Fig. 203.—Abolition of diurnal movement in Tropæolum under constant temperature, and its restoration under normal daily fluctuation. The upper record is of temperature and the lower of plant movement.
Diurnal record of Tropæolum under constant temperature: Experiment 212.—The normal record of geotropically curved Tropæolum is already given in figure 202. In repeating the record I maintained the plant at constant temperature for 24 hours; the result of this is seen in the first part of the record (Fig. 203). The thermal record is practically horizontal, and the diurnal record of the plant shows no periodic movement. The thermal regulator was on the next day put out of operation, thus restoring the normal diurnal variation of temperature. The record of the plant is seen to exhibit once more its normal periodic movement.
I have in the chapter on thermo-geotropism (p. 515) shown that the diurnal movement of a geotropically curved organ is determined in reference to the direction of force of gravity. This will be seen demonstrated in an interesting manner in the two following experiments on the effect of inversion of the plant on daily movement.
DIURNAL MOVEMENT IN INVERTED POSITION.
I have already referred to the distinction that is made between nastic and paratonic movements. In the former the movement is autonomous and in relation to the plant, and in the latter it is due to an external force which determines the direction of movement. In nastic reaction, closure movement would persist as a closure movement[43]; but should the direction of movement be determined by the stimulus of gravity, closure movement would, on inversion, be reversed into an opening movement. Viewed from an external point of view an up-movement in the latter case would, after readjustment on inversion, become an up-movement, though in so doing, the expansion should be transferred from the upper to the lower side of the organ. It is to be understood in this connection, that some time must lapse before this readjustment is possible, and that the former movement may continue, in certain cases, as a persistence of after-effect.
I succeeded in demonstrating the paratonic effect of geotropic stimulus on the periodic movement of the palm leaf, by holding the plant in an inverted position (p. 24). On the first day of inversion, the diurnal record was erratic, but in the course of 24 hours, the leaf readjusted itself to its unaccustomed position, and became somewhat erected under geotropic action. After the attainment of this new state of geotropic equilibrium, the leaf gave the record of down-movement during rise, and up-movement during fall of temperature, movements which in reference to the plant are the very opposite to those in a normal position. But seen from an external point of view, rise of temperature caused in both normal and inverted positions, a down-movement indicative of diminished geotropic curvature; fall of temperature, on the other hand, brought about an erectile movement, thus exhibiting enhancement of geotropic curvature.
Fig. 204.—Effect of inversion of the plant on diurnal movement. (a) Normal record, (b) record 24 hours after inversion and (c) after 48 hours (Tropæolum).
Experiment 213.—A still more striking result exhibiting the phase of transition was given by the geotropically curved stem of Tropæolum. Its diurnal curve and the subsequent changes after inversion are given in figure 204. In (a) is seen the normal diurnal curve; the specimen was inverted, and it took an entire day for the plant to readjust itself to the new geotropic condition. The record (b) was recommenced on the second day after inversion; the persistence of previous movement is seen in the reversed curve during the first half of the second day; but in the second half the record became true, and the third day the inverted plant gave a record which, from an external point of view, was similar to that given by the plant in the normal position.