Cellulose
Cellulose is the main chemical that forms the main structure of plants.  It is the most abundant macromolecule and the most common organic compound on  Earth.  It is also a kind of fiber.  Cellulose is a very complex carbohydrate, consisting of 3,000 or more glucose units.  Cellulose was discovered by Anselme Payen in 1838.  He isolated it from plant matter and determined its chemical formula.