What is a species

The idea of a species is fundamental to gardening and botany, but what is a species? A dictionary will give you a definition but not one which allows you to say this group of plants forms a species, as opposed to say a subspecies. The Cambridge Illustrated Glossary of Botanical Terms describes a species as,

A group of closely related, mutually fertile individuals, showing constant differences from allied groups, the basic unit of classification


As it is this is a nice definition but does really tie down what a species actually is? No, the truth is a species is indefinable. I realise this is probably about as helpful as a fridge in an igloo but once you appreciate what a species actually is it makes complete sense.

So how did we end up in this situation? Well, if anyone is to blame its probably Charles Darwin and Alfred Russel Wallace. These two men independently came to the same conclusion, though they both arrived at it from observing the plants and animals on small islands.

Prior to this people saw species as distinct groups:


A figure illustrating 3 species

What is a species – figure 1


The problem with this was two fold. One where did the species come from originally and second where did the fossils come from people kept finding. The fossils were clearly very old: some similar to living things, while others where very different but still recognisable as plants or animals with characteristics at least similar to living examples.


A figure illustrating 3 species and 2 fossils.

What is a species – figure 2

By the 1850’s when Darwin first published his ideas on the origin of species the notion that living things appeared by evolution was widely accepted but the mechanism which made it work was unknown. What Darwin and Russel suggested was that species changed into new ones by a very gradual, survival driven, process of small changes. Each step in this evolution is so small as to be unnoticeable at the time and could only be appreciated with the benefit of hindsight. This process applied to all the species all of the time; but over many years and generations. The only reason we see separate species is because all of these intermediates have been lost, except for the very rare fossil.


A figure illustrating the evolutionary relationship between 3 species and 2 fossils.

What is a species – figure 3


If this is correct, and all the evidence we have found to date supports this, then there is no real starting point to a species and the only end point can be extinction. What we call a species is merely the extant remnants in a chain of gradual changes.

For this reason a species is rather like fog, you can see where it is, you can say you are in it or not, but it’s impossible to say exactly where it stops and starts. This is why no one has yet come up with a strict definition of what a species is, and in all likelihood they never will.


Castanea sativa

Castanea sativa

Castanea sativa

The Sweet (or Spanish) Chestnut (Castanea sativa Mill.) is probably most know for its edible nuts, traditionally eaten around Christmas time, but in fact it makes a very fine large tree. Clothed in thick glossy leaves, tolerant of a wide range of soil types and relatively free of disease it has been extensively planted through the British Isles. Its quick growth and large size limits its use in all but the largest gardens when grown as a tree, but its success as a coppiced tree would allow the more ornamental forms to be more wildly grown. At present these are not widely grown but Castanea saliva ‘Albomarginata’ is a very attractive form with creamy white edged leaves and C. ‘ Aspleniifolia’ is a vary rare form with the serrated edges of the leaves drawn out into fine filaments.

Castanea sativa 'Albomarginata' foliage

Castanea sativa ‘Albomarginata’ foliage

It is believed that the tree was introduced by the Romans who ate the nuts but in practice the tree fails to provide a crop of edible nuts in all but the warmest parts of the UK. Globally though about 500,000 tonnes of chestnuts are produced annually, about half in the Far East. In France the best nuts are sold as marron and in Italy marron may refer to a cultivar of C. sativa which yields fewer high quality nuts. Though Castanea is only represented by C. sativa in cultivation in the UK the genus is quiet large and widely distributed with several hybrids and cultivars being actively studied because of its economic importance. Of lesser importance is the timber of Castanea due to its tendency to warp and split but its durability in contact with water has long been known and has made it a preferred wood for stakes.

