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 8
feet in height.
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