6	Classification and Nomenclature—grouping and naming

Classification

Biological classification places different organisms and groups of organisms into a logical system, usually a hierarchy. Within the Flowering Plants this process was initially based on similarities in appearance (particularly of the flowers), and sought to reflect a ‘divine plan’. Over time, the range of plant characteristics studied and recorded has increased, and the concept of evolution as a process for change has dramatically influenced our approach to classification. As our ability to observe greater and greater detail has developed, a range of internal features such as anatomy and embryo structure, and biochemistry, has supplemented observations of external morphology. In recent decades these advances have included a capacity to extract and compare DNA sequences, which has significantly enhanced, and sometimes challenged, previous theories about plant relationships.

Most current systems of Flowering Plant classification aim to reflect the evolutionary history of plants and groups of plants, and thus indicate relationships among them. A major aim requires that each present day group be defined such that it represents one evolutionary lineage; such groups are referred to as monophyletic. Researchers have been particularly active in examining the higher levels such as family and above, and a view has been expressed that the limits of these groups are now reasonably established such that further changes are becoming less likely. Much work remains to be done however below the level of family in ensuring for example that all genera are monophyletic.

When depicted diagrammatically, classification schemes usually appear multi-branched and tree-like, ideally three-dimensional. For inclusion in a flora or similar text they must be translated into a linear sequence; as far as possible in this context related groups are placed together.

The sequence of families presented in Chapter 8 closely follows the most recent classification of the Angiosperm Phylogeny Group (APG) as proposed in 2016. (Families are numbered in Name that flower for convenience, but these numbers have no other significance.) The APG is a group of specialists who have collated a large amount of data, including numerous molecular studies published over several decades, to arrive at a classification that has received considerable support within the wider botanical community. Four versions of their classification have appeared between 1998 and 2016, each later version more refined than the ones before. Of course many current published floras are based on previous classification schemes, but the APG proposals have been influential and are being incorporated into some online flora accounts. Indeed there appears to be a general trend for new floras to be produced in an online format; it is possible that the APG classification will never be seen in a traditional printed flora.

Many classification schemes have been put forward over the last several hundred years but, of the more recent of those which predate the APG proposals, perhaps half a dozen have received most attention in the form of subsequent evaluation and criticism. Any one of these is likely to be encountered in an existing flora. The most noticeable consequence for students of plant identification will probably be in the different sequence and contents of the families in various floras. For example, in Flora of Australia (1981–) and Flora of Victoria (Walsh & Entwisle, 1994), the families Rosaceae (Roses and allies) and Liliaceae (Lilies) are both treated in a broadly inclusive sense. By contrast, in Flora of New South Wales (Harden, 1990–) plants in these groups are accommodated in a number of smaller families.

A flora is a volume or series documenting plant life, usually including descriptions of families, genera and species, as well as keys (guides to identification) and often illustrations and photographs. Floras are commonly regional and have been produced for many parts of the world. Australia has a number of state floras as well as for areas such as Central Australia, and the Sydney region. The first to cover the whole continent (Flora Australiensis by George Bentham) was published in 1863–78, but a new Australian flora is in preparation with 33 volumes published in hard copy to date (Flora of Australia, 1981–). Twenty-six of these volumes cover Flowering Plant families.

The production of a flora in electronic form for access via the internet has the advantage of allowing new information to be incorporated as it becomes available, meaning that, resources permitting, floras can be kept up-to-date much more easily. In the past, the publication of a new flora embodying a particular scheme of classification was often a once-in-a-lifetime event. Classification schemes could have an appearance of stability, and users had time to acquire familiarity. With the advent of the electronic age, and increasing speed of change, it seems that ready access to the internet will be as much of an advantage as a well-trained memory.

Despite differences, these pre-APG classification schemes do have a lot in common, for example, in many floras the Magnoliaceae (Magnolia Family) and Ranunculaceae (Buttercup Family) are found towards the beginning of the text. The Asteraceae (Daisy Family) by contrast is often at the end. This is not to say that daisies have evolved from magnolialike ancestors, but rather that magnolias and buttercups and their allies were believed to be more similar to ancestral plants, and diverged as a lineage earlier, than did the daisies. The members of the Daisy Family were believed to differ more distinctly (that is to show a greater degree of change) to ancestral flowering plants than do the magnolias.

When discussing the subject of evolutionary change botanists sometimes use the terms ‘ancestral’ and ‘derived’ referring to the appearance of a plant feature (the character state). An ancestral character state is one thought to have changed little during the course of evolution. Conversely a derived character state is considered to have evolved such that it differs noticeably from ancestral forms. Sometimes a slightly different terminology is used; the discussion might refer to ‘primitive’ and ‘advanced’ plant characteristics.

