According to a display at the San Francisco Exploratorium, tiny surface structures make these nasturtium leaves water-resistant.
The uneven surface creates air pockets between water and leaf, so water rolls right off, taking dirt with it. Research on such small-scale surfaces has inspired revolutionary new materials, including water-resistant fabrics.
This is an example of biomimicry. For more on biomimicry and other examples of innovation inspired by nature, watch this video by Janine Benyus: