BOTANICAL NOMENCLATURE

Before the time of Linnaeus (1707–1778) many plants were known by a double Latin title; however, it is to this great Swedish biologist that we owe the general adoption of the present binomial system, in which the first name denotes the genus, while the second (specific) name denotes the species. All specific names may be written with small initial letters although formerly capitals were used where species were named after persons. Thus the species of Cinchona named after Charles Ledger, who brought its seeds from Brazil in 1865, is now written Cinchona ledgeriana rather than Cinchona Ledgeriana.

The specific name is usually chosen to indicate some striking characteristic of the plant—for example, the hemlock with the spotted stem is named Conium maculatum (maculatus, -a, -um, spotted). Sometimes the reason for the name is not as obvious as in the example just mentioned, but once it is discovered it will serve as a reminder of a characteristic of the plant—for example, Strychnos potatorum (potator, -oris, a drinker) bears a name which is only intelligible when it is known that the seeds of this species are used in India for clearing water. A particular species can also exhibit a number of varieties; these are especially evident with cultivated plants but are also found in the wild. For a medicinal example, see Mentha piperita below.

The modern rules governing the terminology of plant taxonomy are laid down in the International Code of Botanical Nomenclature.

Unlike the names of chemical substances, which are subject to changes which conform to evolving systems of nomenclature, systematic plant names are strictly controlled by rules which give precedence to that name used by the botanist who first described the species. Nevertheless, this seemingly straightforward approach can give rise to various quirks in spelling. The following are three examples involving medicinal plants: Rauvolfia vis à vis Rauwolfia; the former name was given to this Apocynaceous genus by Plumier in 1703, honouring the botanist Leonard Rauwolf. This spelling oversight caused much contention over the years centring on whether Plumier’s obvious intention should be adopted in the name Rauwolfia. Both spellings are commonly found but the rules dictate that Rauvolfia has priority. In another example the downy thornapple may be encountered as either Datura innoxia or Datura inoxia. The former, as Datura innoxia Miller, was used in 1768 (Gard. Dict., edn. 8, Datura no. 5) and this spelling was invariably employed for some 200 years; however in Miller’s original description, the plant was characterized as: ‘Datura (Inoxia) pericarpiis spinosis inoxiis ovatis propendentibus foliis cordatis pubescentibus’ (W. E. Safford, J. Wash. Acad. Sci., 1921, 11, 173) and taxonomists now consider D. inoxia Miller to have priority. Both versions are still commonly encountered. A third example concerns the genus of the coca plant which may appear as Erythroxylum, or in older literature as Erythroxylon. Uppsala Monitoring Centre (a WHO collaborating centre for International Drug Monitoring) has published ‘Accepted scientific names of therapeutic plants and their synonyms’.

SUBDIVISIONS OF THE PHYLA

The branches of the genealogical tree differ so much in size that it is not easy to decide which are of equal systematic importance, and what one biologist may consider as a family another may regard as a subfamily. Similarly, the species of one botanist may be the subspecies or variety of another. The main hierarchical subdivisions of a division, arranged according to Engler’s scheme, may be illustrated by the following example showing the systematic position of peppermint.

It will be noted that in pharmacopoeias and in research publications botanical names are followed by the names of persons or their accepted abbreviations (e.g. Linnaeus and Sole in the case of peppermint given above). These refer to the botanist who first described the species or variety. Students need not attempt to memorize these names, and in the following pages they are usually omitted except in cases where different botanical names have at different times been applied to the same plant and there is possibility of confusion. The source of cloves, for example, is now usually given as Syzygium aromaticum (L.) Merr. et Perry; prior to 1980 the BP used the name Eugenia caryophyllus (Spreng.) Sprague; other synonyms which may be found in the older literature are E. caryophyllata Thunb. and E. aromatica (L.) Baill. Worldwide, not all authors of research papers use the currently accepted name so caution is necessary and botanical sources should be checked.

The letters s.l. following the botanist’s name refers to collective species and varieties and imply ‘in the widest sense’ (sensu latiore), e.g. Thymus serpyllum L.s.l.

BOTANICAL SYSTEMS OF CLASSIFICATION

Before the widespread acceptance of the principle of evolution, biologists, being convinced of the fixity of species and lacking much of the information available today, confined themselves to more or less artificial methods of classification, their systems being frequently based on one or a few characters instead of upon the organism as a whole. These earlier systems are now mainly of historic interest, but certain of their features—for example, the large division of seed plants into monocotyledons and dicotyledons as used by John Ray (1628–1705)—survive today. Linnaeus’ Species Plantarum of 1753 is the starting point for the modern nomenclature of plants, although his actual system of classification is entirely artificial and of little significance today. The Prodromus, started by A. P. de Candolle (1778–1841) and completed under the editorship of his son Alphonse (1806–93), was a massive work of 17 volumes which professed to be an account of every flowering plant then known. The system of classification employed was a modification and extension of that introduced earlier by De Jussieu (1748–1836) and further demonstrated the inadequacies of the Linnaean system which were then becoming apparent. Bentham and Hooker’s Genera Plantarum (1862–1883) was patterned on the de Candolles’ work, each genus being redescribed from herbarium specimens and not consisting of a restatement of earlier literature. Although largely artificial, it was convenient to retain this system as a basis for collections such as the herbaria of Kew and the British Museum, with continuous revision based on molecular systematics.

