The Green Man – you may never have heard of him, though he has been around for centuries. You may never have noticed him, but you can find him easily enough carved on the roof bosses and columns of medieval churches, abbeys, and cathedrals across Europe. He decorates even older temples in the Middle East. You may not know his significance, but then no one really does. What could a man's face with leaves growing out of it (technically a “foliate head”), or with branches issuing from his mouth or eyes, mean? An ancient pagan symbol of rebirth and renewal? Of human reliance on nature? Of fertility? Perhaps he is a spirit of nature, a demon, or some ancient god? Osiris, the Egyptian god of vegetation (along with the Underworld), was usually painted with a green face; could it be him? Sadly, we seem to have forgotten. Perhaps to different cultures he meant different things. For me at least, the Green Man is a reminder of life's inventiveness, for Nature has already found several solutions to the shocking, seemingly impossible mix of plant and animal his face conveys. We are accustomed to hearing that the corals of shallow, tropical seas enter into symbiotic relationships with algae (usually of the zooxanthellae), sometimes adopting strikingly green hues. The corals provide urea (a source of nitrogen) and other nutrients to the algae and receive carbohydrates made by photosynthesis in return. But they are not alone in this type of relationship. Representatives of the foraminifera, radiolaria, jellyfish, anemones, nudibranchs, and even some clams also maintain algal endosymbionts. Depending on the hosting species, the algae are taken up by the developing egg or embryo, or in older organisms via the ectoderm or the cells lining the gut. Even amphibians are in on the act; algae are present in the jelly-like egg capsules of wood frogs (Lithobates sylvaticus) and the spotted salamander (Ambystoma maculatum), giving them a green tinge. After receiving nitrogenous waste released by the embryos, the algae repay them with boosts of photosynthetic oxygen. On 28 July 2010, however, at the Ninth International Congress of Vertebrate Morphology in Punta del Este, Uruguay, Ryan Kerney (now at Gettysburg College, PA) reported algae (Oophila amblystomatis) inside the cells of embryonic spotted salamanders. “[And they] persist in the adult”, he says. “We've amplified algal 18S ribosomal DNA from adult salamander tissues, including from parts of their reproductive system. Our current hypothesis is that some of these algal cells may be passed down from one generation to the next.” Green Men looking down from roof bosses in Norwich Cathedral (UK). But why bother to house whole algal cells if you can get by with just their chloroplasts? Indeed, kleptoplasty – basically stealing the chloroplasts you need to become photosynthetic – is an alternative practiced by certain dinoflagellates, ciliates, foraminifera, and even some metazoans: the sacoglossan sea slugs. These latter animals feed on algae, but they do not digest the chloroplasts. Rather, these organelles are taken up whole by the epithelium of the slug's extensively branched gut, conferring not just a bright green camouflage to the entire animal, but the ability to truly photosynthesize. Some species can retain their stolen chloroplasts only for a few days and need a constant supply of new ones, but others have found ways to preserve them for much longer. The nudibranch Elysia chlorotica, for example, which lives off the Atlantic coast of North America, can maintain its chloroplasts in a functional state for some 14 months and, when fully loaded up, need not feed heterotrophically at all. How this system works, however, remains a mystery, for although chloroplasts carry their own DNA, they require the assistance of algal nuclear genes for photosynthesis to occur. So, have these sea slugs incorporated these genes into their own genome? At least some research suggests they have not (Mol Biol Evol 2011; 28: 699–706). Some animals might have even started down the road toward developing their very own light-harvesting systems. In 2010, it was reported that the Oriental hornet (Vespa orientalis) may take advantage of the xanthopterin in its yellow stripes to capture light energy (Naturwissenschaften 2010; 97: 1067–76) and produce electricity, although just how the insect uses this product is unknown. Nonetheless, it does appear to give them a buzz; these hornets are most active when the light is strongest. Perhaps, then, the Green Man is neither symbol nor spirit nor demon nor god, but an evolutionary prophecy that one day animals will learn the trick of garnering energy from light. I wonder, however, whether he may also be a forecaster of other things to come. In a recent paper (PloS ONE 2011; 6: e18877), Christina Agapakis et al. wrote: “Our results show that it is possible to engineer photosynthetic bacteria to invade the cytoplasm of mammalian cells for further engineering and applications in synthetic biology”. Don't say you were never warned!