Plants are classified according to their assumed relatedness. The species is the basic unit of classification based on the structure of flowers, fruits, and other organs. All species are classified hierarchically. Related species are grouped into a genus; related genera into a family; related families into an order; related orders into a class, e.g., flowering plants whose seed contains two seed-leaves (dicots) and flowering plants whose seed contains one seed-leaf (monocots); related classes into a division, e.g. Ferns and flowering plants; and related divisions into a kingdom.