Castanea sativa foliage and youing fruits

Castanea sativa foliage and youing fruits

Its scientific name dates from the 8th edition of Philip Miller’s Gardener’s Dictionary published in 1768 when he updated it to bring it into line with the Linnaeus system of plant names. He did though take the sweet chestnut back out of the beech genus, that Linnaeus had merged it into, arguing that the male catkins of Castanea are long and those of beech are globular. Castanea is Latin for chestnut while sativa comes from the Latin for cultivated. It is claimed that castanea itself comes from the Greek κάστανα meaning chestnut and a large number of Greek words where borrowed by Latin.

Castanea sativa bark

Castanea sativa bark


Species Plantarum by Carl Linnaeus

Hillers Manual of Trees and Shrubs

Excursion flora of the British Isles by Clapham et al

Inventory of Chestnut Research, Germplasm And References, FAO http://www.fao.org/DOCREP/006/AD235E/AD235E00.HTM [accessed 30th January 2013]

http://apps.kew.org/trees/?page_id=85 [accessed 30th January 2013]

The Royal Horticultural Society Horticultural Database’, available at www.rhs.org.uk [accessed 30th January 2013]

Botanical Latin by William T. Stearn

Tropicos, botanical information system at the Missouri Botanical Garden – www.tropicos.org [accessed 30th January 2013]

Botany for Gardeners by Brian Capon

The danger with this book is that is title may well put off many potential readers. The vast majority of gardeners do not see themselves as botanist: but spending hours studying the plants they grow, can’t grow and aspire to grow. As a gardener you consider the plants shape, colour, needs development and reproduction; yet little thought is given to the science under pinning these characteristics. All too often the science bit it shied away from as being too difficult or not relevant; more likely than not because we were put of the subject at school. The thing is gardening is to a large part applied botany and to deny it is not only short sighted but also making life harder. The problem is at first botany can be off putting in its apparent complexity and little is available in the way of bridging this gap. Brian Capon has set out to do just that with this book.

A professional botanist by training and a gardener by inclination, he has set out to provide a means of introducing gardeners to the how and why of plants. In doing so he has created a book that fills a real gap in gardening literature.

His years teaching botany, often to non-botanists, has given him a natural ability to do mystify his subject. Like all good teachers he has a natural gift for bring his subject to life and sharing his enthusiasm. The different aspects of the subject are approached in a logical order and the book is kept sufficiently concise so that it dose not intimidate those new to the field. It is well illustrated with photographs, line drawings and examples to add understanding. Though no part of the book can go into great depth, doing so would be counter productive, it ends with a list of further reading to encourage the reader to delve further into the subject.

Hopefully the present upsurge in interest in science will encourage gardeners to set aside there feat of it and temp them into learning a little bit more about the plants they grow. The book will not necessarily make them botanist; but it will make them better gardeners.


This is the process of classifying things into groups, and in biology placing them within a hieratical system of classification. It is the study of how living things fit into system of how we name them.


(plural taxa) is the individual levels of taxonomic rank. E.g. genus is one taxon and  species is a second taxon.


This is a group of closely related genera and the names should be written in italics (or underlined) and start with a capital letter. The names of plant families should end in the letters –aceae; but some long established names which do not have this ending are still used, all be it with standardised alternatives used in parallel.

International Code of Nomenclature for algae, fungi, and plants

The International Code of Nomenclature for algae, fungi, and plants (ICN) is as the title says the set of rules that govern how plants, fungi and algae are named and how the names are arranged into ever broader groups. The last published edition was the Vienna code but it is soon to be replaced by the Melbourne code s soon as it is published.

International Code of Nomenclature for Cultivated Plants

The International Code of Nomenclature for Cultivated Plants (ICNCP) lays down a set of rules in an attempt to standardise the way plants produced or selected by humans, as opposed to wild plants, are named. It is in effect a supplement to the International Code of Nomenclature for algae, fungi, and plants (ICN).  The first edition of the code was published in 1953 with the hope of applying some order to the naming of cultivated plants and in 1988 Hortax, a committee if plant taxonomists, was set up to supervise its continued development. The most recent edition, the 8th, was published in 2009.

