The Asteraceae or sunflower family is the world's largest plant family. At one time it was called the Compositae, and this is a clue as to the essential character of the family. What appears to be a single flower is actually a flowering head made up of many ray flowers around the rim and many disk flowers in the center. Some species have no ray flowers and some species have no disk flowers. But all have either both or one or the other. So the flowers are composites, that is, made up of distinct parts. The family contains more than 1600 genera and around 23,000 species, and is sometimes called the aster family after one of its main members. The largest genus is Senecio with 1500 species. It is mainly a herbaceous family but does include a few trees and climbers. The family is likely about 50 million years old and is world-wide.