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Though not the largest kingdom, with a mere 300,000 species catalogued, many might argue that the Kingdom Plantae just may be the most important group of living organisms.
In the process known as "photosynthesis", plants use the energy of the Sun to convert water and carbon dioxide into simple carbohydrates (sugars) and oxygen. This single chemical reaction is the source of virtually all the oxygen in Earth's atmosphere. Also, plants are at the beginning of an energy chain which provides nearly all of the food energy required by living things. Several species of protists and bacteria are also capable of performing photosynthesis, but plants are responsible for the vast majority of it.
The ancestors of modern plants evolved in the seas nearly 700 million years ago. These primitive plants did not have many the structures we tend to associate with plants in general, such as roots, stems, and leaves. The evolution of these structures only occurred after plants appeared on land some 265 million years later. Many scientists believe that the evolution of these specialized structures and the wide variety of forms they can assume largely accounts for the success and diversity of land plants we see now.
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