CHAPTER XVII

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.