Scientists have found and described more than 275,000 kinds of plants, but they believe that many more are yet to be discovered. Plants vary greatly in size and appearance. Some, like single-celled algae, are so small that you can only see them with the help of a microscope. Others, like giant sequoia trees, are so big that you cannot even see the tops of them. Plants are very different from one another because they have developed features—over millions of years—to help them live in the world’s many different environments.