104 Notices of Books. [January, 
by night, whence he infers that the acftion of the light is con- 
fined to retarding one semicircle and accelerating the other, not 
greatly modifying the speed of the whole revolution. It has 
actually been found that in Ipomcea jucunda the semi-circle from 
the light takes 5! hours, whilst the semi-circle towards the light 
is efiedted in 1 hour. In most twiners the branches, however 
numerous they may be, all go on revolving together. In Tamils 
elephantipes only the side branches twine and!not f the main 
stem. In a certain climbing asparagus the case was reversed. 
A plant of Combretum argenteum made a number of short, healthy 
shoots, which showed no signs of revolving, but at last it put out 
from the lower part of one of its main branches a thin sheet, 5 
or 6 feet in length, which revolved vigorously and climbed. 
Polygonum convulvulus, according to Palm, twines only during 
the middle of the summer, but in autumn, even if growing 
vigorously, shows no tendency to climb. Three vegetable species, 
— two Ceropegias and Ipomcea argyrceoides — in their dry home 
in South Africa grow eredl and compact, but seedlings raised 
near Dublin, presumably in a conservatory, twined up sticks from 
6 to 8 feet in height. On these significant fadfs Mr. Darwin 
thus comments : — “There can hardly be a doubt that in the 
drier provinces of South Africa these plants have propagated 
themselves for thousands of generations in an eredt 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.” 
But we must now turn from the twiners to those plants which 
climb by means of prehensile organs, possessing a certain sen- 
sibility or irritability. The simplest and least developed of this 
class are the leaf climbers, which seize hold of any point of 
support either by the foot-stalks of their leaves, or by a pro- 
longation of the midrib. Here, also, there is the power of re- 
volving at various rates. But though no very sharp line 
of demarcation can be drawn between the twiners and the leaf- 
climbers, and some few of the latter “ can ascend by twining 
spirally round a support yet the general objedl of the revolv- 
ing motion is here to bring the foot-stalks or the prehensile tips 
of the leaves into contact with surrounding objedls. The leaves 
are sensitive to a touch and to continued pressure even when 
very slight. Leaf-climbing may be easily understood by observ- 
ing the species of Clematis and Tropceolum , including the 
common nasturtium. This plant, if it meets with a string or 
a thin twig, casts a hitch around it with one of its leaf-stalks, 
and thus secures a point of support. 
More highly specialised are the plants which climb by the aid 
of tendrils, which Mr. Darwin defines as “ filamentary organs, 
sensitive to contacT, and used exclusively for climbing.” These 
organs “ are formed by the modification of leaves with their foot- 
stalks, of flower-peduncles, branches, and possibly stipules. In 
