. These changes synchronise to a certain extent; for, as the sun rises, light appears and the temperature begins to rise. It is therefore difficult to discriminate the effect of light from that of temperature. The only satisfactory method of discrimination would have been in the erection of a large structure with screens to cut off light. The effect of fluctuation of temperature under constant darkness would have demonstrated the effect of one agent without complication arising from the other. Unfortunately screening the tree was impracticable. I shall presently describe other experiments where the action of light was completely excluded.
The curve of movement of the tree, however, affords us material for correct inference as regards the relative effects of light and temperature. The experiment was commenced in March; light appeared at about 5 A.M., the sunrise being at 6-15 A.M.; the sun set at 6-15 P.M., and it became dark by 7 P.M. The incident light would be the most intense at about noon; after this it would decline continuously till night time. If the movement was due to light, its climax, either in up or down movement, would be reached at or about noon, and the opposite climax at midnight. But instead of this we find ([Fig. 3]) the up-movement reaching its highest point not at noon, but at 7 in the morning; after this the fall is rapid and continuous, and the lowest position was reached not in the evening but at 3-15 P.M. The fluctuation of light has, therefore, little to do with the movement of the tree.
Turning next to the element of variation of temperature we are at once struck by the fact that the curve of movement of the tree is practically a replica of the thermographic curve ([Fig. 3]). The fall of temperature is seen to induce a rise in the tree and vice versâ. There is a lag in the turning points of the two curves; thus while temperature began to rise at 6 A.M., the tree did not begin to fall till 7 A.M. There is in this case a lag of an hour; but the latent period may, sometimes, be as long as three hours. The delay is due to two reasons; it must take some time for the thick trunk of the tree to attain the temperature of the surrounding, and secondly, the physiological inertia will delay the reaction. As a result of other investigations, I find that the induced effect always lags behind the inducing cause. It is interesting in this connection to draw attention to the parallel phenomenon, which is described below, of lag in the variation of sensibility of Mimosa in response to variation of temperature. In this case the lag was found to be about three hours. Returning to the Palm, the tree continues to fall in the forenoon with rising temperature. At about 2-30 P.M. the temperature was at its maximum after which it began to decline; the movement of the tree was not reversed into erection till after 3-15 P.M., the lag being now 45 minutes nearly.
I may state here that the movement of the tree is not primarily affected by the periodicity of day and night, but by variation of temperature. In spring and in early summer the rise of temperature during the early part of the day and the fall of the temperature from afternoon to next morning, are regular and continuous; the corresponding movements of the tree are also regular. But at other seasons, owing to the sudden change of direction of the wind, the fluctuations of temperature are irregular. Thus at night there may be a sudden rise, and in the earlier part of the day sudden fall of temperature. And the record of movement of the tree is found to follow these fluctuations with astonishing fidelity, the rise of temperature being followed by a fall of the tree and vice versâ. That the movement is determined by the temperature variation is exhibited in a striking manner in [Fig. 4], where, between 8 and 9 A.M., a common twitch will be noticed in the two curves.