In the plant kingdom, angiosperms, or plants that produce flowers, occupy the majority position, or 90 percent of all species. Named species of flowering plants number close to half a million, with many tropical varieties still unknown. Flowering plants cover all climate zones on earth, including rugged Arctic terrain. They range in size from the tiny Wolffia to the largest known flower, the Rafflesia, or corpse lily, which produces flowers three feet across. Plants that produce spores instead of flowers include ferns and mosses---they are classified as pteridophytes.
Plants flower in order to reproduce. Insects and other pollinators are attracted to flowers because of their appearance and their scent. The flower is simply a sexual organ, containing the male anther and the female stigma. Some plants are bisexual, having both organs, while others are unisexual, with only the male or female organ. Unisexual plants require another plant in order to form seeds, which are the end result of a successfully pollinated flower.
Largest Flowering Plant Families
Approximately 60,000 species of flowering plants, or 25 percent, are contained in three plant families. The sunflower family, or Asteraceae, includes 24,000 species, including sunflowers, asters, daisies, marigolds and chrysanthemums. The orchid family, Orchidaceae, has 20,000 species, and the legume or pea family (Fabaceae) has 18,000 species, which range from the common garden pea to trees.
Plants in the Asteraceae family of flowering plants include herbs, shrubs, trees and vines that grow in a wide variety of climate zones. Their common distinguishing characteristics are their alternate, opposite or whorled leaf structure, bisexual or unisexual flowers often called florets, often containing multiple florets on a receptacle that is surrounded by bracts. Thirteen tribes exist within this family.
Plants in the Orchidaceae family include up to 20,000 species of terrestrial flowering plants that live in many different climates. Orchids are epiphytes, meaning they need a host on which to live, but do not rely on that host for nutrition. Flowers are usually bisexual and rarely unisexual. They possess a single stamen and produce fruit, or seeds, that are capsular.
The Fabaceae, or legume, family contains approximately 18,000 species of flowering plants. Many, like green beans, peas and pinto beans serve as staple foods of the humans and animals that live where they grow. Many varieties of legumes are widely cultivated as food crops. In desert environments, legumes are important members of the environment because they provide food, shelter for birds and other animals, and nitrogen that enriches the soil.