A sound understanding of negative space drawing is critical for watercolor painting, as a watercolor is built through a progressive overlaying of negative-space areas, working from light to dark.
In the photograph below, note how the red-outlined dark areas lock together to reveal the form of the foreground shapes of the leaves. Focusing on the leaf forms is fine for a line drawing, but if you want a shaded value drawing, you'll need to observe the negative spaces as shown, so that you can leave behind the positive space, drawing the leaves lighter, and leaving the white edges and veins of the leaves clear.
