Lichens (地衣) are difficult to see. They don’t move. They often mix into the living place. You might not even recognize one if you are looking right at it. Lichens live on rocks, branches, houses, and even metal street signs. You can find these colorful organisms (微生物) almost everywhere — from deserts to rainforests, and from Antarctica to Africa. They’ve traveled in outer space, and  some scientists think there might even be lichens on Mars. What you probably don’t realize is that a lichen is more than a single thing. It includes two different types of living organisms: a fungus (菌类) and an alga (藻类). Neither of these organisms is a plant, so the lichen isn’t a plant either. Through photosynthesis (光合作用), the alga gets the sun’s energy to make food for the fungus, which is the place for the alga to live in. However  , the alga cannot leave the fungus. Around the world, scientists have found tens of thousands of types of lichens. Many may not even have been discovered yet. As scientists continue to find new kinds of lichens, they are also working to understand how they are connected to one another. By putting together a lichen family tree, they hope to understand why so many different types of lichen  s have grown in so many places around the world.Many researchers are trying to understand basic facts about the organisms and their connections. Researchers are also using lichens to examine the health of the environment. 小题1:What do we know about lichens according to the passage?
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