How do auxins promote the growth of a tendril around a support?
Tendrils are sensitive to touch. When a tendril comes in contact with a support, the part of the tendril in contact with the object does not grow as rapidly as the part of the tendril away from the object.
This happens because auxin diffuses towards the side away from the support. The higher concentration of auxin on that side stimulates the cells to grow longer. As a result, the side away from the support grows faster while the contact side grows slowly.
This differential growth causes the tendril to circle around the object and thus cling to it. This is an example of a directional, or tropic movement in response to touch.
Explanation
The textbook explains that tendrils are sensitive to touch and show differential growth when in contact with a support. The mechanism is similar to phototropism where auxin diffuses to one side and stimulates cell elongation. In tendrils, auxin accumulates on the side away from the support, causing faster growth on that side, which results in curling around the support.