Diurnal movement in inverted position: Experiment 6.—I took a vigorous specimen of Arenga saccharifera growing in a pot, and took its normal record, which as explained before exhibited down-movement during rise, and an up-movement during fall of temperature. The plant was now held inverted, the upper side of the petiole now facing the earth. The diurnal curve of movement should now show an inversion, if that movement was solely determined by the anisotropy of the organ. But the record did not exhibit any such inversion. After being placed upside down, the leaf did not, on the first day, show any diurnal movement; there was, on the other hand, a continuous down-movement on account of the fall of the leaf by its own weight. But in the course of 24 hours the leaf readjusted itself to its unaccustomed position, and became somewhat erected under the action of geotropic stimulus. After the attainment of this new state of geotropic equilibrium, the leaf gave a very pronounced record of its diurnal movement which did not show any reversal; the inverted leaf continued to exhibit the same characteristic movements as in the normal position, that is to say, a down movement during rise, and an up-movement during fall of temperature. As the plant in the inverted position did not show any reversal of the periodic curve, it is clear that the diurnal movement is determined by the modifying influence of temperature on the physiological reaction of the plant to some external stimulus which is constant in direction. I shall presently show that it is the constant geotropic stimulus modified by the action of temperature, which determines the diurnal movement of the tree.
This will be better understood if I refer once more to certain characteristics in the movement of the “Praying” Palm. The neck of the tree was seen to be concave in the morning. The physiological effect of raising temperature is virtually to oppose or neutralise the geotropic curvature as seen in the flattening or slight reversal of curvature in the afternoon. Similarly, various plant organs, growing at an inclination to the vertical, are subjected to geotropic action, and thus assume different characteristic angles. This state of equilibrium is not static but may better be described as dynamic; for it will be shown that this state of geotropic balance is upset in a definite way, by variation of temperature.
That geotropism is an important factor in the diurnal movement is supported by the fact that the Sijbaria Palm with an inclination of 20° to the vertical exhibited a daily movement which was only moderate in extent. But the Faridpur Palm growing at an inclination of 60° was subjected more effectively to geotropic action, and exhibited movements which were far more pronounced. I shall now proceed to describe crucial experiments which will demonstrate the effect of change of temperature on geotropic curvature.
EFFECT OF VARIATION OF TEMPERATURE ON GEOTROPIC CURVATURE.
In the instances of diurnal movement already described the trees or their leaves were already at an inclination to the vertical. I now took a radial and erect shoot of Basella cordifolia growing in a pot and laid it horizontally for two weeks. The procumbent stem curved up and attained a state of equilibrium under the action of geotropic stimulus.
Diurnal curve of Basella cordifolia: Experiment 7.—The plant was completely immersed in a vessel of water, and its diurnal curve recorded. This resembled in all essentials the diurnal curve of the Palm; the slight deviation was due to the fact that owing to difference in the season (August) the temperature maximum was attained at 12-25 P.M. and the minimum at 6 A.M. The geotropic curvature was reduced to its minimum at the maximum temperature, and vice versâ. As in the case of the Palm so also in the procumbent stem of Basella there was a physiological lag, which was 50 minutes in the morning and about the same in the afternoon. The free end of the stem thus exhibited a diurnal movement up and down. The temperature, as stated before, began to rise from 6 A.M. and the down-movement commenced 50 minutes later, i.e., at 6-50 A.M. The temperature, after reaching the maximum, began to fall at 12-25 P.M., and the previous movement of fall of the stem was arrested and reversed into an erectile movement shortly after 1 P.M. There are thus two “turning points,” one at 7 A.M., and the other at about 1 P.M.; at these periods the movement of the plant remains more or less arrested for more than half-an-hour.
Fig. 7. Diurnal curve of movement of procumbent young stem of Mimosa pudica. Successive dots at intervals of 15 minutes.
I obtained records of similar diurnal movements with various procumbent or creeping stems. Figure 7 gives the diurnal record of the procumbent stem of a young specimen of Mimosa pudica.