408 CLIMBING PLANTS. 
ever reach the light. In our temperate countries twiners 
which die down every year would gain nothing as they could 
not reach the summit in a single season. With most twining 
plants all the branches, however many there may be, go on 
revolving together; but, according to Mohl, 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 sboot alone, and not the branches, revolved and 
twined. Some produce shoots of two sorts, one of which 
twines; the others not. In others the uppermost shoots 
alone twine. One twines during the middle of the summer 
but not in autumn. Some grow erect in dry South Africa, 
their native country ; but near Dublin, Ireland, they regu- 
larly twine. 
Leaf Climbers. — The stems of several species of Ole- 
matis are twiners like the hop. But in addition to this mode 
of holding fast, the petioles are sensitive to the touch, 
slowly bend into the form of hooks, and if successful in 
catching a stick they clasp it firmly and soon become greatly 
enlarged and strengthened by an extra growth of woody 
fibre. If they come in contact with no object they retain 
this position for a considerable time, and then bending up- 
wards they reassume their original upturned position, which 
is retained ever afterwards. In Clematis calycina the 
clasped petiole becomes nearly twice as thick as the leaf- 
stalk which has clasped nothing. The petiole of the un- 
clasped leaf is flexible, and can be easily snapped, whereas 
the clasped footstalk acquires an extraordinary toughness 
and rigidity so that considerable force is required to pull it 
into pieces. The meaning of these changes is plain, namely, 
that the petioles may firmly and durably support the stem. 
In some species of Clematis furnished with compound leaves 
the main petiole alone is sensitive, while some have two or 
three sub-petioles, also sensitive; still others have the en- 
tire number, as many as seven, sensitive. Some petioles 
are extremely sensitive to very light weights, as one-eighth 