Prior to about 1990 information on ancestral floral structure gleaned from the fossil record was limited. In general, characteristics taken to represent a more primitive condition included a woody habit, flowers solitary and symmetrical, with numerous free parts spirally arranged, and a superior ovary. Characteristics often seen as representing an evolutionarily advanced or derived condition included a herbaceous habit, with asymmetrical flowers in more complex (often specialised) inflorescences, with reduced numbers of parts arranged in whorls, and an inferior ovary. During the last several decades considerable progress has been made both in the discovery of new fossil deposits, and in examination and interpretation of fossil structure. In particular, numerous kinds of fossilised flowers have been found with remarkable degrees of preservation. A large body of evidence is now challenging older theories as to what features are appropriately considered ancestral or derived. The concept of a ‘magnolia-like’ ancestral form is being reassessed.

The Asteraceae (Daisy Family) and the Orchidaceae (Orchid Family) are two examples of groups with highly specialised flowers and/or inflorescences, compared to say Magnolia (Fig. 22) or Sedum (Fig. 55). Members of these families are often associated with particular pollinators (frequently insects), and it is thought that the evolution of flowering plants is to a large extent a story of increasing specialisation in the relationships between plants and their pollinators.

Traditionally the Flowering Plants have been divided into two main groups, the Monocotyledons (Monocots) and the Dicotyledons (Dicots). These names refer to the cotyledons or seed-leaves attached to the embryo plant within the seed coat, and involved in the supply of food to the young plant immediately after germination. Monocots have one cotyledon, Dicots generally have two. In addition, other characters are typical of, but not confined to, each group as in the following list.

	Monocotyledons	Dicotyledons
Seed	1 cotyledon	2 cotyledons often a tap root with a well-developed lateral root system plants woody or herbaceous venation usually reticulate floral parts usually in 5s, sometimes in 4s
Root system	usually fibrous
Habit	almost all species herbaceous venation usually parallel floral parts usually in 3s
Leaves
Flower

Advances in recent decades in understanding the relationships between the higher groups have seen the Dicots rendered untenable as a group representing one evolutionary lineage. As a result, several rather small groups have been separated from the Dicots, and the remaining species now form an alliance commonly referred to as the Eudicotyledons (Eudicots, see Fig. 19). This name is informal in the sense that it is not formed in compliance with rules set out in the International Code of Nomenclature–it is effectively a common name. The Monocots are considered monophyletic and have retained their traditional circumscription.

All of the main groups of Flowering Plants are further sub-divided into orders, families, genera and species, each of which occupies a particular level or rank in the classification scheme. There are many other ranks available (for example suborder, or tribe or subgenus), should a particular group be large and/or its variation warrant a more detailed view. It is evident that family is a higher rank than genus which in turn is higher than species, the species being the unit of classification; these three ranks are the most relevant in the process of plant identification.

Technical names for the main groups such as the Flowering Plants or the Dicots are not firmly established, and a number of alternatives have been used. Examples for Flowering Plants include Anthophyta and Magnoliophyta at the rank of Division, and Angiospermopsida and Magnoliopsida at the rank of Class. With respect to rank it has been suggested that, when the plant kingdom is viewed as a whole, the Flowering Plants as a group have been placed at a level higher than warranted. Addressing this issue, one proposal places the Flowering Plants at the rank of subclass as shown on the grey background in the following list (Chase & Reveal, 2009).

A classification for Eucalyptus globulus (Blue Gum) showing the main ranks in sequence as used in Flora of Australia (vol. 1, 2nd edn). Many other formal ranks are available (over 20 in all), but not all need to be used for any particular species. The underlined endings are indicative of particular ranks.

There remains discussion on the particular rank that each major group (such as Flowering Plants) should occupy. Seeking a more evenly balanced scheme across the whole plant kingdom (including e.g. the Green Algae), an alternative proposal in 2009 recommended lowering the Flowering Plants from Division to Subclass, and using two ranks between Class and Order, as shown on the grey background.

Fig. 19 Orders and Families of Flowering Plants treated in this book: a diagrammatic representation of the main groups

Diagrams such as these give an indication of the hypothesised sequence of emergence of groups over time.

The names of orders end in -ales; of families in -aceae. The names in white text are commonly used to refer to their respective major groups; they are not formed in compliance with nomenclatural rules.

