The main focus of my research has been the mechanism by which plant cell elongation is controlled. There have been three main phases in this research. The first is a study of the mechanical properties of the cell walls of elongating tissues. I showed that the walls behaved as viscoelastic materials, and that when cell elongation was promoted in stem and coleoptile cells by the hormone auxin, it was by an increase in the extensibility of the walls. A second phase concerned the way in which auxin induces a loosening of the cell wall. With David Rayle, I proposed that it did so by causing cells to excrete H into the cell wall, activating wall loosening proteins. This \"acid-growth theory\"" has largely passed the test of time