21 Mil. DARWIN ON CLIMBING PLANT8. 



the plants wliicli 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, bowever many there may be, go on revol vino- 

 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. purpurcum made nume- 

 rous short healthy shoots ; but they showed no signs of revolv- 

 ing, 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 G 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 Grceca (Palm, S. 43) the uppermost shoots alone twine. 

 Polygonum convolvulus twines only during the middle of the sum- 

 mer (Palm. S. 43, 91) : plants growing vigorously in the autumn 

 show no inclination to twine. The majority of Asclepiadacea> are 

 twiners ; but Asclepias nigra only " in fertiliori solo incipit scan- 

 dere sub volubili caule " (Willdenow, quoted and confirmed by 

 Palm, S. 41). Asclepias vincctoxicum 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 Ceropcgia, 

 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 Ipoma>a argyrceoides 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 



