42 TWINING PLANTS. Cuar. I. 
but Asclepias nigra only “in fertiliori solo incipit 
scandere subvolubili caule” (Willdenow, quoted and 
confirmed by Palm, p. 41). Aselepias vincetoaicwm 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 Convolvulacec 
are excellent twiners; but in South Africa Ipomea 
argyrzoides 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. 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. multijlorus produce (Léon, p. 681) two kinds of 
shoots, some upright and thick, and others thin and 
twining. I have seen striking instances of this curious 