The number of families is shown for each group and gives an indication of relative size, thus there are 77 families of Monocots and, of the 10 families in the order Liliales, only Liliaceae and Colchicaceae are described in Name that flower. The ANA grade groups are briefly mentioned in Chapter 6 but not treated further. The lightest grey branches lead to orders not covered in this book.

A species is recognised as an assemblage of individuals with a unique combination of flower, fruit and vegetative characters and is often thought of as a ‘kind of plant’. The individual members of a species are morphologically more similar to each other than to members of other species. At one time it was thought that a species could be defined as a group of self-perpetuating individuals that did not breed with other groups, but this is not true of all species as some interbreed or hybridise and give rise to intermediate forms.

Sometimes a species is very variable but the variation is insufficient to justify the recognition of additional species. The lower categories that may be used to show this, in order of rank, are subspecies (ssp. or subsp.), variety (var.), and form (f.). One example is Banksia ericifolia subsp. macrantha.

Related species are grouped into genera. It is usually clear that the species in a genus are related, as they are often similar in general appearance with a number of features (generic characters) in common. In some cases the characters that separate two similar species are only observed after close examination. Related genera are grouped into families. However, the characters shared by all members of a family are fewer than the number shared by members of a genus. In spite of this, many families are clearly recognisable.

The terms grade and clade are sometimes seen in discussions dealing with plant classification. Both refer to groups that are recognised for a particular reason, or simply under discussion at the time, but neither has formal rank in a classification scheme. A clade is a monophyletic group (i.e. representing one evolutionary lineage), a grade is not monophyletic.

The ANA grade is a group of about 175 species dispersed in three orders, Amborellales, Nymphaeales and Austrobaileyales (hence ANA, Fig. 19). They have retained a number of features considered similar to hypothetical ancestral Flowering Plants, and in recent diagrammatic representations of classification are placed in a basal position. The order Austrobaileyales is made up of trees, shrubs or lianes. Amborellales includes only Amborella trichopoda, an endemic shrub from New Caledonia. Nymphaeales are aquatic herbs.

Nomenclature

The Swedish naturalist Linnaeus (1707–1778) established the use of the characteristics of the sexual reproductive organs as the basis for grouping plants. He then named them according to a binomial system which gives each species a two-word name. The first word is the name of the genus, or generic name, and the second is the trivial name or specific epithet, which belongs only to one species in the genus. The generic name is written with a capital initial letter and the specific epithet in lowercase. The binomial is usually underlined in written or typed work and placed in italics in printed publications.

The specific epithet is not restricted to one genus; thus Eucalyptus alpina, Acacia alpina and Grevillea alpina are three distinct species that share the same epithet.

After a binomial has been written in full, in subsequent references to the genus it is usual to abbreviate the generic name to the first letter only. For example, a second reference to Acacia alpina would be written A. alpina, and other species of Acacia could be A. stricta or A. suaveolens.

An abbreviated word often follows the binomial, as in Acacia microcarpa F. Muell. and Hakea ulicina R.Br. The addition of these initials to the binomial indicates the person who described and named the species: F. Muell. is short for Ferdinand von Mueller and R.Br. for Robert Brown. The author’s name is called ‘the authority’, and it always appears in floras and official publications. When a name is changed, the authority for the original name is placed in brackets and is followed by the author of the alteration. Therefore, when Mimosa suaveolens Sm. was transferred to Acacia by Willdenow, the citation became Acacia suaveolens (Sm.) Willd. Another common form of authority citation includes the word ‘ex’, as in Daviesia genistifolia A. Cunn. ex Benth. The ‘ex’ connecting the two abbreviations means that Bentham described and published a name for the species first recognised, but not published, by Cunningham.

Many names have Latin or Greek roots and often indicate a real or imagined feature of the plant: Leptospermum comes from leptos = slender, sperma = seed; grandifolium from grandis = large, folius = leaf; and microcarpa from micros = small, carpus = fruit.

Other names are associated with famous botanists, explorers or collectors. Banksia is named after Sir Joseph Banks, who accompanied Cook on his first voyage. Brunonia and the specific epithets brownianus and brownii all honour Robert Brown, who was the naturalist on Flinders’ expedition and who later described and named many plants sent to England by other collectors. Several specific epithets honour Allan Cunningham, who was sent to Australia by Banks to collect plants for the Kew Botanic Gardens, and accompanied Oxley and King on their expeditions. Cunningham made many other exploring trips in Australia, New Zealand, and Norfolk Island, and later became Superintendent of the Sydney Botanic Gardens. He described and named much of his material, and the authority ‘A. Cunn.’ appears regularly in our floras. For dictionaries of plant names and their meanings see Sharr (1996), and Wapstra et al. (2010) which is particularly informative. For two accounts of people involved in early Australian botany see Carr and Carr (1981a, b).

