Systems of Classification

From Convenience to Evolution: The Range of Classification Systems

Plant classification systems have changed considerably as botanical knowledge has grown, and it helps to see the range of approaches side by side. An Artificial system classifies plants using one or a very small number of conveniently observed characters — such as habit alone, or the number of stamens alone — without regard to true biological relationship; such a system can therefore group together plants that only superficially resemble each other in the chosen character while separating genuinely close relatives. A Natural system improves on this by considering a much larger set of characters, drawn from both the vegetative and the floral parts of the plant, so that the resulting groups reflect a broader and more genuine similarity; the Natural system followed for the family descriptions in this chapter is the one devised by the British botanists George Bentham and Joseph Dalton Hooker in their work Genera Plantarum. A Phylogenetic system goes further still, aiming to arrange plants according to their actual evolutionary descent and drawing on fossil and comparative evidence to reconstruct lines of ancestry. Numerical Taxonomy brings a quantitative, computer-assisted method to bear: a large battery of characters is scored for each specimen, and statistical procedures then calculate overall similarity between specimens, reducing the subjectivity that comes from a taxonomist simply judging "overall resemblance" by eye; its scope and significance lie precisely in this objectivity and in its ability to manage very large data sets of characters and specimens together. Cytotaxonomy draws on cytological evidence such as chromosome number and behaviour during division, since closely related plants commonly share similar chromosome complements, while Chemotaxonomy draws on the chemical constituents produced by a plant, on the reasoning that related plants tend to share classes of secondary compounds.

Bentham and Hooker's Scheme

Of these, the Natural system of Bentham and Hooker is the framework used throughout the family descriptions later in this chapter, so its structure deserves closer attention. It recognizes three major classes among seed plants: Dicotyledonae, Gymnospermae, and Monocotyledonae — with Gymnospermae placed between the two flowering-plant classes, reflecting how botanical knowledge stood at the time the system was devised. The class Dicotyledonae is split into three subclasses according to the state of the corolla: Polypetalae, where the petals are free from one another; Gamopetalae, where the petals are fused; and Monochlamydae, where only a single floral envelope is present or the perianth is much reduced. Each subclass is then broken down further into series and cohorts, and finally into individual orders, which is the classical term used at roughly the rank of a family or a closely related family group. Family Fabaceae sits under the Polypetalae subclass because of its free petals, while family Solanaceae sits under Gamopetalae because of its fused corolla.

The Overall Scale of the System …