An intriguing way to demonstrate gravitropism in action over the course of a lesson, using the hypocotyls (stalks) of small seedlings.   

Using black film canisters, a freshly cut seedling stalk is attached to the lid, the lid replaced, and the resulting direction of growth of the seedling assessed after approximately one hour. The stalk will grow towards the vertical, away from the pull of gravity, demonstrating negative gravitropism. (The upward growth of plant parts such as stems is called ‘negative gravitropism’ and the downward growth of plant roots is called ‘positive gravitropism’).

Students may be surprised to see how quickly the seedlings respond to gravity.

The process by which this occurs is currently being researched, but it is generally understood to take place as follows. The gravitropic response is triggered as a result of statoliths (specialised amyloplasts) located within the endodermal cells. Statoliths are denser than the cytoplasm within the cell, and so sediment as a result of gravity. In response to this, concentrations of the plant hormone auxin are redistributed, differential growth in the plant cells takes place, and the hypocotyl bends.

Download the student sheet and teachers' notes from the links on the right.