From about 300 BCE to about the middle of the 18th century plants were grouped together according to their habit as trees, shrubs, herbs and so on. The purpose of the classification was to group plants according to simple and obvious characteristics of the plants themselves rather than any particular utility they might have for humans. Theophrastus’s division of plants into ‘herbs, shrubs,subshrubs and trees’ was a practical classification that persisted into the 18th century. Today formal scientific classification groups plants in relation to one-another in terms of their presumed evolutionary (phylogenetic) history, sometimes known as phylogenetic systematics. The one-time morphological characters observable with the naked eye have been supplemented by microscopic and chemical-genetic information that is analysed using complex computer algoriths. Classification that concentrates on genetic characters is referred to as molecular systematics.
But this was a brief intellectual flame that was quickly extinguished for over 1000 years as the basis of plant study returned to the utilitarian relationship of plants and humans; classifications that characterized the post-classical educated world of materia medica, apothecaries, monasteries, and herbals that existed across both Europe and Asia.
Through the era of European printed herbals that lasted for about 200 years from 1470 to 1670, it was plant medicinal properties that took precedence, albeit diminishing, over the description of the plants themselves.
From around the 15th century the main task of botany had moved towards a global plant inventory, a task whose foundations had been largely laid by the European colonial powers in the mid-19th century at about the time of publication of ‘On the origin . . . ‘.
From about the mid-15th century into the mid-19th century the special study of plants was focused on the task of establishing kinds, what I refer to as ‘botany’. It is a narrowed-down focus of botanical attention (and therefore of classifications) to the delineation of plant kinds – to the way that we are to represent the different kinds and groupings of plants that exist in the world. This is what is at the heart of the desire for a global plant inventory.
In Linnaeus’s day the relationship was judged in terms of the overall similarities and differences between kind categories. This would indicate what were referred to as ‘natural affinities’. Classifications based on these plant characteristics (selection criteria) were then called ‘natural classifications’. However, it was often easier to group plants according to simple and obvious characters, like flower colour rather than overall similarity and difference. These classifications were called ‘artitificial classifications’. Linnaeus used a system of artificial classification based on the number of stamens and styles in the flower. He was aware that his artificial system would be superseded by natural classifications devised by botanists in France and England, but his method was simple and practical.
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.
As plant knowledge rapidly increased during the 18th century this artificial system was further refined using a much larger set of characters. Emphasis was now placed on overall similarities and differences between the plants, rather than characters that were simple and practical to manipulate. This became known as natural classification because it grouped plants according to the way they seemed to be related in nature. Botanists were no longer imposing order on the plant world by using an arbitrary human choice of convenient characteristics, instead through the close examination of many characteristics they were discovering the order that existed within nature itself. Most botanists, like Linnaeus, believing that this was the ‘natural order’ established by god which they were gradually revealing.
The comparison of similarities and differences was sometimes referred to as a general-purpose (phenetic) classification since as many characters as possible were taken into account with no character regarded as more important than any other.
During the 18th century with a more universal terminology that included a broad range of plant parts, the selection of possible characters became more numerous as plants were further divided using a system of artificial classification which used just a few simple and convenient characters that facilitated plant identification to a finer resolution of taxa, as the number of plant species rose from about 1000 at the end of the classical era, to around 10,000 in Linnaeus’s day. The best known of these artificial systems was the ‘sexual system’ of Linnaeus which grouped plants according to the numbers of sexual parts in the flower. The selection criteria, though based on differences between plants themselves, were nevertheless practical characters of human convenience.
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.