It sounds easy, but is it? If you are distinguishing between human faces, you have some fixed feature to start with - eg your computer can probably identify eyes relatively easily, and from there start making measurements to mouth and nose and chin and so on and therefore pick up differences. But there's a lot more variety in a plant leaf or flower than in a human face, and it may not be easy for the computer to get a fix on a feature to start making comparisons. And that's assuming you have taken a carefully posed photo of just a flower and a leaf, rather than a shot of the whole plant with leaves and flowers all over the place.