Page:Darwin - On the movements and habits of climbing plants.djvu/25

 the plants which I have observed being nearly the same during the night and the day, I infer that the action of the light is confined to retarding one semicircle and accelerating the other, so as not to greatly modify the whole rate. This action is remarkable when we reflect how little the leaves are developed on the young and very thin revolving internodes. It is the more remarkable, as botanists have thought (Mohl, S. 119) that twining plants are but little sensitive to the action of light.

I will conclude my account of twining plants by collecting 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 (S. 4), the main stem of Tamus elephantipes does not twine—only the branches. On the other hand, with the Asparagus, given in the table, 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 from its leaves being little developed, and this shoot revolved vigorously and twined. So that this plant produces shoots of two sorts. With Periploca Græca (Palm, S. 43) the uppermost shoots alone twine. Polygonum convolvulus twines only during the middle of the summer (Palm. S. 43, 94): plants growing vigorously in the autumn show no inclination to twine. The majority of Asclepiadaceæ are twiners; but Asclepias nigra only "in fertiliori solo incipit scandere sub volubili caule" (Willdenow, quoted and confirmed by Palm, S. 41). Asclepias vincetoxicum does not regularly twine, but only occasionally (Palm, S. 42; Mohl, S. 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 Convolvulaceæ are excellent twiners; but Ipomæa argyræoides in South Africa 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. Seedlings, on the other hand, raised near