Tendril

Lore & Background

This work coined the term circumnutation to describe the motion of growing stems and tendrils seeking supports. Darwin also observed tendril perversion, where tendrils adopt the shape of two sections of counter-twisted helices with a transition in the middle. In the garden pea, only the terminal leaflets are modified to become tendrils; in yellow vetch (Lathyrus aphaca), the whole leaf is modified while stipules carry out photosynthesis. Members of the genus Clematis use the rachis of a compound leaf as a tendril.

Reader's Guide

Tendrils are significant as a key adaptation in climbing plants, enabling them to reach sunlight in dense canopies and contributing to the diversification of flowering plants. They derive from various morphological structures—stems, leaves, or inflorescences—and are found primarily in angiosperms, with few examples in ferns. The coiling mechanism involves circumnutation, a circular oscillatory movement that increases the chance of contacting a support; tendrils can change direction based on the presence of a support stimulus. Contact coiling is driven by thigmotropism, with a signaling cascade involving gamma-Aminobutyric acid (GABA) and jasmonate phytohormones, leading to turgor pressure changes and gelatinous fiber contraction. Plants also exhibit self-discrimination, using chemoreception to avoid coiling around conspecific plants, as demonstrated in Cayratia japonica. This behavior reduces competition and provides more stable support, offering an evolutionary advantage.

Did You Know?

• The chestnut vine (Tetrastigma voinierianum) can have tendrils up to 20.5 inches (52 centimeters) in length.
• Tendrils of the parasitic plant Cuscuta are guided by airborne chemicals and only twine around suitable hosts.
• The aardvark cucumber (Cucumis humifructus) can have as many as eight tendrils at a single node.

What a Tendril Is and How It Varies

A tendril is a thread-like modification of a plant's stem, leaf, or petiole that serves as a grasping organ for climbing species and, in parasitic plants like Cuscuta, as a vehicle for cellular invasion into a host. The botanical world offers a remarkable roster of tendril-bearing plants, ranging from familiar garden crops such as sweet peas and grapes to the showy Chilean glory-flower and the passionflower. These structures react to both physical touch and airborne chemical cues, responding by curling, twining, or firmly adhering to whatever surface or organism they encounter. Their dimensions span an impressive spectrum: most measure only a few centimeters, yet the Nepenthes harryana produces tendrils reaching 27 inches, and the chestnut vine (Tetrastigma voinierianum) can extend tendrils to 20.5 inches. In terms of number, a typical node bears a single simple or branched tendril, though the unusual aardvark cucumber (Cucumis humifructus) may produce as many as eight at a single node, illustrating just how diverse this one organ can be across the plant kingdom.

Darwin and the Birth of Tendril Science

The scientific study of tendrils traces its roots to a single, ambitious monograph. Within its pages, Darwin introduced the term circumnutation to capture the continuous circular oscillation that growing stems and tendrils perform as they search for something to grasp. He also documented a curious geometric phenomenon he called tendril perversion, in which a tendril takes on the shape of two counter-twisted helical sections joined by a transitional zone in the middle. These observations were not merely descriptive; they laid the conceptual groundwork for everything that followed. Modern researchers still build on Darwin's vocabulary and his recognition that tendril movement is a purposeful, goal-directed process rather than random growth. His work established the framework through which botanists would later dissect the cellular, hormonal, and evolutionary mechanisms that make a tendril capable of finding, recognizing, and securing a support.

The Mechanics of Coiling

The process by which a tendril wraps around a support unfolds in two linked phases. The first is circumnutation, a circular oscillatory movement around the tendril's own axis that maximizes the odds of encountering a physical structure. The second phase, contact coiling, is triggered by thigmotropism. In pea tendrils, highly sensitive cells on exposed cell-wall surfaces detect touch and launch a calcium wave that cascades into a hormonal response involving gamma-aminobutyric acid and jasmonate. In grapevine tendrils, GABA alone can drive coiling, while jasmonate acts as the initiating hormonal signal. These pathways ultimately activate a plasma-membrane proton pump that creates an osmotic gradient, causing selective loss of turgor pressure. The resulting asymmetry in cell size produces the visible curl, a process further reinforced by gelatinous fibers that contract and lignify in response to the same signaling cascade.

Evolutionary Origins and the Ability to Tell Self from Other

Tendrils are not a single evolutionary invention. They have been independently derived from stems, leaves, and even inflorescences, and researchers have identified seventeen distinct types based on their ontogenetic origins and growth patterns, with each type appearing multiple times across angiosperms. Common examples illustrate this variety: the watermelon tendril is a modified stem, the pea tendril arises from modified terminal leaflets, and the grapevine tendril is a transformed whole inflorescence. Although climbing behavior appears in gymnosperms and ferns, tendrils are overwhelmingly an angiosperm feature, and their evolution is tied to the drive to reach the canopy for greater sunlight and to promote diversification in flowering plants. Perhaps most intriguingly, tendrils possess a form of self-discrimination. When a tendril touches a neighboring plant of the same species, signaling molecules released by that plant bind to chemoreceptors on the tendril, effectively blocking the thigmotropic pathway and preventing coiling. Experiments with the climbing plant Cayratia japonica confirmed that tendrils will not wrap around a conspecific, revealing a chemotropic recognition system that keeps each plant from anchoring itself to a genetic neighbor.