Mask Along a Path

[raven+crow]

Hey, all. I'm wondering how best to mask along a path (I think that would be the right terminology).

See atatched—I know how to animate a motion tween along a custom path, so I know how to make the leaf of the vine move like it's growing, but how best would I mask a path along the underlying vine to make it look like the whole thing is growing as one?

Thanks in advance!