A vine twined around a tree limb or a trellis is a familiar sight, but did you ever wonder how they get that way? As the vine twines, in this Moment of Science.
Tangled
The purpose of the twining is to anchor the vine, and in some cases the vines themselves do the twining, while in others the vines have small appendages called tendrils that coil around twigs or trellises.
How does the vine know to twine, and in what direction? The fact is the vine doesn't know anything; its coiling is a blind reaction, so to speak, caused by chemical changes inside it.
Obstacles
The vine twines because the cells on one side grow faster than the cells on the opposite side. When the growing tip of the vine touches an obstacle, such as a twig, this stimulates the cells of the vine on the side opposite the twig to grow faster than those next to the twig.
As the side of the vine opposite the twig gets longer, and the side touching the twig grows very little, the vine is pushed around the twig. Most vines are not particular about which way they twine. If the outside of the vine's tip touches an obstacle the vine will change direction.