By Michael Allaby
The moment I walk out of the door I enter a world of bewildering complexity, filled with objects of widely differing shapes and colours, some still, some swaying in the breeze, others moving evidently of their own volition. If I am to survive here I must discover some means of categorizing all the things I see. At the very least, I need to know which are harmless and which present a threat.
So my brain makes selections. It pays more attention to some things than to others, ranking them according to their perceived importance and thereby imposing a kind of order on the scene. To do this it seeks patterns that repeat. Patterns allow like things to be grouped, reducing the complexity. Our brains have evolved to recognize patterns. They are very good at it.
We are also great communicators. We share our experiences and thoughts with others, and when I see something that interests me I want to describe it. But that presents me with a problem because in order to describe an object I must give it a name, and it must be a name that others will understand. This is simple enough for broad categories such as buildings, buses, bishops, and bicycles. And plants? Are we sure we all know what a plant is, compared, say, to a mushroom (which is not a plant)? What about trees? Can we distinguish confidently between a tree and a shrub? Yet plants, trees, and shrubs are also broad categories. If I want to tell my friend about a particular kind of tree I need to distinguish it from all the many other kinds — and in language my friend will understand. Extrapolate to the task facing an author who plans a book about plants and the need for a classificatory system becomes obvious.
In 323 B.C.E. Alexander the Great died and in the following years the people of Athens turned against Macedonians. Aristotle, though Greek, had been born in Macedonia and had tutored Alexander. He was charged with impiety, a capital offence, and moved away from Athens leaving his school, the Lyceum, to his friend Theophrastus. Theophrastus was a popular teacher with a wide range of interests. One was the study of plants and two of the books he wrote have survived: On the History of Plants and On the Reasons for Plant Growth. These have led to Theophrastus being called the father of botany.
Theophrastus grew plants in his own garden and he described more than 500 different types, categorizing them as trees, shrubs, subshrubs, and herbs. He noted differences in seed leaves (cotyledons) and distinguished between monocotyledons and dicotyledons. He was observing and recording patterns, grouping plants by their shared features and introducing terms we still use.
Many others sought to categorize plants over the succeeding centuries, but most concentrated on their practical uses, especially medicinal uses, rather comprehensively classifying them. A botanist studying an unfamiliar plant would check features of the specimen one by one against a standard list of characters, the decision at each point dividing the path and leading to a positive identification by eliminating all the alternatives.
This began to change in the late seventeenth century with the work of John Ray (1627–1705). Ray had been interested in plants from childhood. In 1650 he fell seriously ill and during the six years it took him to recover he devoted himself to studying the plants around Cambridge, where he lived, publishing his results in 1659 as a catalogue of plants: Catalogus plantarum circa Cantabrigiam nascentium. He later toured England, publishing in 1670 a catalogue of the plants of England and adjacent islands.
Ray’s most important work, Historia generalis plantarum (a general account of plants) appeared in three volumes, each with approximately 1,000 pages, in 1686, 1688, and 1704. In the first volume he wrote: ‘In order that an inventory of plants may be begun and a classification of them correctly established, we must try to discover criteria of some sort for distinguishing what are called “species”. After a long and considerable investigation, no surer criterion for determining species had occurred to me than distinguishing features that perpetuate themselves in propagation from seed.’ Ray had decided that the term ‘species’ should describe plants and animals that are descended from a common ancestor. He continued: ‘. . . we do not regard caryophylli [carnations, pinks, sweet William] with full or multiple blossoms as a species distinct from caryophylli with single blossoms, because the former owe their origin to the seed of the latter and if the former are sown from their own seed, they once more produce single-blossom caryophylli.’ Ray was the first naturalist to offer a definition of ‘species’ and he based it on features that are inherited. His Historia generalis described more than 18,600 species.
John Ray influenced the Swedish botanist Carl Linnaeus (1707–78), who continued the work of classification and popularised the binomial system of nomenclature, devised by the Swiss brothers Caspar (1560–1624) and Jean (1541–1613) Bauhin. This allocates two Latin names, one generic and the other trivial, to every species, removing all ambiguity. Linnaeus, however, classified plants according to their reproductive organs — similarities in their flowers — rather than their ancestry. So the work continued to expand and, where necessary, correct Linnaeus.
Over the course of the nineteenth century biological classification moved away from Linnaeus toward a more natural system based on relationships, a system that nowadays we would call phylogenetic. The vision behind that movement found its fullest expression in 1947 with the publication of Grundzüge einer Theorie der phylogenetischen Systematik (English translation, Phylogenetic Systematics, published in 1960) by the German biologist Willi Hennig (1913–76). Hennig, compared by some to Aristotle and Darwin, showed how biological classification could be built on a truly genetic foundation. In the case of plants, his work has led to a classification being developed by taxonomists calling themselves the Angiosperm Phylogeny Group that is rapidly superseding all previous systems and that covers gymnosperms as well as flowering plants.
It is such changes as these that necessitate the regular updating of dictionaries. Little by little, scientists have moved from attempts to impose order on the apparent chaos of nature, to revealing the genuine order that is already present. Thanks to them, as I step from my door the spectacle that greets me becomes less bewildering.