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Plant classification

Philosophers speak of science as dealing with natural kinds, which are categories that reflect the character of the natural world rather than the subjective intuitions and interests of human beings. Part of the success of science is that we believe its classifications and taxonomies correspond well to real kinds in nature.[1]

The ‘kinds’ of science (not just taxa, but all scientific categories) are subjected to intensive intellectual scrutiny and, as a result, we are able to predict (and therefore manage with a high degree of confidence) the circumstances of the world outside our minds.

Today, plant practitioners give special credence to scientific classification because it lives up to Theophrastus’s ideal of objectivity by reflecting, as far as possible, the way plants are related to one-another – not to human utility[7], or unrelated (or extremely distantly related) organisms, and the inorganic constituents of the environments where they are found.

A scientific classification therefore minimizes the influence of both human special interest and human subjectivity.

Classification & progress

The advance of science is not just about experiments, observations, and discoveries. It is also about the refinement of the web of ideas that provide our explanation and understanding of the world.

Classifications help us to order and refine our thoughts more effectively and, when they are steadily improved (making our interaction with the world more efficient and effective) as part of our collective lerarning, then they are progressive.

Not all classifications are progressive. Going shopping requires all kinds of mental ordering of our purchases, but we are not making scientific progress. The advantage of scientific categories is that they are shared (not individual or personal) categories whose refinement is a matter of broad agreement within the community of scientists.

There have been, throughout history, obvious ways of refining scientific categories – as in the discrimination of ever smaller particles of matter – and ever larger expanses of space. Biologically we have achieved an ever finer resolution of the numbers of taxa, their geographic distribution, and evolutionary relationships of the organisms of the world. Indeed the resolution of categories has has been so rapid that the number of individual disciplines has rocketed.

Progressive scientific classification has been so successful that we might assume it is in some way a superior form of classification surmounting all others since it is more accurate, reliable, and predictive – of all systems of classification, scientific classification is the best. But this could be a mistake, and a look at scientific plant classification will tell us why.

Evolution of scientific plant taxonomy

If we view scientific plant classifications as those groupings of plants that have been adopted by the plant experts of their day – plant practitioners and academics of various kinds – then we can see a historical path of development in the selection criteria used for plant classifications in general. This change, it would appear, reflects both a predictable path of collective learning as well as the changing interests and perspectives of the day.

First came plant utility, the use of plants for food, medicine, structural materials and so on. With plants the common factor in so much of importance to human life and activity, the advantages of distinguishing one kind of plant from another were obvious. Though it was plant properties that were of primary interest (their key selection criteria) there were obvious benefits in being more proficient at distinguishing one kind of plant from another. In very general terms, the emphasis of plant practitioners had moved from the relationship of plants to people, to the relationship of one plant to another.

The need for plant ‘units’ served the same purpose as the need for units of perception, cognition, meaning, and language. It provided the means for linking, manipulating, and improving, our understanding of the relations between plants and the other categories our our understanding – the web of ideas that is part of our collective learning.

The study of plants had emerged from more general human activity in a process of naming, description, and classification – a phase of plant study appropriately known as ‘botany’ which emerged as an academic discipline in the mid-16th century.

Before Darwin, in the mid-19th century, botanists had noticed that plants shared similarities and differences and that they therefore shared different degrees of affinity with one-another. Even so, most people believed that god had created each species de novo as separate and immutable. The great Linnaeus believed that his work on plant classification in the mid-18th century was the revelation of plant order placed on earth by god.

In the mid-19th century Charles Darwin changed all this when he proposed that the entire community of life had arisen from a common ancestor. Modern classifications therefore seek to reflect, not only static similarities and differences, but also the dynamic changes that have occurred in the course of evolutionary history.

At the time of Darwin, plant study was beginning to broaden out once again as it linked once again to the human community. Agricultural science emerged stimulated by discoveries in chemistry and the new plant physiology, while plant description was putting a new emphasis on plant spatial distribution as new plants were drawn in from geographically distant colonial empires, and scientists like Alexander von Humboldt were initiating new scientific categories of plant distribution across the world and in relation to other environmental factors. The categories of ecology were now open to the process of collective scientific refinement.

The study of plants had entailed three core relationships: between plants and humans, plants and plants, and plants and their environment.

Plants in relation to humans
A giant leap in the study of plants was taken in ancient Greece. This, so far as we can tell, was the true dawn of plant science when Theophrastus, who followed Aristotle as Head of the Lyceum in ancient Athens, recognized that, for all prior history, people had investigated plants from a human perspective, classifying and studying them according to their uses . . . as food, medicine, materials, and so on. This was the study of plants as a human resource, and classifications therefore employed utilitarian selection criteria.

Aristotle, who founded the Lyceum, famously declared:

‘How tiresome it is to keep asking of natural things – what is its use? . . . Once we have got what we need to survive, we should turn our attention to understanding nature in terms of its own ends and goods‘.[7]

The great Aristotle-Theophrastus insight, then, was that plants could be studied for their own sake . . . their structure, function, relationship to one-another, their environments and more . . . without regard to human interests.

