What chemical reaction, if any, is going on here?

Leaf leaves leafprint in wet pavement
Originally uploaded by Jodiepedia

Here's a 'ghost leaf' on a pavement in London. I presume that the leaf landed on the pavement when it was wet and more rain landed on the leaf. When the leaf blew away, or was kicked away by a pedestrian, this was the pattern that was left.

Is it just dirt and oils on the leaf's surface that's making contact with the water on the pavement resulting in this image, or is something 'eluting' from the leaf under rainfall. I'm guessing there's not much in a leaf that's water soluble though so am going with the leaf's waxy outer coating as being the answer. Whatever it is it doesn't come off the pavement very easily with a boot dragging across it.

Anyway it always pleases me to see this sort of 'foliography' for want of a better word.

Here's what I said about it originally, on Flickr, but I know think I'm wrong and that it's just the wax layer being transferred.

Via Flickr:
This looks like the leaf of a London plane which has presumably had something leached out of it and onto the pavement below during the rain. Scraping my boot over the leaf print didn't make a big difference to it, wonder what chemical is creating the pattern, or chemical reaction.