City Trees

28 T he natural world is packed with surprises, and when we can stop to notice the small and seemingly mundane components of it, we are usually rewarded. Consider the lichen: sometimes mistaken for mosses or other plants, lichens are a synthesis of two distinct life forms, a mutually beneficial union between fungi and algae. In 1877, the German botanist Albert Bernhard Frank used the term symbiosis (from the Ancient Greek meaning "to live together") to describe the lifestyle of lichen. At the time, the idea of two unrelated species engaged in a closely-twined, communal existence was new, and quite surprising. Lichen were one of the first such unions to make themselves known to science, and with the new paradigm and a word to describe it, symbiotic relation- ships began to be recognized in all sorts of places. Since then, a good deal of attention has been paid to these small, multifarious creatures, and we now know that symbiotic relationships are quite common on our planet. Though lichen's contributions to their environment, or to human life, may not be obvious, they play important roles and their study has given us both a better understanding of the complexities of living things and of the effects that our human activities have on the life around us. Lichen are found pretty much everywhere. The Earth hosts over 14,000 named "species" of lichens, in a great variety of fungal/algal combinations, on mostly every corner and kind of surface available. Lichen may live happily on rocks, on trees, on other lichen, even on the backs of insects. The union consists of a fungi, mostly from the group known as the "cup fungi" (the Ascomycetes), the ones whose fruiting bodies are generally shaped like saucers or shallow cups. The algal component is often referred to as the photobiont, due to the role that it plays in the synthesis. Photobionts are photosynthesizers; they transform sun- light, water, and carbon dioxide into sugars and other carbohydrates to fuel both themselves and their fun- gal partners. Photobionts come from many different groups; they can be green algae (Chlorophyta), golden algae (Chrysophyta), or brown algae (Phaeophyta)—three groups of eukaryotes that house their chlorophyll inside chloroplasts. Photobionts can also come from the cyano- bacteria, a group of prokaryotes with a more ancient lin- eage and simpler structure; they lack chloroplasts and so their chlorophyll is free-floating in the cell. (Cyanobacteria were known as the blue-green algae before their lineage was understood.) Though there are many possible combinations of fungi

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