I can't remember where I got them, but google Hortus plants. They aren't PSDs, but they are TIFFs. and using channels, you can make upacity and bump maps etc. It's work but they come out quite well. I don't know of ANY PSDs out there. I make most of my opacity mapped plants and trees by getting large format photos and using the lassoo selection tool painstakingly cut them out, etc etc.

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