How Was This Done

I'm in the process of redesigning our website and came across this website: http://www.swallow.net.au/main.htmlI'm trying to get the same effect they did with the growing vines. Being a light Flash user, I can't quite figure out how to accomplish this. I found some reference files that use masking and reveals, but these files only work on very linear effects. As the vine effect is organic, straight-forward reveals don't work. Can someone help me out with this? I've been butting my head against the wall and the bruises are starting to show.