A novel shape description method is proposed for mobile retrieval of leaf images to aid in plant recognition. In this method, traveling the shape contour, the convexity and concavity properties of the arches of various levels are measured, respectively, to generate a multiscale shape descriptor. Its performance has been tested on two leaf datasets and the experimental results indicated higher recognition accuracies than the state-of-the-art approaches with a speed improvement of more than 170 times. The proposed method has been successfully applied to develop a prototype system of online plant leaf identification working on a consumer mobile platform.