58 
POPULAR SCIENCE REVIEW. 
in tlie dryer provinces of S. Africa these plants must have 
propagated themselves for thousands of generations in an 
erect condition ; and yet during this whole period they have 
retained the innate power of spontaneously revolving and 
twining, whenever their shoots elongated under proper con- 
ditions of life.” 
II. Leaf Climbers — Mr. Darwin commences his 2nd 
class by remarking “ that it has long been observed that 
several plants climb by the aid of their leaves, either by the 
petiole or by the produced midrib.” He observed nine 
different genera ; and of two, Clematis and Tropceolum, eight 
species of each in order to discover what amount of difference 
there might be within the same genus ; and this, it appears, is 
considerable. 
Clematis glandulosa. The thin upper internodes revolved, 
against the sun, at an average rate of 3h. 48m. The leading 
shoot twined round a stick placed near it, first in one di- 
rection, then ascending straight, and that portion becoming 
rigid, twined in an opposite course. This peculiarity was 
common with other species of this genus. Ho use is made of 
the leaves while twining up a vertical stick : “ nevertheless if 
the footstalk of a young leaf be rubbed with a thin twig a few 
times on any side, it will in the course of a few hours bend to> 
that side; afterwards, however, straightening itself. When 
first developed, the petioles are upturned, parallel to the 
stem; they then slowly bend downwards, remaining for a 
short time at right angles to the stem, and then become so 
much arched downwards that the blade of the leaf points to 
the ground with its tip curled inwards, so that the whole 
petiole and leaf together form a hook. The young leaves are 
thus enabled to catch twigs when brought into contact with 
them by the revolving movement of the internodes. The 
petioles which have clasped any object soon become much 
thickened and strengthened (as may be seen by reference to 
Pl. y„, fig. 1). If they come into contact with no object, they 
retain their downward position for some time, and then 
bending upwards re- assume their original position, which 
is retained ever afterwards.” 
Space will not allow us to mention particulars of other 
species of clematis, to show what amount of variability 
obtains in this genus ; but we introduce a figure of a young leaf 
of Clematis viticella (PI. V., fig. 2), to compare with that of C . 
glandulosa . In this species, the whole petiole, which with the 
sub-petioles is sensitive, acts as a hook, being rectangularly 
bent at the extremity. We may remark that there is a gra- 
dation of sensitiveness in the petioles of species of this genus* 
In montana it is confined to the main petiole, while in 
