A walk through a woodland in spring or a summer meadow--even a close inspection of your own backyard--reveals the diversity of flowering plant species found on earth. The flowering plants are the most abundant of the four types of plants, taking forms as varied as the grass beneath your feet and the trees towering overhead. Classification of this diverse group of organisms reveals their evolution and relationships.
In the 18th century, Carolus Linnaeus practiced medicine, but his passion was for botany. Linnaeus collected and named thousands of plants, using his work to develop a new system for classifying species. Prior to Linnaeus' work, botanists tended to arrange plants hierarchically, based on complexity. In this so-called "Great Chain," plants were ordered linearly without accounting for the complex, branching relationships between species.
Linnaeus' work heralded the modern science of taxonomy, the classification of organisms based on their relationships to each other. Rather than arranging plants in a line, Linnaeus's system began with a single broad classification--the kingdom--and branched into increasingly more specific classifications, ending with species. Flowering plants belong to the plant kingdom along with species as diverse and moss and algae. Below the kingdom level, taxonomists sort plants into 10 divisions or phyla, each of which is further subdivided many more times until ending at the species level. Flowering plants have a phylum to themselves, called the angiosperms.
Flowering plants share traits in common that set them apart from the other nine phyla, aside from the obvious fact of producing flowers where other plants do not. Flowering plants are the most recently evolved phylum, so many of their distinct traits improve adaptations found in earlier phyla, accounting for the flower plants' dominance of the plant kingdom today. Along with several other plant phyla, flowering plants produce seeds, but only flowering plants protect those seeds inside of an ovary. Flower seeds contain extra content called endosperm that nourishes the developing seedling. Finally, flowering plants have advanced vascular structures that form as long tubes inside of the stems and transport water and minerals throughout the plant.
Beyond the phylum, flowering plants are further subdivided into class, order and, finally, family. Botanist William A. Niering, author of "The Audubon Society Field Guide to North American Wildflowers," notes that most people begin to see strong resemblances between flowers belonging to the same family. Large flowering plant families include the sunflowers, orchids and legumes.
Genus and Species
Very closely related species are grouped within families by genus, the final grouping before arriving at the individual species. Although there is some contention about how species should be defined, generally, members of the same species can reproduce and produce fertile offspring. Within an individual species, there are often multiple types or cultivars of a flower. Although these individuals may look very different from each other, they can still cross to produce fertile seed.