42 TWINING PLANTS. Chap. I. 



but Azclepias nigra only " in fertiliori solo incipit 

 scanclere 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 

 bear 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 ConvolvulaceaB 

 are excellent twiners ; but in South Africa Ipomcea 

 argijrdeoides 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 sj)ecies 

 of Phaseolus are twiners ; but certain varieties of the 

 P. multijlorus 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 



