This set of demonstrations into the effects of tropisms on seedlings is both simple and extendable. Seeds sown on damp cotton wool in (plastic) Petri dishes are grown either flat, or stuck vertically to a wall. If you have access to a clinostat, this could be used as another comparison.

Students enjoy the dramatic effect of sticking Petri dishes to the wall, and you will have well-grown seedlings within a week, showing the different effects of gravitropism and phototropism.

Why not get your students to come up with some of their own ideas for extension activities, and try these out in your classroom?