To ascertain how often the same tendril would become curved when touched, I kept a plant in my study, which from being cooler than the hot-house was not very favourable for the experiment.The extremity was gently rubbed four or five times with a thin stick, and this was done as often as it was observed to have become nearly straight again after having been in action; and in the course of 54 hrs.it answered to the stimulus 21 times, becoming each time hooked or spiral.On the last occasion, however, the movement was very slight, and soon afterwards permanent spiral contraction commenced.No trials were made during the night, so that the tendril would perhaps have answered a greater number of times to the stimulus; though, on the other hand, from having no rest it might have become exhausted from so many quickly repeated efforts.
I repeated the experiment made on the Echinocystis, and placed several plants of this Passiflora so close together, that their tendrils were repeatedly dragged over each other; but no curvature ensued.I likewise repeatedly flirted small drops of water from a brush on many tendrils, and syringed others so violently that the whole tendril was dashed about, but they never became curved.The impact from the drops of water was felt far more distinctly on my hand than that from the loops of thread (weighing one thirty-second of a grain) when allowed to fall on it from a height, and these loops, which caused the tendrils to become curved, had been placed most gently on them.Hence it is clear, that the tendrils either have become habituated to the touch of other tendrils and drops of rain, or that they were from the first rendered sensitive only to prolonged though excessively slight pressure of solid objects, with the exclusion of that from other tendrils.To show the difference in the kind of sensitiveness in different plants and likewise to show the force of the syringe used, I may add that the lightest jet from it instantly caused the leaves of a Mimosa to close; whereas the loop of thread weighing one thirty-second of a grain, when rolled into a ball and placed gently on the glands at the bases of the leaflets of the Mimosa, caused no action.
Passiflora punctata.--The internodes do not move, but the tendrils revolve regularly.A half-grown and very sensitive tendril made three revolutions, opposed to the course of the sun, in 3 hrs.5 m., 2 hrs.40 m.and 2 hrs.50 m.; perhaps it might have travelled more quickly when nearly full-grown.A plant was placed in front of a window, and, as with twining stems, the light accelerated the movement of the tendril in one direction and retarded it in the other; the semicircle towards the light being performed in one instance in 15 m.less time and in a second instance in 20 m.less time than that required by the semicircle towards the dark end of the room.Considering the extreme tenuity of these tendrils, the action of the light on them is remarkable.The tendrils are long, and, as just stated, very thin, with the tip slightly curved or hooked.The concave side is extremely sensitive to a touch--even a single touch causing it to curl inwards; it subsequently straightened itself, and was again ready to act.A loop of soft thread weighing one fourteenth of a grain (4.625 mg.) caused the extreme tip to bend;another time I tried to hang the same little loop on an inclined tendril, but three times it slid off; yet this extraordinarily slight degree of friction sufficed to make the tip curl.The tendril, though so sensitive, does not move very quickly after a touch, no conspicuous movement being observable until 5 or 10 m.had elapsed.
The convex side of the tip is not sensitive to a touch or to a suspended loop of thread.On one occasion I observed a tendril revolving with the convex side of the tip forwards, and in consequence it was not able to clasp a stick, against which it scraped; whereas tendrils revolving with the concave side forward, promptly seize any object in their path.
Passiflora quadrangularis.--This is a very distinct species.The tendrils are thick, long, and stiff; they are sensitive to a touch only on the concave surface towards the extremity.When a stick was placed so that the middle of the tendril came into contact with it, no curvature ensued.In the hothouse a tendril made two revolutions, each in 2 hrs.22 m.; in a cool room one was completed in 3 hrs., and a second in 4 hrs.The internodes do not revolve; nor do those of the hybrid P.floribunda.
Tacsonia manicata.--Here again the internodes do not revolve.The tendrils are moderately thin and long; one made a narrow ellipse in 5hrs.20 m., and the next day a broad ellipse in 5 hrs.7 m.The extremity being lightly rubbed on the concave surface, became just perceptibly curved in 7 m., distinctly in 10 m., and hooked in 20 m.
We have seen that the tendrils in the last three families, namely, the Vitaceae, Sapindaceae and Passifloraceae, are modified flower-peduncles.This is likewise the case, according to De Candolle (as quoted by Mohl), with the tendrils of Brunnichia, one of the Polygonaceae.In two or three species of Modecca, one of the Papayaceae, the tendrils, as I hear from Prof.Oliver, occasionally bear flowers and fruit; so that they are axial in their nature.
The Spiral Contraction of Tendrils.
This movement, which shortens the tendrils and renders them elastic, commences in half a day, or in a day or two after their extremities have caught some object.There is no such movement in any leaf-climber, with the exception of an occasional trace of it in the petioles of Tropaeolum tricolorum.On the other hand, the tendrils of all tendril-bearing plants, contract spirally after they have caught an object with the following exceptions.Firstly, Corydalis claviculata, but then this plant might be called a leaf-climber.
