Rewriting the Plant Family Tree

May 28, 2019

There has never been a better time to study plant sciences. That may sound odd, or like an arrogant twenty-first century disparagement of the skills and intelligence of our forebears. After all, people have been studying plants, botany in the modern sense, at least since the days of Ancient Greece in Europe, and in India for centuries before that. Herbalists have studied the therapeutic properties of plants for even longer, and the earliest records of plants and their uses are as old as plant domestication and the invention of writing itself.

All of that is true, but today’s plant scientists have at their disposal tools of which their predecessors could not even have dreamed, tools that allow them to investigate the innermost secrets of their subjects, to delve to genuinely unprecedented depths. Our ancestors were able to observe the morphology of plants, the habitats in which they grow, the similarities and apparent relationships between types of plants, and plant behaviour. Today, scientists are probing beyond what plants do and discovering, in considerable detail, how they do it and its significance. Early light microscopes revealed structures, organisms, and cells too small to be visible to the unaided human eye. Modern electron microscopes go far beyond, to reveal, at high resolution, objects as small as leaf stomata with their guard cells. Chemists can isolate, identify, and trace the sequences of reactions involved in plant metabolism and in the operation of plant immune systems. Researchers can explore the ways in which plants communicate among themselves to warn of attack by herbivores. Radar can penetrate the soil to show the arrangement and areal extent of plant roots, contributing to research into how plants seek and partition resources, and all without disturbing the plants.

Underlying these advances, in a sense dominating them, perhaps, has been the exponential leap in the understanding of genetics and the techniques for its study made possible by the speed and comparative affordability with which genomes can now be sequenced and tiny scraps of DNA can be extracted and multiplied to provide samples large enough for examination. Comparisons of ancient DNA from early domesticates and pre-domesticates with DNA from their modern descendants has revealed much about the processes involved in plant domestication and the spread of crop cultivation. This study is of relevance to archaeology and anthropology. It tells of our history as well as that of the plants, of how our distant ancestors arrived in the places where we live now, and of the plants they brought with them. It informs our gardening, by tracing the journeys to our gardens and allotments taken by the plants we grow for their beauty or utility.

Genome comparisons also underlie modern plant taxonomy, resolving a puzzle half as old as time itself. Their similarities make it obvious that certain plants are related to certain others. But how, exactly, are those relationships determined? Morphologies may mislead, as only distantly related plants find similar responses to similar growing conditions. Think, for instance, of superficial similarities between some cacti and some euphorbias. That is what doomed Linnaeus’s simple and straightforward comparisons of reproductive structures. Clearly, relationships must be based on descent. I am descended from my parents, grandparents, and so on, but you could not unravel that chain of descent simply by comparing my physical appearance with that of my neighbours. Inevitably, therefore, the application of genetic studies has led to major revisions in plant taxonomy. Species once thought to be related turn out not to be so, old groups are revealed as polyphyletic and have to be split, and others, apparently dissimilar but monophyletic, are united. Taxonomy, you might say, is being turned on its head, and this is still very much a work in progress.

A dictionary records the use of words and expressions and it does so at the particular time of its compilation. It provides a snapshot, if you will, of a moment in the landscape of its area of interest. But as time moves on the scene changes and that is why revisions become periodically necessary. The fourth edition of the Oxford Dictionary of Plant Sciences appeared recently. In preparing it, which was my job, I have taken the opportunity to reflect as many of these changes as I could, and there were many. It is a major revision.

In particular, I have checked, altered where necessary, and greatly augmented the Dictionary’s coverage of taxonomy, relying for reference on the work of the Angiosperm Phylogeny Group (APG). This involved moving old plant families to new designations, adding many families missing from earlier editions, and completing the listing of orders and clades. Always, where I moved an entry, I left behind enough information to explain what I’d done and to guide the reader to the new location.

While revising and reordering the taxonomic entries I also removed all the in-text cross-references to common names. These had tended to accumulate in great lumps; think of the number of plants with a name beginning ‘common’. So all of them are now listed in an appendix, with references to the entries in which they are described. The second advantage in this change was that I could add many more common names. As we all know, it was the vast proliferation of common names that made a coherent system for naming plants a vital necessity. Taxonomy began, of course, as a ‘language’ that would allow botanists in different parts of the country and the world to converse.

The other major addition to the new edition is the inclusion of drugs and medicines derived from plants. There were always some of these, but I’ve added many more. This, I suppose, is a form of self-defence. There has never been a more propitious time to study plants sciences, but still there is a perceived need to show that such study is relevant in our modern world, that plants and the study of them really do matter to all of us.

So here it is, the fourth edition. Read and enjoy!

Michael Allaby has written many books on nature and environmental science. He is the General Editor of the Oxford Dictionaries of Zoology, Ecology, and Geology and Earth Sciences, and co-author of the Dictionary of Environment and Conservation. Other publications include the Encyclopedia of Weather and Climate, the DK Guide to Weather and, most recently, Plant Love (Filbert Press).