Satin stitch can be used to fill in any shape, and leaves are no exception. In the freestyle embroidery project, I used satin stitch to make some simple leaves.
Using two strands of floss in a matching colour, I put leaves on the sides of the sides of the couched stems. I just eyeballed the shape, having one longer stitch to be the point at the end of the leaf. Most of the leaves have five stitches, but I just concentrated on filling in the space that was there rather than making them all identical.
The leaves fill in some of the space and add interest to the long stems.