172 
CLIMBING PLANTS 
forming a firm but elastic support, and at the same time 
dragging the stem upwards and thus economising its 
materials. The direction of the twist reverses at some 
point in the spiral ; this is a mere mechanical result. To 
twist a piece of wire, fast at both ends, into a spiral, it will 
be found necessary to reverse the twist at the middle. 
Some tendrils, e.g. in Vitis sp., have adhesive discs at 
the end, others, e.g. in Cobaea, hooks. 
Tendrils, morphologically considered, may be of various 
nature — stems (usually modified inflorescence-axes), leaves 
or parts of leaves, or even roots, e.g. the aerial roots of Vanilla 
and perhaps other plants. Modified stipules form the tendrils 
of Smilax, &c., modified leaves or parts of leaves those of 
many Leguminosae (e.g. Lathyrus, Vicia, &c.), Bignoniaceae 
(e.g. Bignonia) and Cucurbitaceae, Mutisia, Cobaea, Cory- 
dalis, &c. ; modified stem-structures occur in Vitis, Passiflora, 
Antigonon, Landolphia, &c. A special form of tendril is 
the sensitive hook that occurs in so many tropical climbers. 
Inflorescence-axes, modified in structure to form recurved 
hooks, project from the stem ; when a hook catches a sup- 
port, it clasps it tightly and becomes lignified, e.g. in Arta- 
botrys, Hugonia, Uvaria, Ourouparia, Unona, Ancistrocladus, 
Landolphia, Strychnos, Uncaria, &c. Sometimes they are 
long, thin and flat, and are rolled up like watch-springs, as in 
some Sapindaceae (Paullinia, &c.), Gouania, Bauhinia, &c. 
Many plants climb by aid of sensitive leaves. In Glo- 
riosa, Littonia, &c., the tip of the leaf is sensitive, acting 
like a tendril. The petiole is often sensitive to contact, 
usually clasping once round its support, and then frequently 
becoming woody, e.g. in Tropaeolum, Clematis, Hablitzia, 
Maurandia, Rhodochiton, &c Cf. also the leaf-climbers, 
Fumaria, Adlumia, and Nepenthes (see Part II.). 
Other plants climb by aid of sensitive lateral branches, 
which bear ordinary leaves, e.g. Securidaca, Hippocratea, 
Uvaria, Salacia, Machaerium, &c. Sometimes the branches 
are leafless and tendril-like, but they are always capable of 
producing leaves. 
III. Hook-climbers sprawl over other vegetation, and 
have hooks, usually recurved, which aid in their support. 
The only British hook-climbers are Galium and Rubus, 
both of them with small hooks arising as mere emergences 
