Science Bank 15: Part 3 covers Plant Tropisms (from 9m 25s to 14m 30s):
A 3 minute introduction to tropisms (from Twig World).
Plant Control – Paul Andersen explains how plants use hormones to respond to their environment. We only need to know about auxins (to 3m 40s).
IGCSE Biology: Coursebook, chapter 10, pp. 139 – 142