Climbers

The woody vine, wisteria (Wisteria sinensis), is a popular garden plant for climbing up walls, which it does by twining its stems around a support.

There are many species of flowering plants that have developed the ability to climb up other plants or structures. Their stems are not strong enough to be self-supporting. Climbers redirect some of their energy into producing structures such as tendrils, which assist them in climbing up their vital supports.

Climbers come from a great range of plant families, from the cucumbers (Cucurbitaceae) and grapevines (Vitaceae) through representatives of tropical families such as the bignonia (Bignoniaceae) and passionflowers (Passiflo-raceae). Nearly all climbers are dicotyledons, with the exception of the rattans, which are monocotyledons of the palm family (Palmae).

In many habitats, and particularly in dense jungles, climbing is the method these plants use to reach the sunlight that they need for photosynthesis. The energy saved by not developing large stems can be used to grow faster, farther, and higher than neighboring plants, which allows them to stay in the light and gain even more energy. Their flowers are then more likely to be produced in an environment that is conducive to fertilization. Many climbers, however, are also able to reproduce vegetatively by means of organs such as underground stems.

Plants may climb using suckers, as does Virginia creeper, or modified branches that have become tendrils, as on the grapevine. Some plants, for example clematis, wind their whole stem around a support. Adventitious roots on the cablelike stem of ivy attach the plant to a surface. Trailing climbers, such as brambles, hook onto supports with their downwardpointing thorns.

Means and methods of climbing

Climbing is accomplished by a variety of methods, and some plants possess special organs for this purpose. The simplest way in which a plant climbs is by twining itself around a support that can be either natural—for example, another plant—or artificial. Most twining climbers are left-handers—that is, the direction in which they twine when viewed from above is counterclockwise. A few—notably the hop (Humulus lupulus) and the honeysuckle (Lonic-era periclymenumJ—are right-handers.

Shoots, leaves, and even roots of flowering plants may be modified to assist the climbing process. Shoots may bear sharp down-curved spines, often reinforced with woody tissues, such as those of roses (Rosaceae), which hook onto the surrounding vegetation. Special shoot outgrowths may be equipped with suckers like those of the Virginia creeper (Par-thenocissus quinquefolia). Tendrils, which are generally formed as modified leaves, may be a simple spiral or may be armed with hooks. In some species, the tendril bears a leaflike structure, such as that of a vetch [Vida sp.), which may have a simple tendril in place of the uppermost leaflet. The rapidly climbing ivys and their relations produce special roots from cablelike stems, which anchor the plants so that they can grow up the surface of their support. These roots are borne in groups or rows but only on the shaded side of the stem. If the support is a living substrate, such as a tree, the roots may penetrate it and take up nutrients. This process is not likely to kill the support, but its eventual death may be hastened as a result.

The wild hop (Humulus sp.) climbs by twining its stem around anything it can. It is a right-handed climber, climbing in a clockwise direction, unlike most other twining climbers.

How climbers work

The tissues of flowering climbers are often modified in their arrangement to give maximum strength and flexibility. For this purpose, a large number of sclerenchyma fibers may be contained in the stem. Many climbers, particularly the woody jungle lianas, have stems in which rays of phloem-containing pith run through the woody xylem. Liana stems do not cling to their supports, however, but attach themselves more than 150 feet (46 meters) above the ground, with the “trunk” left to hang free.

The xylem vessels contained in the woody tissues of the tallest climbers are very large-in the rattans, for example, they may be 10 to 20 feet (3 to 6 meters) long. These and similar vessels conduct water up the climber at a spectacular rate, sometimes as fast as 6 feet (1.8 meters) per minute. If a section is cut out of a tropical climber and then held upside down, the water in it pours out.

Climbing movements

Apart from their physical structure, climbing plants are largely reliant on innate physiological mechanisms. These mechanisms depend on the plant’s ability to sense light, gravity, touch, and temperature, and to respond accordingly by a movement, which generally involves growth. Like those of most plants, the shoots of climbers respond positively to light (phototropism) by growing toward it and negatively to gravity by growing away from it. The roots behave in an exactly opposite way. There are exceptions, however, such as the wild grape (Vitis sp.), the tendrils of which are negatively phototropic. They end in disks which attach themselves to their support.

As a shoot grows it can be seen to move about in circular arcs. This natural tendency seems to arise from an inner control on the part of the plant—the stimulus is received at sites in the epidermal cells and is rapidly conducted across the whole tendril. Shoots are also affected by sensitivity to touch. Once a support has been touched by the encircling shoot, a growth spurt is triggered on the surface of the shoot directly opposite the point of contact. This surface grows longer and more rapidly than the side of contact, causing the shoot to bend inwards toward the contact and grow around the support. The growth is so rapid that it can be seen over a period of a few hours. Once begun, the shoot continues to grow in the same direction.

The tropical climber balsam pear (Momordica cha-rantia) uses spiraling tendrils to attach itself to other plants. This annual plant is a member of the cucumber family, Cucurbitaceae, and grows in the Old World tropics. Its orange fruit splits to reveal an edible pulp and to disperse the seeds.