I updated Project Broccoli II above a few days ago. I've been focusing on other things, but thought I'd complete the leaf placement on the Leafy Tree to just get it out of the way. Only had a short time to mess with it, so it's obviously not finished with anti-aliasing and proper highlighting (and perspective is off, should have gaps for branches, and as always, ignore the trunk, etc), but it clues one in to how such an approach could look.

As advised by people here, I'm moving on to smaller plants, learning how to shade individual leaves, focusing on the basics and making some progress drawing trees that are trees that look nothing like trees yet are interpreted as trees. Updates will be unlikely for a bit.
I think a thread on shading semi-basic shapes that aren't easily googled and don't follow basic rules (from what I've been told) - donuts, flat topped discs and the like - would be more helpful, perhaps.