Why Multiple Systems Exist
Over the history of botany, several different systems of plant classification have been proposed, each reflecting the knowledge and the priorities of its time. A system built purely for convenience, using one or two easily observed characters, is called an Artificial system; such systems group plants that merely look alike in a chosen feature (for instance, habit or number of stamens) without regard to true relationship, so they often place unrelated plants together and separate close relatives. A Natural system, by contrast, uses a large number of characters — vegetative as well as floral — and tries to group plants according to their overall resemblance, so that the resulting groups come closer to reflecting genuine natural affinity. The Natural system used in this chapter's classical framework is the one proposed by the British botanists George Bentham and Joseph Dalton Hooker, published in their work Genera Plantarum. A Phylogenetic system goes a step further and attempts to arrange plants according to their evolutionary descent, so that the classification reflects ancestor-descendant relationships and the course of evolution, drawing on fossil evidence and comparative morphology. Numerical Taxonomy takes a more quantitative approach: a large number of characters (often dozens or more) are scored for each plant, and the plants are then grouped using statistical and computational methods that calculate overall similarity, with the help of computers to handle the volume of data. Its scope and significance lie in reducing the subjectivity of judging "overall resemblance" by eye, giving a more objective and repeatable basis for grouping, and in being able to handle a very large number of characters and specimens at once, which becomes valuable as more taxonomic data accumulates. Cytotaxonomy uses cytological information — chromosome number, chromosome structure, and behavior during cell division — as taxonomic evidence, since closely related plants often share similar chromosome complements. Chemotaxonomy uses the chemical constituents of plants, such as secondary metabolites, proteins, or other biochemical markers, as a basis for judging relationships, on the reasoning that closely related plants tend to produce similar classes of chemical compounds.
Bentham and Hooker's Classification in Detail
Bentham and Hooker's system is the natural system followed for the classical family descriptions in this chapter. It divides the seed-bearing plants into three major classes: Dicotyledonae, Gymnospermae, and Monocotyledonae. Placing Gymnospermae between the two great angiosperm classes reflects the state of botanical understanding at the time the system was devised, before the modern distinction that separates gymnosperms from angiosperms at a higher rank. …