This droplet of water describes a near-perfect sphere. It does this because the surface of the leaf does not allow for a lot of wetting.
Wetting isn't just the thing that occurs with blue liquid during tasteful diaper commercials. It actually has a scientific meaning. Wetting is the ability of a liquid to glom (scientific term) onto a surface. Variable degrees of wetting is why, when you spill droplets of water (or other liquid) on some surfaces, they form plump round little droplets, while when you spill them on others, the drops flatten themselves out as they sink into the solid.
Some plants have an ability to repel water drops along their surface. Although the bottom of a water droplet will touch the plant, the water molecules everywhere else are repelled from the surface of the plant and bundle themselves up, via surface tension, to form a mostly spherical droplet.
Via Science Direct