During the last 100 years a considerable number of phylogenetic systems of classification have been propounded; these systems arrange taxa (any groups used for classification such as orders, families, genera, etc.) to indicate the possible relationship of one taxon to another. Such systems are clearly susceptible to change with increasing knowledge, and no final system acceptable to all taxonomists is in sight; indeed, for some practical purposes a stable, workable phenetic system is often preferable. A close examination of the phylogenetic systems reveals that certain taxa form precise groups, others have less well-defined boundaries and other groups are difficult to accommodate phylogenetically. The work of Engler (1844–1930) in association with other German systematists is still adhered to in this connection. Engler’s scheme of classification largely embodied the fundamental concepts of Eichler (1839–87) and was exemplified in the 20-volume work (1887–89) Die natürlichen Pflanzenfamilien, by Engler and Prantl. Subsequent to this, there appeared many editions of Engler’s Syllabus der Pflanzenfamilien, the eleventh by Engler and Diels in 1936. The last version of the Syllabus, produced by Melchoir as two volumes, was published in 1964; the plant families in Chapter 5 of this textbook are arranged in this order. The immediate popularity of Engler’s works was due to their applicability to plants of the whole world; they afforded a means of identifying all known genera.

Obviously, large works such as the above are not easily compiled and many taxonomists have produced phylogenetic schemes directed at various levels of classification without the complete systematics of the Engler series. Of the schemes, those of Cronquist (1981) and Takhtajan (1959) are generally similar whereas that of Hutchinson (1992) differs in that the dicotyledons are divided into two large groups—those characteristically and primitively woody (Lignosae) and those characteristically and primitively herbaceous (Herbaceae). These schemes incorporate data often not accessible to the earlier taxonomists; thus Cronquist, while emphasizing classical morphological characters and following the strobilar theory of Angiosperm evolution also takes account of micromorphological data (e.g. embryology and pollen structure), chemical data (e.g. secondary metabolites and serology) and the fossil record.

Dahlgren’s proposals (1983), which involve a taxonomic method termed cladistics, demonstrate the distribution of characters and his cladograms of the orders of Angiosperms can be conveniently used for illustrating the occurrence of secondary metabolites throughout the higher plants. In this method (cladistics), clade is a group of plants at any level sharing a common ancestor and formed by a splitting to give two new species, which themselves in the course of time may split again. Clades may be very large or small, with clades within clades; as they comprise hypothetical relationships, they are subject to change as new knowledge becomes available. Evolutionary changes as envisaged in cladistics are sudden and widespread vis à vis the continuous gradual evolution taking place by small changes over a long period of time, as postulated by Darwin. Cladistics are now widely employed by modern taxonomists.

A modern replacement for Engler’s classical work, now in the course of compilation, is The families and genera of vascular plants [K. Kubitzki et al. (eds)]. So far (2007), nine volumes have been published.

TAXONOMIC CHARACTERS

All plants possess hundreds of characters of a morphological, histological, embryological, serological, chemical and genetic nature which are potentially available for building up a classification of the plant kingdom. In the artificial schemes the characters employed were those that experience had shown could be used to produce suitable groups or taxa. The eventual scheme, into which could be inserted new plants as they were discovered or in which any plant could easily be traced, resembled a catalogue with a ‘telephone directory’ arrangement of plants in which the groups of individuals listed together did not necessarily have any phylogenetic relationship, but merely possessed certain common features.

Phylogenetic classifications, which endeavour to indicate the relationship of one taxon to another, imply the use of characters that are capable of showing such relationships. Because some groups of plants are more primitive than others on the evolutionary scale, certain characters will also be primitive, whereas other characters will have evolved from them. Thus, woody plants are generally regarded as more primitive than herbaceous ones and flowers with few parts more advanced than those with many parts.

The difficulties facing the taxonomist are appreciable. The appearance of a particular character in certain plants does not necessarily imply a relationship between these plants, because at some time in the past, under favourable conditions, whole groups of unrelated plants could have undergone this change (e.g. the development of fused corollas from polypetalous flowers; this is known as convergence). Alternatively, related plants may, in some point of time, have started to diverge in their characteristics so that the modern phenotypes appear very dissimilar—this is divergence. Parallelism refers to the similar evolution of characters in related plants or related groups of plants. Having decided which characters are of value and how many can be used, the taxonomist then has to consider whether each character should be given equal value or whether a ‘weighting’ system should be employed. Computers have an obvious role in dealing with large numbers of characters applied to thousands of plants, not only from the aspect of storage and retrieval of information, but also for the science of numerical taxonomy, which will probably play an increasing role in the development of systematics. For a fuller discussion of this subject the reader is referred to Heywood’s Plant Taxonomy.

