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 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 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.
In some plants tendrils are _leaflets_ (Chap. XI). Examples are the
sweet pea and 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 cobea,
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.
[Illustration: FIG. 171.--LEAVES OF PEA,--very large stipules, opposite
leaflets, and leaflets represented by tendrils.]
=Twiners.=--The entire plant or shoot may wind about a support. Such a
plant is a twiner. Examples are bean, hop, morning-glory, moonflower,
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 behavior 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 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 get up in the world? =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.