To take on the universal character of a science, botany needed a community of communicating botanists with a commonly accepted toolbox of ideas. Before classification could proceed there needed to be an agreed set of basic biological units of classification (taxa), along with an agreed terminology for their structures, along with a method of inventory and description that included a system of nomenclature and classification. In the modern era it took over 200 years to develop these scholastic tools whose integration into an internationally-acceptable system was the great achievement of Carl Linnaeus. It was Linnaeus’s forging of this common scientific ground that constituted his great achievement, not just his reinforcement of the system of binomial nomenclature.
In Linnaeus’s day the relationship was judged in terms of the overall similarities and differences that would indicate affinities. Classifications based on these characters were called natural classifications. However, sometimes it was easier to group plants according to simple and obvious characters like flower colour, and these were known as artificial classifications. Linnaeus himself used a system of artificial classification based on the number of stamens and styles in the flower.
When Darwin published On the Origin of Species . . . in 1859, the meaning of ‘natural’ changed. Biological classifications became progressively focused on hypothetical evolutionary relationships, not just superficial resemblance. As it happened, those organisms with similar characteristics were usually closely related evolutionarily but not always, as in the case of parallel evolution. The selection criteria now became more focused on characters indicating common descent (hypothetical evolutionary trees now calculated by sophisticated computer programs using a wide range of characters) rather than simple similarity and difference. As it happened, descent with modification fitted in very well with the boxes-within-boxes strict nested hierarchy form of classification that had been used by Linnaeus.
Of all the various projects that might be subsumed under the category ‘study of plants’, up to the mid-19th century the key endeavour remained the process of inventory- the naming, classification and description of plants. This project has, unfortunately, dominated beyond all reason the study of plants and persisted into the present day. This was partly a consequence the preoccupation of the best botanical minds in Britain with recording of plants within its empire. But, just as botany reluctantly shook itself from the clutches of medicine, to descriptive botany has been slow to extricate itself from the wider relationships of plants, humans, and the environment. German botany began the fragmentation of plant study with major breakthroughs in plant physiology that were of consequence to agriculture, bringing plant study back into the world while the new ecology, the role role of plants within the environment as a whole, had also emerged in Germany with the work of Alexander von Humboldt, amplified by Ernst Haeckel.
We need to know the units that underlie any study, but the course of historyand its environmental concerns long ago outstripped the demand for taxonomy.
However, on the taxonomic front increasing numbers of characters were used to map, with ever finer resolution, the similarities and differences that existed between different kinds of plants.
The advent of computers and gene technology has subsequently given plant classification greater discriminatory precision, and therefore increased predictive power – all part of the progressive accumulation of plant collective learning.
While the selection criteria use to generate general plant classifications depended, very broadly, on three kinds of relationship: that between plants and people (by far the most common way that we have always grouped plants), plants and plants (as adopted by botanists and plant scientists), plants and the environment (as emerged with the scientific adoption of ecological ideas.
In the 19th century the project of plant scientists moved beyond the delineation and description of taxa into other fields of learning.
It remains to acknowledge that as a consequence of the historical development of of the study of plants of the many scientific plant classifications that could have been, the one we speak of as ‘plant classification’ is the one that organizes plant kinds. It is this path that we follow now, from its beginnings.