So, up to the time of Greek science this perspective reflected the relationship between plants and humans, mostly in relation to their medicinal properties. Though it seems likely that plant use for foods would take precedence, it was the medicinal properties that comprised the specialized ‘scientific’ knowledge of respected community plant practitioners – the precursor ‘educated academics’ of old, be they the shaman, priest, or scribe.

At Theophrastus’s Lyceum in ancient Greece it became the relationship between plants and plants that became the key criterion of classification.

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 world of materia medica, apothecaries, monasteries, and herbals that existed across both Europe and Asia.

Plants in relation to plants
Detachment from human concerns is often regarded as an indication of sound science. But it would be nearly 1600 years after Theophrastus before the study of plants in relation to plants would return. Theophrastus had studied plants in many kinds of relationship but from the Renaissance to the at least the mid 19th century it would be the naming, classification and description of plant kinds that would dominate the study of plants. The study of plant morphology and identification exemplified this scientific approach to plants but it was still generally also allied to utilitarian medicinal properties. Though questions about plant function and interaction with the environment were asked, it would be some time before carefully reasoned scientific answers would be given.

So, the scientific classification of plants was devised (in line with the original intentions of Theophrastus) to study the similarities and differences between plants themselves so as to organize them as efficiently as possible into groups based on their physical characteristics.

The purpose or goal of scientific plant classification is therefore to ascertain the number, kinds, and relationships between the world’s plants. This scientific endeavour was at its most pressing during the Ages of Discovery and Enlightenment when Europeans were mapping the physical boundaries of the world’s continents, no doubt an incentive to accelerate the inventory of the world’s biota. At that time the economic and cultural potential of the world’s unknown plant resource was of foremost concern. Plants like spices had made men fortunes and given some countries power over others.
After the Renaissance, as emphasis on comparative morphology increased, there was a return once again to the ancient Greek classification based on the relationship between plants and plants. This is the approach that has, by-and-large, continued until the present day although a profoundly different perspective emerged with the publication of On the Origin of Species . . .. Before Darwin, grouping were made based on morphological similarities and differences. Though this continued after Darwin, this took on a different meaning. Formerly similarities and differences were mostly treated as differences that arose when god created new and immutable species. This was essentially the view of plant classifications greatest exponent of these times, Carl Linnaeus. Humans, in their taxonomy, were organizing god’s creation.

Darwin presented a totally new account of the community of life as descent, with modification, from a common ancestor. The purpose of scientific classification then is to provide insight into the relationships of all organisms as they have evolved from a common ancestor. Closely related organisms possess similar properties and this becomes valuable knowledge in the establishment of predictability. Botanists now make a very clear distinction between artificial plant classifications such as those based on flower colour or edibility (a subjective classification) and natural classifications based on flower structures that reveal the evolutionary relationships of plants (much more objective).

plants in relation to their environment
There is a third kind of plant classification, essentially that introduced by Alexander von Humboldt. He selected as a key criterion of purpose and perspective, the relationship between plants and nature. This form of plant classification has been largely ignored or, perhaps better expressed as distinctly subsidiary and of little consequence for classifications based on plant/plant comparisons. In my view, this has had a profoundly unfortunate consequence for the history of botanical science as so much money and resources has been poured into plant systematics that could have been directed towards plant ecology and the attempt to better ascertain the scientific relationship between humans and the world’s vegetation.

The purpose of a scientific plant classification, then, is to mirror as accurately as possible what exists in the world. This, then, facilitates prediction and therefore the understanding and management of the objects it contains.

It turns out that the classification systems used in science are not unique to science – they are universal: they are the way we organize objects in our minds, and the way we organize them in our language. What then are the various ways of classifying things – what are their goals, structural properties, strengths and weaknesses?

Theophrastus’s division of plants into ‘herbs, shrubs, 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.

It is virtually impossible today to define what we mean by ‘science‘, but one devious way would be to say that it is the refinement of the classification of all those categories that we use in the field of activity that we refer to as ‘science’. So, for example, science is involved with the classification of physical objects like animals and plants. But science is much more than the classification of physical objects it also deals with more mental categories like names, laws, hypotheses, definitions, principles, theories and so on, and part of the scientific process is the refinement or improvement of these categories as well.

There are, then, three major phases of plant study: first, the close observation of plants in the general realm of human activity but especially through the specialist skills of medicine men and priests who had both knowledge and skills in the plant properties and their relationship to the spiritual world; second, a phase of plant inventory and description best described as botany; third, the expansion of studies beyond morphological and anatomical description into physiology and ecology as plant science.

Species

Though we might seem to have clear and distinct concepts of what constitutes an individual plant, the idea of a species is controversial. Are species ‘real’ entities existing in nature, or are they creations of the human mind – or a bit of both?

