This is interesting, called Leafsnap it’s an iPhone application that is able to identify a tree by it’s leaves. To use the application you need to take a picture of the leaf on a white background. The application then uses face recognition software to identify the species. It is produced by the Columbia University, the University of Maryland and the Smithsonian Institution. It uses the same technology as face recognition software to identify the species. It’s an interesting idea and having tried it, it is useful, however, it currently is mainly US based so is not so good with UK species.