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1. PLANT TAXONOMY Taxonomy is the study of the rules, principles and practices of classifying living organisms. Systematic or Taxonomic Botany is the study and practice of classifying plants. It is not a new science by any means; Aristotle recognised two kingdoms plants and animals - in the 4th Century BC. Dioscorides and Pliny the Elder were recording hundreds of different ‘kinds’ of plants in the 1st Century BC. Plant names, over the centuries, became increasingly complicated until 1753 when Carl von Linné, better known as Linnaeus, devised a classification system that included the binomial system of two names: genus and species. This has been greatly refined over the years, but is still the basic system we use today. 1.1 Botanical Classification Botanical classification attempts to group members of the plant kingdom together with those that have the most characteristics in common. This grouping of plants may aid identification and also assist the grower in his knowledge of the ideal growing conditions or even the pests and diseases to which a specified plant may be susceptible e.g. plants of the family Ericaceae prefer acid soil; many members of the Subfamily Maloideae (previously known as the Pomoideae) - the pome fruits such as pear, apple and Pyracantha, are susceptible to fireblight. The plants are divided and sub-divided into… …with further precise divisions beyond the species, which are detailed in section 1.2. ©HCC RHS LVL3 PGHAP L1 2011 A5 13 An example using the Hedgehog Rose (Rosa rugosa) may be written as such: Kingdom: Division: Plantae Spermatophyta (Plants) (Seed plants) Class: Angiospermae (Flowering plants) Subclass: Dicotyledoneae (The Dicots) Order: Rosales Family: Rosaceae (The rose family) Genus: Rosa (The ancient Latin name for the rose) Specific epithet: rugosa (A descriptive word meaning ‘wrinkled’ - referring to the leaves) Further examples will be seen in the text. To begin with, we will start at the top with the Kingdom, working through to the Subclass. There are a number of classification systems which one may encounter in historic and modern literature. The five-kingdom system put forward by Whitaker (1969) is commonly accepted, although recently more modern systems have proposed six kingdoms, dividing Monera further to create a new kingdom; Archaebacteria. The first systems considered only two or three kingdoms and by default, many organisms fell into the kingdom Plantae. The classification system according to Whitaker is briefly considered below: Kingdom: Kingdom: Animalia Plantae (Animals) Kingdom: Fungi (Fungi) Kingdom: Protista (Protazoa, slime moulds & single-celled algae) Kingdom: Monera (Bacteria & blue-green algae - Cyanobacteria) (Plants) The plant Kingdom is divided into subordinate ranks called Divisions. Some studies have recognised as many as 24 Divisions. The system devised by Adolf Engler is accepted by many and divides the Kingdom into 17 Divisions. Bentham and Hooker’s system is presented in ‘Genera Plantarum’, and most of the British herbaria are based on this system, as opposed to the system devised by Adolf Engler, which has become the standard in Europe and North America. ©HCC RHS LVL3 PGHAP L1 2011 A5 14 Mosses and liverworts are not specified in the syllabus. These have a relatively simple structure: Division: Phycophyta Division: Mycophyta Algae Fungi Modern classifications place these in separate kingdoms; formerly they were grouped in the Plantae Division: Bryophyta (Mosses and liverworts) Plants in this division are diminutive and commonly found in shaded, damp areas where moisture levels remain sufficient to prevent desiccation and allow for fertilisation by swimming sperm cells. There is an alternation of generations within which the haploid gametophyte generation is dominant. The gametophyte possesses no true roots, leaves or stems. Reproduction is by spores. Liverworts can be a serious weed of container grown crops. Gemma Cup containing Gemmae (tissue which produces new plants) ♀ Archegoniophore ♂ Thallus (body of the plant) Rhizoid (root-like structures) Figure 1 Liverwort (Marchantia polymorpha) Division: Tracheophyta This is an older collective term covering vascular plants of nine previously recognised divisions. The term ‘tracheophyte’ commonly encompasses two main divisions, which alternatively are considered as divisions in their own right: Division: Pteridophyta ©HCC RHS LVL3 PGHAP L1 2011 A5 Seedless, vascular plants e.g. ferns and horsetails 15 Division: Spermatophyta Gymnosperms Angiosperms Division: Pteridophyta