Experiment 109.—In order to subject the question to quantitative test I applied feeble stimulus of the same intensity on upper and lower side alternately. Successive stimuli were kept more or less uniform by employing the following device. I took a flat strip of wood 1 cm. in breadth, and coated 2 cm. of its length with shellac varnish mixed with fine emery powder. On drying the surface became rough, the flat surface was gently pressed against the area of the tendril to be stimulated, and quickly drawn so that the rough surface 2 cm.×1 cm. was rubbed against the tendril in each experiment. Stimulation, thus produced, induced a responsive movement of each side of the organ. The extent of the maximum movement was measured by the microscope micrometer. The following results were obtained with four different specimens.
TABLE XXVII.—SHOWING THE RELATIVE INTENSITIES OF RESPONSES OF THE UPPER AND UNDER SIDE OF TENDRIL (Passiflora).
| Movement induced by stimulation of under side, A. | Movement induced by stimulation of upper side, B. | Ratio B⁄A. |
| (1) 85 divisions | 14 divisions | 1⁄6 |
| (2) 106 " | 15 " | 1⁄7 |
| (3) 60 " | 8 " | 1⁄7 |
| (4) 80 " | 10 " | 1⁄8 |
It will thus be seen that the upper side of the tendril is not totally inexcitable, its power of contraction being about one-seventh that of the under side.
NEGATIVE CURVATURE OF THE TENDRIL.
I shall now describe certain remarkable results which show that under certain definite conditions the tendril moves away from the stimulated side. I have explained, how in growing organs the effect of unilateral stimulus longitudinally transmitted, induces an expansion higher up on the same side to which the stimulus is applied, resulting in convexity and movement away from the stimulus (cf. Laws of Tropic Curvatures, p. 286). As the reaction of tendril is in no way different from that of growing organs in general, it occurred to me that it would be possible to induce in it a negative curvature by application of indirect unilateral stimulus.
Experiment 110.—A tendril of Passiflora was held in a clamp, as in the diagram (Fig. 106) in which the left is the more excitable side of the organ. The responsive movement of the tendril is observed by focussing a reading microscope on a mark on the upper part of the tendril. Direct mechanical stimulation at the dotted arrow makes the tendril move in the same direction, the response being positive. But if stimulus be applied on the same side below the clamp the tendril is found to move away from stimulus, the response being now negative. This reversal of response, as previously stated, is due to the fact that the transmitted effect of indirect stimulus induces an acceleration of growth higher up on the same side, which now becomes convex. The result though unexpected, is in every way parallel to the response of the flower bud of Crinum, in which the normal positive response was converted into negative by changing the point of application of stimulus, so that it became indirect (p. 216).
SUMMARY.
The response of tendril is in no way different from that of growing organs in general.
Direct stimulus, electrical or mechanical, induces an incipient contraction; the after-effect of a feeble stimulus is an acceleration of growth above the normal. Indirect stimulus induces an enhancement of the rate of growth.