Heather Gordon of Gore asks :-
Why do the tips of runner beans turn towards the fence to climb?
Ross Bicknell, a plant developmental geneticist at Crop & Food Research, responded.
Tendril activity and perception is only just becoming understood. Growth in all plant cells and tissues takes place as a result of differences in turgor (water pressure within) and cell division. The plane of cell division is the principal determinant in final tissue form. The final spiral shape of the tendril about its support is therefore set in place by non-uniform cell division leading to more cells on the outside of the twist.
How do they perceive the presence of a support and how do they move around? Movement is largely driven by changes in turgor. Much like more familiar hydraulic systems the plant alters water pressures within its tissues in a coordinated manner to drive the movement of the tendril through space. The direction of this appears to be under genetic control as there are variants in some systems that go backwards (eg peas) but what the genes are and how they work is still unknown. It is apparent that once a support is encountered changes in hormone levels and hormone transport (typically auxins) lead to the altered planes of cell division that lock a particular form in place. The nature of the perception is still a tantalizing mystery.