Grasping at Straws

Vining plants have an amazing ability to grab onto a  trellis, fence, or a twig on another plant by curling around it. It's an adaptation that allows the vine to grow rapidly upward using other objects for support. This gives them a distinct advantage over tree or shrub saplings that need to build their own woody support as they grow upwards. But how does it work? 

The process is called thigmotropism, or touch-induced growth response.  Specialized organs called tendrils, or sometimes the stem of a young plant itself, can sense contact with a nearby object and alter their growth pattern so as to bend toward it.  If the object is rigid enough and not too thick, the tendril or stem will continue to bend and coil around it. 

The tendrils of a bitter melon vine stretch out ahead of the shoot apex.

When a tendril encounters an object, such as this actual straw 
recruited for the demonstration, it will grasp it by wrapping
around it.

The tendril of a passion fruit vine seems to have tied itself into some kind of nautical knot 
to secure its support on a fence.

 Thigmotropism is similar to phototropism and gravitropism, which are the bending responses to light and gravity respectively.  

In the light response, light-sensitive pigments create an inhibition of  the growth hormone, auxin, on the lit side and then the opposite side grows faster, bending the stem toward the light. 

Gravitropism comes into play underground, causing roots to grow downward and buried shoots to grow upward. For example, if a root emerges from a sprouting seed sideways, tiny crystals in the cells of the root tip, called statoliths, settle downward to the lower surface of the root, causing the upper side of the root to grow faster and bending the tip downward.

The mechanism in thigmotropism is not as clear and not always the same. Since thigmotropism occurs in different kinds of organs in different plants, it has certainly evolved independently many times.  For example, just within the Legume Family, peas have evolved to climb by tendrils, while  beans climb by twining their stems around a support.

 In general though, the touch of an object deforms the surface of the epidermal cells, and growth closest to the object is suppressed. Continued growth on the opposite side causes the stem or tendril to coil around the support. 

In the noxious weed, skunk vine, the stems themselves wrap around
the support. Bean plants twine in the same way.

Tendrils may be separate organs, or in the case of this climbing lily,
Gloriosa, just the tip of the leaf. Photo by SAPlants, posted on Wikipedia,
CC-BY-SA-4.0.

The genus Clematis is unique in the Buttercup Family,  Ranunculaceae,
in its vining habit. Its young compound leaves are thigmotropic and can
wrap around slender objects.

Some climbing plants use a completely different means of 
attaching to a support. The most unusual I've ever seen is this
climbing Sundew from southwestern Australia, which re-purposes
some of its sticky insect-catching leaves for attachment.