Severian's friend the Green Man always seemed quite plausible to me. I hadn't realized just how plausible. Apparently a sea slug has incorporate algae genes into its genome and produces and uses chlorophyll inside its body: http://www.sciencenews.org/view/generic/id/53496/title/Sea_slug_steals_genes_for_greens,_makes_chlorophyll_like_a_plan