Plant classification

So far we have found that plants may be grouped in countless ways and that the outcome, the resultant classification, depends of two major factors: the selection criteria on which the classification is based, and the particular system of classification that we use. It was suggested that the range of topics with the potential to become subjects of serious plant study fall into three groups. First, those that involved the relationship between plants and people and among which would be studies of plants as food, medicine, and the materials of potential use in daily life, like building materials, dyes, resins and so on. Second, for its own intrinsic interest, and to assist any other studies involving plants, there was the need to establish an inventory of the plant world as, essentially a comparative study of plants in relationship to one-another. Thirdly, there were questions to be answered about the way plants functioned (physiology), their spatial distribution across the world, and their relationship to other organisms and their environments. Some work on all these topics was, of course, being carried out from the earliest days. However, it might be concluded, in retrospect, that it is unfortunate so much time was spent by the botanical scientific community on descriptive botany when there were, in addition, so many other important and challenging matters to be addressed.

From about the mid-15th century into the mid-19th century the special study of plants was narrowed down to the task of establishing kinds, what I refer to as ‘botany’. It is this narrowed down understanding of plant classification (and what is commonly understood by the expression ‘plant classification’) as the delineation of plant kinds, that will now be briefly addressed. Text-books on this mode of plant classification are part of any biological course so I shall be brief.

Historical development

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.

Habit

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.

Artificial classification

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.

Artificial classification

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.

Natural general-purpose classification

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.

Flowering Plant Phylogeny diagram

Flowering plant phylogeny as at 2020
Including the currently recognized 435 plant families

The colours of the branches and the outer circles represent the major angiosperm clades (branches of the plant evolutionary tree) as indicated in the caption (ANA, Amborellales + Nymphaeales + Austrobaileyales). The illustrations around the circumference show 32 plant families and their position in the tree.

The flowering plants (angiosperms) arose in the Early Cretaceous about 145–100 million years ago, supplanting the earlier ferns and conifers (pteridosperms and gymnosperms). Palaeobotanical evidence indicates that this replacement was gradual, flowering plants not becoming dominant until the Palaeocene about 66–56 million years ago. The timing, geographic sequence, and plant composition of this diversification remains uncertain. There appear to have been substantial time lags, mostly around 37–56 million years, between the origin of families (stem age) and the diversification leading to extant species (crown ages) across the entire angiosperm tree of life. Families with the shortest lags occur mostly in temperate and arid biomes compared with tropical biomes. The ecological expansion of existing flowering plants, it seems, occurred long after their phylogenetic diversity originated during the Cretaceous Terrestrial Revolution.

Courtesy public domain license. Adapted from the originals provided by S.M., the Peter H. Raven Library/Missouri Botanical Garden and the Mertz Library/New York Botanical Garden

Phylogenetic special-purpose classification

After the mid-19th century and the publication of Darwin’s theory of evolution by natural selection classifications acknowledged the relationship between plants as descent with modification from common ancestors. Similarities and differences now became evidence of ancestral evolutionary relationships (phylogeny) with god no longer a necessary source of biological order.

Now, in determining phylogenetic relationships, some characters become more important than others. For example, vertebrates are, unsurprisingly, united in their branch of evolution by possessing vertebrae. The production of hypothetical evolutionary trees by computing characters (now often molecular) is known as cladistics, a system now employed throughout biology.

Through the history of the classification of plant taxa certain kinds of characters have been thought of as the key to a fully natural classification – whether they be those of pollen, chemicals etc. It would, however, appear that with the advent of genomics we are currently getting close to the finalization of a final ‘tree of life’ which includes some 350 plant families, and over 350,000 plant species.

From medicine to sustainability

With the ‘tree of life’ near completion, scientific resources once devoted to taxonomy are now addressing other plant questions. The study of plants, across the world, emerged out of the use of plants for medicinal purposes.

Botany
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 . . . ‘.

Plant science
There then followed a period of plant science that addressed the remaining questions of plant function through an intense period of laboratory experimentation in plant physiology complemented by advances in field studies as a more scientifically based study of environmental relationships that would become ecology.

Sustainability
By the mid-20th century the mysteries of plant structure and function were, in principle, resolved. The success of the human enterprise, not only in botany, was now becoming evident as an explosion in population with its associated consumption of nature. The world, much of which, at the dawn of the European Age of Discovery only a few generations ago, was wild and unknown, is now mostly converted to human use in an age we now know as the Anthropocene.
Our priority today is the gathering of information that will ensure that environmental damage bequeathed to future generations is minimized.

So, what are the important plant categories, plant classification systems (and their selection criteria) appropriate for the Anthropocene?

The categories we would use for such a classification would relate plants not to one-another, or directly to people, but to human environmental impact. But what would these classification(s) look like?

Bit

Science & fuzzy categories

Categories in a taxonomy are related to one-another via degrees of group inclusion with the most inclusive also being the most abstract and abstract while the lowest levels are the least inclusive and abstract and with the greatest degree of specificity and within-category similarity. Passing from greater to lesser generality and inclusivity: we have, for example . . . tree, eucalyptus tree, coolabah tree (classifications using analysis and synthesis)

Among the multitude of categories of thought, emotion, and perception, are those denoting physical objects in the world. These can be referred to as ‘kinds’ and one subset of these kinds is that of the living organisms which constitute the community of life.

Scientific categories aim for maximum economy and precision in their expression and this begins with the metaphysical observation that we can consider the world of scientific objects in toto as consisting of kinds, their properties, and relations.

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