The flower-peduncles, which stand up above the end of the shoot, are carried round and round by the revolving movement of the internodes;and when the stem is securely tied, the long and thin flower-peduncles themselves are seen to be in continued and sometimes rapid movement from side to side.They sweep a wide space, but only occasionally revolve in a regular elliptical course.By the combined movements of the internodes and peduncles, one of the two short hooked tendrils, sooner or later, catches hold of some twig or branch, and then it curls round and securely grasps it.These tendrils are, however, but slightly sensitive; for by rubbing their under surface only a slight movement is slowly produced.I hooked a tendril on to a twig; and in 1 hr.45 m.it was curved considerably inwards; in 2 hrs.30 m.it formed a ring; and in from 5 to 6 hours from being first hooked, it closely grasped the stick.A second tendril acted at nearly the same rate; but I observed one that took 24 hours before it curled twice round a thin twig.Tendrils which have caught nothing, spontaneously curl up to a close helix after the interval of several days.Those which have curled round some object, soon become a little thicker and tougher.The long and thin main peduncle, though spontaneously moving, is not sensitive and never clasps a support.Nor does it ever contract spirally, although a contraction of this kind apparently would have been of service to the plant in climbing.Nevertheless it climbs pretty well without this aid.The seed-capsules though light, are of enormous size (hence its English name of balloon-vine), and as two or three are carried on the same peduncle, the tendrils rising close to them may be of service in preventing their being dashed to pieces by the wind.
In the hothouse the tendrils served simply for climbing.
The position of the tendrils alone suffices to show their homological nature.In two instances one of two tendrils produced a flower at its tip; this, however, did not prevent its acting properly and curling round a twig.In a third case both lateral branches which ought to have been modified into tendrils, produced flowers like the central branch, and had quite lost their tendril-structure.
I have seen, but was not enabled carefully to observe, only one other climbing Sapindaceous plant, namely, Paullinia.It was not in flower, yet bore long forked tendrils.So that, Paullinia, with respect to its tendrils, appears to bear the same relation to Cardiospermum that Cissus does to Vitis.
PASSIFLORACEAE.--After reading the discussion and facts given by Mohl (p.47) on the nature of the tendrils in this family, no one can doubt that they are modified flower-peduncles.The tendrils and the flower-peduncles rise close side by side; and my son, William E.
Darwin, made sketches for me of their earliest state of development in the hybrid P.floribunda.The two organs appear at first as a single papilla which gradually divides; so that the tendril appears to be a modified branch of the flower-peduncle.My son found one very young tendril surmounted by traces of floral organs, exactly like those on the summit of the true flower-peduncle at the same early age.
Passiflora gracilis.--This well-named, elegant, annual species differs from the other members of the group observed by me, in the young internodes having the power of revolving.It exceeds all the other climbing plants which I have examined, in the rapidity of its movements, and all tendril-bearers in the sensitiveness of the tendrils.The internode which carries the upper active tendril and which likewise carries one or two younger immature internodes, made three revolutions, following the sun, at an average rate of 1 hr.4m.; it then made, the day becoming very hot, three other revolutions at an average rate of between 57 and 58 m.; so that the average of all six revolutions was 1 hr.1 m.The apex of the tendril describes elongated ellipses, sometimes narrow and sometimes broad, with their longer axes inclined in slightly different directions.The plant can ascend a thin upright stick by the aid of its tendrils; but the stem is too stiff for it to twine spirally round it, even when not interfered with by the tendrils, these having been successively pinched off at an early age.
When the stem is secured, the tendrils are seen to revolve in nearly the same manner and at the same rate as the internodes. The tendrils are very thin, delicate, and straight, with the exception of the tips, which are a little curved; they are from 7 to 9 inches in length.A half-grown tendril is not sensitive; but when nearly full-grown they are extremely sensitive.A single delicate touch on the concave surface of the tip soon caused one to curve; and in 2 minutes it formed an open helix.A loop of soft thread weighing one thirty-second of a grain (2.02 mg.) placed most gently on the tip, thrice caused distinct curvature.A bent bit of thin platina wire weighing only fiftieth of a grain (1.23 mg.) twice produced the same effect;but this latter weight, when left suspended, did not suffice to cause a permanent curvature.These trials were made under a bell-glass, so that the loops of thread and wire were not agitated by the wind.The movement after a touch is very rapid: I took hold of the lower part of several tendrils, and then touched their concave tips with a thin twig and watched them carefully through a lens; the tips evidently began to bend after the following intervals--31, 25, 32, 31, 28, 39, 31, and 30 seconds; so that the movement was generally perceptible in half a minute after a touch; but on one occasion it was distinctly visible in 25 seconds.One of the tendrils which thus became bent in 31 seconds, had been touched two hours previously and had coiled into a helix; so that in this interval it had straightened itself and had perfectly recovered its irritability.
