rtens itself by coiling up
spirally, and so draws the shoot of the growing plant nearer to the
supporting object. But the tendrils of the Virginia Creeper (Ampelopsis,
Fig. 93), as also the shorter ones of the Japanese species, effect the
object differently, namely, by expanding the tips of the tendrils into a
flat disk, with an adhesive face. This is applied to the supporting
object, and it adheres firmly; then a shortening of the tendril and its
branches by coiling brings up the growing shoot close to the support.
This is an adaptation for climbing mural rocks or walls, or the trunks
of trees, to which ordinary tendrils are unable to cling. The Ivy and
Poison Ivy attain the same result by means of aerial rootlets (78).

[Illustration: Fig. 93. Piece of the stem of Virginia Creeper, bearing a
leaf and a tendril. 94. Tips of a tendril, about the natural size,
showing the disks by which they hold fast to walls, etc.]

100. Some tendrils are leaves or parts of leaves, as those of the Pea
(Fig. 35). The nature of the tendril is known by its position. A tendril
from the axil of a leaf, like that of Passion-flowers (Fig. 92) is of
course a stem, i. e. a branch. So is one which terminates a stem, as in
the Grape-Vine.

101. =Spines= or =Thorns= (Fig. 95, 96) are commonly stunted and
hardened branches or tips of stems or branches, as are those of
Hawthorn, Honey-Locust, etc. In the Pear and Sloe all gradations occur
between spines and spine-like (spinescent) branches. Spines may be
reduced and indurated leaves; as in the Barberry, where their nature is
revealed by their situation, underneath an axillary bud. But prickles,
such as those of Blackberry and Roses, are only excrescences of the
bark, and not branches.

[Illustration: Fig. 95. A branching thorn of Honey-Locust, being an
indurated leafless branch developed from an accessory bud far above the
axil: at the cut portion below, three other buds (_a_) are concealed
under the petiole.]

[Illustration: Fig. 96. Spine of Cockspur Thorn, developed from an
axillary bud, as the leaf-scar below witnesses: an accessory leaf-bud is
seen at its base.]

102. Equally strange forms of stems are characteristic of the Cactus
family (Fig. 111). These may be better understood by comparison with

103. =Subterranean Stems and Branches.= These are very numerous and
various; but they are commonly overlooked, or else are confounded with
roots. From their situation they are out of ordinar