Plant Systematics

The history of plant systematics - the biological classification of plants - stretches from the work of ancient Greek to modern evolutionary biologists. As a field of science, plant systematics came into being only slowly, early plant lore usually being treated as part of the study of medicine. Later, classification and description was driven by natural history and natural theology. Until the advent of the theory of evolution, nearly all classification was based on the scala naturae. The professionalization of botany in the 18th and 19th century marked a shift toward more holistic classification methods, eventually based on evolutionary relationships.

Early botanical works

Historians of botany generally begin the history of botanical classification with folk taxonomy or with Theophrastus’s Historia Plantarum, the earliest surviving treatise on plants. Theophrastus, a student of Aristotle, did not articulate a formal classification scheme; instead he relied on the common groupings of folklore combined with growth form: tree shrub; undershrub; or herb. The Materia medica of Dioscorides was also an important early compendium of plant descriptions (over five hundred); it was in use from its publication in the 1st century until the 16th century.

Early-modern plant classification

In the 16th century, works by Otto Brunfels, Hieronymus Bock, and Leonhart Fuchs helped to revive interest in natural history based on first-hand observation; Bock in particular included environmental and life cycle information in his descriptions. With the influx of exotic species in the Age of Exploration, the number of known species expanded rapidly, but most authors were far more interested in the medinical properties of individual plants than an overarching classification system. Later influential Renaissance books include those of Caspar Bauhin and Andrea Cesalpino. Bauhin described over 6000 plants, which he arranged into 12 books and 72 sections based on a wide range of common characteristics. Cesalpino based his system on the structure of the organs of fructification, using the Aristotelian technique of logical division.

In the late 17th century, the most influential classification schemes were those of English botanist and natural theologian John Ray and French botanist Joseph Pitton de Tournefort. Ray, who listed over 18,000 plant species in his works, is credited with establishing the monocot/dicot division and some of his groups — mustards, mints, legumes and grasses — stand today (though under modern family names). Tournefort used an artificial system based on logical division which was widely adopted in France and elsewhere in Europe up until Linnaeus.

The book that had an enormous accelerating effect on the science of plant systematics was the Species Plantarum by Linnaeus, although this work does not deal with the relationships of plants, as such. It assumed that plant species were given by God and that what remained for humans was to recognise them and use them (a Christian reformulation of the scala naturae or Great Chain of Being). The Species Plantarum presented a complete list of the plant species then known, ordered for the purpose of easy identification, by the number and arrangement of the male and female sexual organs of the plants. Of the groups in this book, the highest rank that continues to be used today is the genus. However, the consistent use of binomial nomenclature and the fact of having a complete listing of all plants provided a huge stimulus for the field.

Linnaeus was quite aware that the arrangement of species in the Species Plantarum was not a ‘natural system’, i.e. did not express relationships. Elsewhere Linnaeus did present some ideas of plant relationships. The earliest system of plant classification probably was that by de Jussieu (inspired on the work of Adanson) and the early nineteenth century saw the start of the work by de Candolle, culminating in the Prodromus.

Evolutionary systematics

A major influence on plant systematics was the theory of evolution (Charles Darwin published Origin of Species in 1859), resulting in the aim to group plants by their phylogenetic relationships. To this was added the interest in plant anatomy, aided by the use of the light microscope and the rise of chemistry, allowing the analysis of secondary metabolites. A fairly late development was the advent of cladistics, which only came into its own with the availabity of the computer and the enormous flood of molecular data: the APG II system is only the latest in a long line of systems (see the list of systems of plant taxonomy).
» Theophrastus. Historia Plantarum. 
» A. Cesalpino (1583). De plantis libri XVI. 
» John Ray (1686). Historia Plantarum. 
» Linnaeus (1753). Species Plantarum. 
» M. Adanson (two volumes, 1763). Familles des plantes. 
» A.L. de Jussieu (1789). Genera Plantarum, secundum ordines naturales disposita juxta methodum in Horto Regio Parisiensi exaratam. 
» A. P. de Candolle et al. (1824–1873). Prodromus systemati naturalis regni vegetabilis sive enumeratio contracta ordinum, generum specierumque plantarum huc usque cognitarum, juxta methodi naturalis normas digesta. 
» Lindley, John (1846). The Vegetable Kingdom. 

Notes and references

- Mayr, Ernst. The Growth of Biological Thought: Diversity, Evolution, and Inheritance. The Belknap Press of Harvard University Press: Cambridge, Massachusetts, 1982. pp. 154-155.
- Mayr, The Growth of Biological Thought, pp. 155-161
- Mayr, The Growth of Biological Thought, pp 162-165.