Heather Gordon of Gore asks :-

Why do the tips of runner beans turn towards the fence to climb?

Rainer Hofmann, a plant biologist at Lincoln University, responded.

An explanation for this observation is provided by the phenomenon of `tropismĀ“, indicating growth or movement of a plant.

Many tropisms are mediated by plant hormones in response to environmental cues. There are a number of plant tropisms, including phototropism (growth directed by light), gravitropism (growth directed by gravity) and thigmotropism (growth directed by contact). Many vines use the latter mechanism to climb up walls and other objects (e.g. on other plants).

However, to locate these objects they can utilise the mechanism of negative phototropism, whereby plants grow away from light. Through this, they can grow towards shade and dark areas created by other objects, thus increasing the chances to encounter a tree or wall to climb on. Auxins are important plant hormones involved in these tropisms and are commonly produced in young, dividing tissues such as plant tips.