How Do Plants Climb? Cheating Their Way to the Light
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Building a trunk strong enough to stand tall costs an enormous amount of wood, and a climbing plant avoids that cost entirely by using somebody else's trunk. It invests in length and in gripping devices instead of in support, which lets it reach the canopy with a stem a fraction of the thickness a self-supporting plant would need.
The economics of not standing up
A free-standing stem must resist bending under its own weight and under wind, and the resistance required rises steeply with height, which is why a tall tree puts most of its production into a trunk that does no photosynthesis at all. A climber transfers that structural burden to its host and spends the saving on rapid extension growth and on leaves. The result is a characteristic anatomy: climbing stems are long, flexible and often surprisingly narrow for the mass of foliage they supply, with wide vessels that move water efficiently over long distances, and they are frequently unable to stand without support even when mature. The trade is not free. Climbers must find a support, which means searching, and a plant that extends into open space without contacting anything has wasted the investment. They also depend on their host surviving, and because a heavy load of climbers increases wind resistance and weight, they can contribute to the failure of the tree they depend on, which is one reason they are controlled in managed woodland.
The gripping strategies
Climbing has evolved repeatedly and independently, and the mechanisms fall into distinct groups:
- •Twining stems, where the whole shoot circles a support, with each species turning consistently clockwise or anticlockwise, so a plant wound the wrong way around a pole will unwind itself
- •Tendrils, which are modified leaves, leaflets or stems that circle slowly through the air until they touch something and then coil around it
- •Adhesive pads, as in Virginia creeper, where tendril tips flatten against a flat surface and cement themselves, allowing the plant to climb bare walls and glass
- •Adventitious roots, as in ivy, which grow from the stem into crevices and secrete an adhesive, gripping a surface without needing to encircle anything
- •Hooks, thorns and backward-pointing spines, which do not grip actively but catch on surrounding vegetation, which is how many climbing roses and rattans ascend
- •Leaning and scrambling, the least specialised approach, simply growing up through dense vegetation and relying on it for support
How a tendril knows
A tendril searches by circumnutation, a slow circular sweeping motion produced by growth on one side of the organ moving progressively around it, so the tip traces a circle through the air over a period of hours. When it contacts a suitable object, touch-sensitive cells trigger a rapid response: growth slows on the contacted side and accelerates on the opposite side, curving the tendril around the support within minutes to hours, which is one of the faster movements plants make. Once anchored, the free portion between plant and support coils into a helix, and it does so in two opposite-handed halves separated by a reversal point, because the ends are fixed and a single-handed coil would require the tendril to rotate. That coiling shortens the tendril, drawing the plant towards its support, and it acts as a spring that absorbs shock from wind rather than snapping. The tendril then lignifies and becomes woody and strong. Darwin studied these movements in detail and published a book on climbing plants that established much of what is still taught.
Lianas and what they do to forests
Woody climbers, called lianas, are a major structural component of tropical forest and can account for a large share of woody stems, linking the crowns of neighbouring trees into a connected canopy that many animals use as a pathway. Their ecological effect on their hosts is substantial and largely negative, since they compete for light above and for water and nutrients below, reduce tree growth and fruit production, increase the chance of a tree being pulled down when a neighbour falls, and slow the recovery of disturbed forest by smothering regeneration. Evidence has accumulated that liana abundance has been increasing in neotropical forests, with proposed explanations including rising carbon dioxide, increased forest disturbance and fragmentation, and changing rainfall seasonality, and the consequence matters for carbon storage because lianas hold less carbon than the tree growth they suppress. Rattan, a climbing palm, is among the most economically valuable non-timber forest products in Southeast Asia, which makes climbing plants both a management problem and a livelihood in the same forests.
The takeaway
Skipping the trunk transfers the structural cost to a host and lets a thin stem reach the canopy, paid for with extension growth and gripping organs. Twining stems, tendrils, adhesive pads, aerial roots and hooks are independent solutions to the same problem. A tendril sweeps in slow circles until it touches something, then curves around it within hours and coils into two opposite-handed helices that pull the plant in and act as a spring.