HOW PLANTS CLIMB
We have found that plants struggle or contend for a place in which
to live. Some of them become adapted to grow in the forest shade,
others to grow on other plants, as epiphytes, others to _climb to the
light_. Observe how woods grapes, and other forest climbers, spread
their foliage on the very top of the forest tree, while their long
flexile trunks may be bare.
There are several ways by which plants climb, but most climbers
may be classified into four groups: (1) =scramblers=, (2) =root
climbers=, (3) =tendril climbers=, (4) =twiners=.
=Scramblers.=—Some plants rise to light and air _by resting their
long and weak stems on the tops of bushes and quick-growing herbs_.
Their stems may be elevated in part by the growing twigs of the
plants on which they recline. Such plants are scramblers. Usually
they are provided with prickles or bristles. In most weedy swamp
thickets, scrambling plants may be found. Briers, some roses,
bed-straw or galium, bittersweet (_Solanum Dulcamara_, not the
_Celastrus_) the tear-thumb polygonums, and other plants are familiar
examples of scramblers.
=Root Climbers.=—Some plants climb by means of _true roots_. These
roots seek the dark places and therefore enter the chinks in walls
and bark. The trumpet creeper is a familiar example (Fig. 36).
The true or English ivy, which is often grown to cover buildings,
is another instance. Still another is the poison ivy. Roots are
distinguished from stem tendrils by their _irregular or indefinite
position_ as well as by their mode of growth.
[Illustration: FIG. 169.—TENDRIL, to show where the coil is changed.]
=Tendril climbers.=—A slender coiling part that serves to hold a
climbing plant to a support is known as a =tendril=. The free end
swings or curves until it strikes some object, when it attaches
itself and then coils and _draws the plant close to the support_.
The spring of the coil also allows the plant _to move in the wind_,
thereby enabling the plant to maintain its hold. Slowly pull a
well-matured tendril from its support, and note how strongly it holds
on. Watch the tendrils in a wind-storm. Usually the tendril attaches
to the support by _coiling about it_, but the Virginia creeper and
the Boston ivy (Fig. 170) attach to walls by means of _disks_ on the
ends of the tendrils.
[Illustration: FIG. 170.—TENDRIL OF BOSTON IVY.]
Since both ends of the tendril are fixed, when it finds a support,
the coiling would tend to twist it in two. It will be found, however,
that the tendril _coils in different directions_ in different parts
of its length. In Fig. 169, showing an old and stretched-out tendril,
the change of direction in the coil occurred at a. In long tendrils
of cucumbers and melons there may be several changes of direction.
Tendrils may represent either _branches_ or _leaves_. In the
Virginia creeper and the grape they are branches; they stand opposite
the leaves in the position of fruit clusters, and sometimes one
branch of a fruit cluster is a tendril. These tendrils are therefore
homologous with fruit clusters, and fruit clusters are branches.
[Illustration: FIG. 171.—LEAVES OF PEA,—very large stipules, opposite
leaflets, and leaflets represented by tendrils.]
In some plants tendrils are _leaflets_ (Chap. XI). Examples are the
sweet pea and the common garden pea. In Fig. 171, observe the leaf
with its two great stipules, petiole, six normal leaflets, and two
or three pairs of leaflet tendrils and a terminal leaflet tendril.
The cobaea, a common garden climber, has a similar arrangement. In
some cases tendrils are _stipules_, as probably in the green briers
(smilax).
The _petiole_ or _midrib may act as a tendril_, as in various kinds
of clematis. In Fig. 172, the common wild clematis or “old man vine,”
this mode is seen.
=Twiners.=—The entire plant or shoot may wind about a support.
Such a plant is a =twiner=. Examples are bean, hop, morning-glory,
moon-flower, false bittersweet or waxwork (_Celastrus_), some
honeysuckles, wistaria, Dutchman’s pipe, dodder. The free tip of the
twining branch _sweeps about in curves_, much as the tendril does,
until it finds support or becomes old and rigid.
Each kind of plant usually coils _in only one direction_. Most plants
coil against the sun, or from the observer’s left across his front to
his right as he faces the plant. Examples are bean, morning-glory.
The hop twines from the observer’s right to his left, or with the sun.
[Illustration: FIG. 172.—CLEMATIS CLIMBING BY LEAF-TENDRIL.]
SUGGESTIONS.—=136.= Set the pupil to watch the behaviour of any
plant that has tendrils at different stages of maturity. A vigorous
cucumber plant is one of the best. Just beyond the point of a young
straight tendril set a stake to compare the position of it. Note
whether the tendril changes position from hour to hour or day to
day. =137.= Is the tip of the tendril perfectly straight? Why? Set
a small stake at the end of a strong straight tendril, so that the
tendril will just reach it. Watch and make drawing. =138.= If a
tendril does not find a support what does it do? =139.= To test the
movement of a free tendril draw an ink line lengthwise of it, and
note whether the line remains always on the concave side or the
convex side. =140.= Name the tendril-bearing plants that you know.
=141.= Make similar observations and experiments on the tips of
twining stems. =142.= What twining plants do you know, and which
way do they twine? =143.= How does any plant that you know shoot
up? =144.= Does the stem of a climbing plant contain more or less
substance (weight) than an erect self-supporting stem of the same
height? Explain.