TENDRIL

A tendril is a slender stem- or petiole-like structure that is used by vines and lianas (climbing plants) to wrap around or to hook a support. Although we often think of tendrils attaching a climber to a more sturdy host plant, quite regularly tendrils also form strong linkages within their own stems on the vine (Example: grape ivy, Cissus rhombifolia), forming a tangled or fishnet-like shoot.

The tendril is, in most cases, a flexible modified shoot, for example, a modified leaf or portion of a leaf, for which thin blade never forms while the central axis elongates greatly via apical and especially intercalary growth. Tendrils that can wrap around a support are thigmotropic, meaning that the young, flexible, relatively straight tendril is sensitive to touch. When a tendril comes into contact with a support, there is faster growth for cells on the opposite side so that the tendril rapidly forms a coil around support, and making a tendril look like a watch spring. Spring-like tendrils function to cinch the vine close to the host axis, and their coils can absorb and dissipate energy when the plant is subjected to strong wind.

Tendril climbing should be a very efficient mode of ascending into a low canopy, because tendrils can grow laterally from a vertical main stem, thereby minimizing amount of stem tissue necessary for climbing. However, even though tendrils develop high friction for grasping structures, they are relatively weak and thus lack adequate support for a heavy liana climbing high into a forest canopy. It is not surprising, therefore, to find tendrilar climbers most common in open disturbed habitats or remaining in full-sun environments atop host species and moving between host individuals efficiently.

Types of Tendrils

Each type of tendril can be described best by noting its origin from a particular plant part.

1. leaf tendril: developed from the entire leaf primordium.
2. leaflet tendril: developed from a single leaflet of a compound leaf (as in many Bignoniaceae) or several of the most distal leaflets (as in sweetpea, Lathyrus)
3. stipular tendril: developed from a stipule attached at the leaf base (Example: Smilax, a lianaceous monocotyledon)
4. leaf tip tendril: developed from the apex of the developing leaf (Examples: glory lily, Gloriosa and another lily, Littonia modesta). This type of leaf apex is termed cirrhose.
5. prophyll tendril: developed from a prophyll, which is the first leaf on a shoot
6. stem tendril or shoot tendril: developed from a shoot apical meristem, and possibly having minute leaf primordia (Example: evergreen grape, Rhoicissus capensis)
7. pedicel tendril or peduncle tendril: developed from the axis that subtends a flower

Families with Tendrils

Relatively few plant families have evolved tendrils, in contrast to the many families of climbers with twining but leafy stems and petioles. The following is a list of the families or isolated genera where you are most likely to see tendrils.

1. Antirrhinum, most notably certain California species of this genus of snapdragons (Family Scrophulariaceae), have tendrilar structures.
2. Bignoniaceae is a family that includes many species of tropical lianas with leaflet tendrils. Many of the leaflet tendrils in this family are branched near the tip (e.g., trifid), and a few are very highly branched, as in Pithecoctenium. A leaf of a lianaceous species may not form a leaflet tendril, but in other cases the leaflet tendril forms and may be exceedingly long.
3. Clematis are temperate climbers (Family Ranunculaceae) with leaflet tendrils, such as virgin's bower, C. lasiantha in Southern California.
4. Cobaea is a vine of the phlox family (Family Polemoniaceae) having the upper portion of a pinnately compound leaf forming as a leaf tendril.
5. Cucurbitaceae, the gourd family, has many vines with tendrils, including the very common man-root of the Los Angeles hillsides, Marah macrocarpus, and a foul-smelling gourd called calabazilla, Cucurbita foetidissima, which often can be found around dry gravel pits of Los Angeles County . In eastern North America, a fairly common cucurbit is the gourd Echinocystis lobata.
6. Dioscorea, yams (Family Dioscoreaceae) may form tendrils. This is Dioscorea bulbifera.
7. In Fabaceae, there are certain lineages of legumes, e.g., peas (Pisum), sweetpea (Lathyrus), and vetch (Vicia), with leaflet tendrils, and Bauhinia has tendrilar hooks (Examples: young tendrils, fully grown tendrils, and old, wooden hooks).
8. Glory lily, Gloriosa, a type of lily (Family Colchicaceae) has leaf tip tendrils, as do other climbers of that family, such as Littonia modesta.
9. Gouania lupuloides is a scrambling neotropical plant (Family Rhamnaceae) with hooked peduncle tendrils.
10. Mutisia spp. can have leaf tip tendrils.
11. Nepenthes are Asian pitcher plants (Family Nepenthaceae) on which a tendril forms from the leaf tip and then the tip of the tendril develops as the water-holding pitcher.
12. Passifloraceae, the passionflowers, have shoot tendrils (Examples: P. auriculata and P. vitifolia). These arise from a structure that forms both the tendril and one or two floral meristems from an initial axillary bud. Its exact nature is still not entirely understood.
13. Sapindaceae, such as Serjania mexicana, climb by means of tendrils and prickles along the shoot.
14. Smilax (Family Smilacaceae) has stipular tendrils (another species).
15. Vitaceae, the grape family, which have "stem" tendrils, although some authors consider these to be prophyll tendrils. In this family, interpreting the tendril is very difficult, because a tendril is present opposite the leaf, unlike a true axillary bud, and only forms at particular nodes and not others. The same primordium that forms the tendril will on an older plant become an inflorescence, because either forms from an uncommitted structure. The vitaceous tendril may produce a bract and branch. Some of the species form a water-releasing hydathode at each tendrilar tip.

Within the family Vitaceae, many of the cultivated species have beautiful examples of tendrils (Examples: Tetrastigma and grape ivy, i.e., Cissus rhombifolia), but the most bizarre by far are the ones of Parthenocissus, including Boston ivy (P. tricuspidata) and Virginia creeper (P. quinquefolia), which have intricately branched tendrils possessing adhesive suckers (glandular disks), enabling this vine to climb masonry.