The Cambridge Illustrated Glossary of Botanical Terms by Michael Hickey and Clive King

This may not seem an obvious choice to include in this list and retailing at about £38 for a paperback it is not exactly cheap; but it does plug a very important gap. This book provides an accessible explanation of the often confusing technical terms used to describe plants. These are words used in botanical and horticultural books and other literature to describe plants and their characteristics. Their use causes considerable confusion and, sadly, puts off many; but are vital to help describe plants in an unambiguous way. The result is that many plant descriptions rely on words which make no sense to most people or very specific uses of words with much broader colloquial means. Frustrating as this is the accurate naming and description of plants would be impossible without them; as they allow the necessary degree of precision when trying to describe the difference between plants.

To achieve this, the book is divided into two sections; the first provides a list of words with a brief explanation of the meaning and cross references to the appropriate illustration in the second section. This second section justifies the large size of the book: being A4 in size. This is a selection of clear black and white line drawings illustrating the many terms. Most books attempting to describe this with illustrations rely on photographs, but such a technique could never achieve the clarity that these accomplish.

While of limited use to many people, and hence the publishers high price, anyone serious about understanding plants and there descriptions would do well to get hold of a copy. This is not the sort of book you are going to sit and read it is one you are going to find you are repeatedly pulling down of your shelves to refer to.

Buddleja davidii

Buddleja davidii 'Burgundy'

Buddleja davidii ‘Burgundy’

Buddleja davidii Franch.is a popular garden shrub which has escaped cultivation to become established as a naturalised plant in the wild, where it can often be found on waste ground around towns and cities. It will grow happily in most garden soils and can even be seen growing out of cracks in masonry on buildings. It does prefer a sunny position as none of the buddlejas do well in shade. It grows well in even poor soil sending up long arching branches which are topped by the flowers in ranged in a cyme in late summer. Being a late flowering shrub it responds to early spring pruning and when pruned hard in March it produces far more flowers. These flowers give it its common name of butterfly bush as there strong scent is irresistible to butterflies.

Buddleja x weyeriana 'Honeycomb'

Buddleja x weyeriana ‘Honeycomb’

Its ease of cultivation and natural variability has lead to a host of named varieties being selected and introduced varying from white through to purples and very dark blues. The RHS plant finder currently lists 121 different ones, including some which are forms of Buddleja x weyeriana – a hybrid between B. davidii and B. globosa.

The genus Buddleja L. was named by Carl Linnaeus in ‘Species Plantarum’, naming it after the Reverend Adam Buddle but of reasons unknown he spelt it with a “j” when strictly speaking it should be spelt with an “i”. This has lead to considerable confusion over the

Buddleja davidii 'Harlequin'

Buddleja davidii ‘Harlequin’

years but as he repeatedly spelt it this way that is how it should be spelt. The only Buddleja known to Linnaeus was B. Americana L.which William Houstoun had sent back to Europe from the Americas and even then the spelling was causing confusion as in one of his illustrations he spelt it incorrectly both times even though he attributes the naming to Linnaeus. The species B. davidii itself was first sent back to Europe from China by the French Catholic missionary Armand David having found it in  August of 1869 and  Adrien René Franchet working in the herbarium of the natural history museum in Paris published a description of it in 1887 naming it in after the discoverer.

Buddleja davadii 'White Profusion'

Buddleja davadii ‘White Profusion’









Species Plantarum by Carl Linnaeus

Hillers Manual of Trees and Shrubs

Excursion flora of the British Isles by Clapham et al

Nouvelles Archives Du Muséum D’Histoire Naturelle series 2

Reliquiae Houstounianae sive Plantarum in America meridionale … Collectarum Icones by William Hostoun

The International Plant Names Index (2012). Published on the Internet http://www.ipni.org [accessed 12th December  2012]

Tropicos, botanical information system at the Missouri Botanical Garden – www.tropicos.org [accessed 12th December  2012]

The Royal Horticultural Society Horticultural Database’, available at www.rhs.org.uk [accessed 12th December  2012]