Other options for a leaf center vein:
You can fold the leaf in half, gently hammer the fold, When you open, the crimp from the fold creates the center vein.
Solder on a small piece of silver wire for a center vein.
I like Billie Ann's suggestion of using a paper clip - hammer it into the leaf shape to create the center vein.
You can also use a sharp scribe to scratch the veins into the leaf. This has a quiter look, but it can look great.
Lisa