Botanists, who are scientists who study plants, have identified and described over 3,500 types of plants. Plants have different shapes and sizes, some are so small they can only be seen with a microscope, while others are so tall that it is hardly possible to identify their foliage; many have very colourful flowers others do not have any. There is such a large variety of plants because they live in different environments of the Earth: in the water, in thick forests or wide plains, in cold climates or blazing deserts. Since millions of years they have had to and have known how to adapt to their environment. Plants provide much of the oxygen that animals and humans breathe, and a large part of the food they eat.