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Martin Quigley, The Ohio State University, Horticulture and Crop Science; James A. Chatfield, Kenneth D. Cochran, |
Plants, like all other organisms on earth, are classified in a hierarchical system, starting with the species and making it all the way up to one of the kingdoms of life. This system moves from related species in a genus to related genera in a family, on through order, class, subphylum, phylum, and kingdom. For practical horticulture, the most important of these classification units are the species, the genus, and the family. Thus Pyrus calleryana is the Latin name for the species known as Callery pear. Pyrus calleryana and Pyrus communis are different species in the genus Pyrus; and Pyrus (pears), Malus (apples and crabapples), Aronia (chokeberries), and Rosa (roses) are some of the genera in the rose family (the Rosaceae).
The most fundamental of these classification categories is the species. The idea of a species is sometimes hard to describe, often vaguely defined as "the basic unit of classification" or "plants of one kind." One of the most useful concepts of a species, however, relates to the fact that a species is a reproductively isolated population of organisms. This, too, is imperfect, of course, and respectable species like red maples and silver maples fail to read our books, and cross-fertilize anyway.
Nevertheless, even though we may not be able to always precisely define species, we often pretty much know one when we see it. Thus, human beings, gypsy moths, the fungus that causes apple scab disease, the dawn redwood, and the coast redwood are all seen as distinctly different species.