Cultivated or garden plants also have botanical names. Some are popularly known by their generic name, others by common name. Examples include Zinnia elegans Jacq. (Zinnia), Antirrhinum majus L. (Snapdragon), Papaver nudicaule L. (Iceland Poppy, Pl. 4f, g) and Quercus robur L. (English Oak). The authority ‘L.’ here stands for Linnaeus.

With a few exceptions, family names are derived from that of a genus in the family, which is then referred to as the type genus. Proteaceae comes from the South African genus Protea, and the genera Banksia, Hakea and Persoonia are some Australian members of the family. In earlier times, some families were given names associated with obvious characteristics, so Compositae described the inflorescence of the daises and Labiatae the two-lipped flowers prevalent in the Mint Family. Many more recent texts use the family name Asteraceae, from the genus Aster, instead of Compositae, and Lamiaceae (from Lamium) as an alternative to Labiatae. This preserves consistency, with all family names then using the prescribed family name ending of ‘aceae’ (usually pronounced ‘–ay-see’). Authors who value the history and tradition inherent in nomenclature however often maintain the original names, a practice sanctioned for some eight families by the Botanical Code (ICN). Related families are grouped into orders, the names of which end in ‘ales’ (usually pronounced ‘–ay-lees’).

Name changes are a common cause of irritation to anyone working with plants. They indicate the continuing efforts to refine the classification system. One reason for a change is the discovery, during the revision of a family or genus, of an earlier name and description of a plant than the one currently in use. The International Code of Nomenclature for Algae, Fungi and Plants (ICN, Turland et al., 2018) establishes the rules for naming plants, one of which gives priority to the earliest name published in accordance with the code. Sometimes plants were classified incorrectly, often as a result of the work being done with insufficient material, and subsequent revision requires a change of name. The delimitation of families is often a matter of opinion or interpretation, so a genus that does not quite fit in one family may be moved into a closely related one.

To comply with the rules, all changes or new names must be notified in the proper form in an acceptable publication or, since the adoption of a resolution at the 18th International Botanical Congress (held in Melbourne, Australia, in 2011), may be published electronically. If generally accepted, new names or other changes will be adopted into general use and incorporated into floras. Another resolution adopted at the Congress allows the description (or diagnostic notes) accompanying the publication of new species to be either in Latin or in English, whereas in the past Latin was mandatory and an English translation was optional.

Cultivated plants originating from a selection process (which may also involve hybridisation) are known as cultivars (cv.) and the names of these are distinguished by a cultivar epithet, which is enclosed in single quotation marks. Cultivars from Prunus persica (Peach) include P. persica ‘Magnifica’ (Double-white Peach) and P. persica ‘Roseoplena’ (Double-pink Peach). Some cultivars are the result of selection from wild populations and their geographic origin may be suggested by the cultivar epithet, for example, Grevillea confertifolia ‘Major Mitchell’ is a form from the plateau of that name in the Grampians, Victoria. Preferred selections may be perpetuated by taking cuttings, as is done with grevilleas, or grafted onto another stock plant, which is the method for most fruit trees.

Hybrids are shown with a multiplication sign either between the names of the two parents, if they are known, or before the specific epithet if the hybrid has been formally named. Magnolia × soulangeana (Fig. 22) is a hybrid between M. denudata and M. liliflora.

In addition to their botanical name, many plants have a common name in the language of the country or state in which the plant grows. However, as common names are not universal and may differ from region to region, their use is prone to errors and confusion. In situations where accuracy is important, use of the botanical name is recommended.

For examples of classification schemes, see APG (1998, 2003, 2009, 2016), Kanis et al. (1999) and Takhtajan (2009). Standard texts such as Judd et al. (2016), Lawrence (1951) and Raven (1986) provide a more general introduction. Reveal & Pires (2002), and Chase (2004) discuss progress in Monocot classification and phylogeny. The APG website has a wealth of detailed, concisely presented information. Crane et al. (2004) and Friss et al. (2011) discuss fossils and plant phylogeny. An overview of the ‘Plant Tree of Life’ is provided by Palmer et al. (2004). And see also Lewis & Court (2004) and Soltis & Soltis (2004).

Mabberley (2017) provides a list of then-recognised APG orders and families, and very concise summaries of family characteristics.

On nomenclature, see Brickell et al. (2016), Gledhill (1989), Spencer et al. (2007), and Turland et al. (2013 and 2018).