How do plants sense and respond to gravity? Here's gravitropism explained by Isaac Newton in two minutes or summarized by Churchill in one sentence.

For research in this area here are somereviews of current ideas about plant hormones and gravity sensing:

Baldwin, Strohm and Masson (2013) American Journal of Botany 100: 111-12

Toyota and Gilroy (2013) American Journal of Botany 100: 111-125