48 A TEXT-BOOK OF BOTANY 
in other cases, as the grape-vine and star-cucumber, tendrils 
are formed, which twine or hook about the supports (Fig. 
48); in still other cases, as the woodbine, the tendrils pro- 
- duce suckers that act as 
holdfasts and enable the 
plant to cling to trees or 
walls (Figs. 49 and 50). 
It is in the dense forests 
of the tropics that climb- 
ing plants become espe- 
cially conspicuous. There 
great woody vines fairly 
interlace the vegetation, 
and are known as lianas 
or lianes. 
If a young morning- 
glory or twining bean be 
watched, it will be dis- 
Fic. 48.—Branch of star-cucumber, with its covered that the elonga- 
tendrils in various conditions. 
ting stem is unable to 
stand upright, and that, as it bends over, the inclined part 
begins to swing through a horizontal curve, which may bring 
the stem in contact with a suitable support. If this hap- 
pens, the stem, continuing to swing in a curve and growing 
in length at the same time, winds itself about the support. 
This movement of the portion of the stem which is in a hori- 
zontal position is thought to be brought about by a peculiar 
response of the plant to gravity. The influence of gravity 
in directing plant organs will be considered later. 
Tendrils are illustrations of plant structures that are 
unusually sensitive to contact. When the tip of a tendril 
in moving about touches a suitable support, the side touched 
becomes concave and the tendril hooks or coils about the 
support. This is only the first response of the tendril to 
contact, for presently the rest of it begins to curve—a move- 
