第12章 TWINING PLANTS(11)

80 m., and that towards the light only 1 hr.Lonicera brachypoda revolved, in a reversed direction to the Ipomoea, in 8 hrs.; the semicircle from the light taking 5 hrs.23 m., and that to the light only 2 hrs.37 m.From the rate of revolution in all the plants observed by me, being nearly the same during the night and the day, Iinfer that the action of the light is confined to retarding one semicircle and accelerating the other, so as not to modify greatly the rate of the whole revolution.This action of the light is remarkable, when we reflect how little the leaves are developed on the young and thin revolving internodes.It is all the more remarkable, as botanists believe (Mohl, p.119) that twining plants are but little sensitive to the action of light.

I will conclude my account of twining plants by giving a few miscellaneous and curious cases.With most twining plants all the branches, however many there may be, go on revolving together; but, according to Mohl (p.4), only the lateral branches of Tamus elephantipes twine, and not the main stem.On the other hand, with a climbing species of Asparagus, the leading shoot alone, and not the branches, revolved and twined; but it should be stated that the plant was not growing vigorously.My plants of Combretum argenteum and C.

purpureum made numerous short healthy shoots; but they showed no signs of revolving, and I could not conceive how these plants could be climbers; but at last C.argenteum put forth from the lower part of one of its main branches a thin shoot, 5 or 6 feet in length, differing greatly in appearance from the previous shoots, owing to its leaves being little developed, and this shoot revolved vigorously and twined.So that this plant produces shoots of two kinds.With Periploca Graeca (Palm, p.43) the uppermost shoots alone twine.

Polygonum convolvulus twines only during the middle of the summer (Palm, p.43, 94); and plants growing vigorously in the autumn show no inclination to climb.The majority of Asclepiadaceae are twiners;but Asclepias nigra only "in fertiliori solo incipit scandere subvolubili caule" (Willdenow, quoted and confirmed by Palm, p.41).

Asclepias vincetoxicum does not regularly twine, but occasionally does so (Palm, p.42; Mohl, p.112) when growing under certain conditions.So it is with two species of Ceropegia, as I hear from Prof.Harvey, for these plants in their native dry South African home generally grow erect, from 6 inches to 2 feet in height,--a very few taller specimens showing some inclination to curve; but when cultivated near Dublin, they regularly twined up sticks 5 or 6 feet in height.Most Convolvulaceae are excellent twiners; but in South Africa Ipomoea argyraeoides almost always grows erect and compact, from about 12 to 18 inches in height, one specimen alone in Prof.

Harvey's collection showing an evident disposition to twine.On the other hand, seedlings raised near Dublin twined up sticks above 8feet in height.These facts are remarkable; for there can hardly be a doubt that in the dryer provinces of South Africa these plants have propagated themselves for thousands of generations in an erect condition; and yet they have retained during this whole period the innate power of spontaneously revolving and twining, whenever their shoots become elongated under proper conditions of life.Most of the species of Phaseolus are twiners; but certain varieties of the P.

multiflorus produce (Leon, p.681) two kinds of shoots, some upright and thick, and others thin and twining.I have seen striking instances of this curious case of variability in "Fulmer's dwarf forcing-bean," which occasionally produced a single long twining shoot.

Solanum dulcamara is one of the feeblest and poorest of twiners: it may often be seen growing as an upright bush, and when growing in the midst of a thicket merely scrambles up between the branches without twining; but when, according to Dutrochet (tom.xix.p.299), it grows near a thin and flexible support, such as the stem of a nettle, it twines round it.I placed sticks round several plants, and vertically stretched strings close to others, and the strings alone were ascended by twining.The stem twines indifferently to the right or left.Some others species of Solanum, and of another genus, viz.

Habrothamnus, belonging to the same family, are described in horticultural works as twining plants, but they seem to possess this faculty in a very feeble degree.We may suspect that the species of these two genera have as yet only partially acquired the habit of twining.On the other hand with Tecoma radicans, a member of a family abounding with twiners and tendril-bearers, but which climbs, like the ivy, by the aid of rootlets, we may suspect that a former habit of twining has been lost, for the stem exhibited slight irregular movements which could hardly be accounted for by changes in the action of the light.There is no difficulty in understanding how a spirally twining plant could graduate into a simple root-climber;for the young internodes of Bignonia Tweedyana and of Hoya carnosa revolve and twine, but likewise emit rootlets which adhere to any fitting surface, so that the loss of twining would be no great disadvantage and in some respects an advantage to these species, as they would then ascend their supports in a more direct line.