CHEMICAL PLANT TAXONOMY

This subject has recently attracted much attention and has, after many years, brought the plant chemist back to systematic botany. The concept that plants can be classified on the basis of their chemical constituents is not new; for example, early workers classified the algae into green, brown and red forms, but it is only during the last 40 years that modern techniques of isolation and characterization have led to the chemical screening of many thousands of plant samples. Compared with morphological characters, chemical constituents are often more precisely definable and can be of more fundamental significance for classification purposes. Plant taxonomists, in general, hold the view that chemical characters are yet another type of character to be considered alongside those used traditionally, but it does not necessarily follow that taxa constructed on a purely chemical basis, if such were possible on the data at present available, would necessarily coincide with those arrived at by classical methods.

The characters employed in chemical taxonomy need to be those of intermediate distribution in the plant kingdom. The presence of such ubiquitous compounds as the essential amino acids and common sugars is of little diagnostic value and, at the other extreme, the occurrence of coniine in the single species Conium maculatum of the large family Umbelliferae is also of little taxonomic significance. Characters most studied in this connection are therefore secondary metabolites (alkaloids, isoprenoids, flavonoids, characteristic glycosides, etc.), many of which are of established pharmaceutical interest.

However, as discussed later, secondary metabolites may be subject to considerable variation in the living plant, depending on environmental and ontogenetic factors, and more stable chemical characteristics are offered by those closely associated with DNA composition of the species. Increasingly, it is becoming possible to use DNA hybridization, serotaxonomy and amino acid sequencing techniques for taxonomic purposes. One pharmacognostical application (Y. Mino et al., Phytochemistry, 1993, 33, 601) has been the determination of the complete amino acid sequence of one of the iron sulphur ferredoxins present in varieties of Datura stramonium. The results support the view that the white and purple forms are varieties of a single species and that the tree daturas (e.g. D. arborea) are best regarded as constituting one section of the genus Datura and not a separate genus (Idem., ibid., 1994, 37, 429; 1995, 43, 1186); work with D. quercifolia and D. fastuosa also suggests that the amino acid sequence depends not on the species, but on the section. Similar studies were subsequently applied to Physalis (Y. Mino and K. Yasuda Phytochemistry, 1998, 49, 1631).

A second example (H. Mizukami et al., Biol. Pharm. Bull., 1993, 16, 388) shows that restriction fragment length polymorphisms (RFLPs) can be used as a simple and efficient method for distinguishing between Duboisia leichhardtii, D. myoporoides and the hybrid of the two species (RFLPs are produced by digestion of DNA with restriction endonucleases and vary in number and size according to genus, species etc.) However, the same group found (ibid., 1993, 16, 611) that the technique did not distinguish between the various geographical strains of the traditional Chinese drug Glehnia littoralis (Umbelliferae) containing different furanocoumarin compositions but did so with Bupleurum falcatum (ibid., p. 279).

The differentiation between samples of Panax ginseng (Oriental ginseng), P. quinquefolium (American ginseng) and adulterants can be difficult by conventional means and F. Ngan et al. (Phytochemistry, 1999, 50, 787) have reported on the authentication and differentiation, one from another, of six species of Panax and also their adulterants, using RFLPs involving the DNA sequences in a selected ribosomal region; see also J. Wang et al., Planta Medica, 2001, 67, 781 and Z. Zhao et al., Planta Medica, 2006, 72, 865 for the authentication of Chinese herbal medicines. Salvia divinorum, which contains the hallucinogenic diterpenoid salvinorin A not present in other species of Salvia (e.g. the sage plant), can be identified unequivocally by the combined use of analytical chemistry (HPLC-MS) and molecular DNA fingerprinting (C. M. Bertea et al., Phytochemistry, 2006, 67, 371).

Random amplified polymorphic DNA analysis has been used to distinguish between the various subspecies of Melissa officinalis common on the pharmaceutical market and now included in the BP/EP. Previously, samples have been classified according to the distribution pattern of compounds present in the lemon balm oil (H.-T. Wolf et al., Planta Medica, 1999, 65, 83).

Recent examples of the correspondence of genetic profiles and chemical constituents for the delineation of closely related plant species and chemotypes is illustrated by research on Withania somnifera (R. S. Dhar et al., Phytochemistry, 2006, 67, 2269), Zingiber officinalis and related species (H. L. Jiang et al., Phytochemistry, 2006, 67, 1673), and Hypericum spp. (A. Smelcerovic et al., Phytochemistry, 2006, 67, 171).

Serotaxonomic studies of Acacia gum exudates have demonstrated the value of such immunological tests in the chemotaxonomic analyses of these economically important products (T. C. Baldwin et al., Phytochemistry, 1990, 50, 599).

A standard work on chemotaxonomy (in German) is that of Hegnauer (see ‘Further reading’); it comprises 11 volumes published over nearly 40 years. A four-volume work in English is that of Darnley Gibbs, published in 1974; unfortunately, it does not appear to have been updated.