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.
The class Dicotyledonae is further divided into three subclasses based on the condition of the corolla: Polypetalae (flowers with free, separate petals), Gamopetalae (flowers with fused petals), and Monochlamydae (flowers with a single floral envelope, or with the perianth absent or much reduced, so that a clear distinction into calyx and corolla is not evident). Each of these subclasses is further broken down into smaller groups called series and cohorts, which in turn contain the individual orders (in this classical usage, "order" corresponds to what would today usually be called a family or a related cluster of families). Family Fabaceae, for instance, is placed under the subclass Polypetalae, while family Solanaceae falls under the subclass Gamopetalae.
Scale of the System
Bentham and Hooker's classification is large in scope: it accommodates roughly 165 orders under Dicotyledonae, a further 3 orders under Gymnospermae, and 34 orders under Monocotyledonae, giving a total scheme that was, for its time, one of the most complete natural classifications of flowering plants available. Even though it predates modern molecular and phylogenetic methods, its systematic, hierarchical structure — class, subclass, series/cohort, order — and its practice of grouping plants by a broad suite of shared floral and vegetative characters made it the standard reference system for herbaria and taxonomic description for a very long period, and it remains the classification framework used for describing families such as Fabaceae, Solanaceae, and Liliaceae in this chapter.