Ferns, horsetails, clubmosses & quillwarts) Plants in this division possess the vascular tissues of phloem and xylem, have true roots, stems and leaves, but reproduce by spores. They undergo an alternation of generations2 with a haploid gametophyte and diploid sporophyte generation, of which the latter is dominant. The presence of the pteridophytes can be seen in the fossil record spanning tens of millions of years, predating the evolution of the flowering plants. Figure 2 Equisetum arvense - a troublesome weed (Reproduced under the Wikimedia Commons Licence) Horsetails are a very problematic weed and are able to reproduce in abundance by the production of spores. Of this same division are the ferns, the propagation and sale of which are significant to the horticultural industry. Ferns possess chlorophyll, leaves, roots and stems, but of course no flowers and they do not produce seeds. 2 See additional sheet in Appendix -‘Alternation of Generations’. ©HCC RHS LVL3 PGHAP L1 2011 A5 16 Pinna Viewed at x10 magnification Sorus comprised of Sporangia (spore-producing structures) Leaf Vein Figure 3 Underside of two pinnae of Polypodium Division: Spermatophyta (The seed plants) In terms of horticultural significance our greatest concern, perhaps, lies with this division. The origin of the gymnosperms begins around 300 million years ago, with the evolution of the angiosperms being much later. The alternation of generations is much reduced in the spermatophyta, but again is dominated by the diploid sporophyte generation. Excepting Cycads and Ginkgo, swimming sperm are not present and so fertilisation may take place in the absence of free water. There are complex vascular tissues which are present in the roots, stems and leaves. The spermatophyta is comprised of two groups; the gymnospermae and the angiospermae. Gymnospermae These are woody plants, mostly evergreen trees and shrubs. Their seeds are not enclosed in an ovary, but instead are naked (the name is derived from the Greek words for ‘naked seed’). This class includes the conifers (such as firs, pines, cedars, cypresses, junipers and yews), cycads and the Ginkgo. ©HCC RHS LVL3 PGHAP L1 2011 A5 17 (A) Ripe cones hanging from a branch (B) Ripe cone with seeds visible within Figure 4 Cones of Cupressus arizonica var. glabra ‘Fastigiata’. Seeds may be winged (for example, the seeds of pines) and enclosed in woody, ripe cones. Gymnosperms have several to many cotyledons. There are generally considered to be five subdivisions. These are often treated as Classes (in which case the words would end with -psida instead of -phytina): Cycadophytina: Cycads Gnetophytina: Containing only three genera; Gnetum, Ephedra and Welwitschia Ginkgophytina: Ginkgo Pinophytina: Includes most conifers Taxophytina: Four or five genera including Yew The latter three are included as one by some authorities – Coniferophytina. This, as a Class, is Coniferoptera. Angiospermae (or Class - Angiospermidopsida/Magnoliopsida) Angiosperms are the flowering plants; their seeds are enclosed in the ovary. The flowering plants are complete, in that they have all these: chlorophyll, roots, leaves, stems, flowers and seeds. The seeds are enclosed in a case derived from the ovary (e.g. pods, capsules, etc.) and the name angiosperm comes from the words for ‘covered seed’. ©HCC RHS LVL3 PGHAP L1 2011 A5 18 Figure 5 Open seed pod of Pisium sativum ‘Early Onward’ This group of plants includes the broad-leaved trees and most garden plants. Angiosperms have normally only one or two cotyledons (seed leaves) and so the Angiospermae is further divided into the subclasses Monocotyledoneae and Dicotyledoneae. Monocotyledons and Dicotyledons Subclass: Monocotyledoneae (Liliopsida) - commonly called Monocots. These bear a single cotyledon and include many plants with strap-shaped leaves. Examples include grasses and bulbs, such as the lily and the daffodil. Subclass: Dicotyledoneae (Magnoliopsida) - commonly called Dicots3. These bear two cotyledons and include many broad-leaved trees, shrubs, annuals and herbaceous perennials. Examples include Magnolia, beech and sunflower. The chief differences between these groups are described overleaf: 3 The term dicot is slowly being replaced by the term eudicot, literally meaning “true dicot”. The former is, however, likely to remain in circulation for a considerable time. This new term makes reference to the fact that some plants classified in the dicotyledoneae share characteristics in their petal arrangements and pollen grains which suggests they may be an early divergent form of the monocots. ©HCC RHS LVL3 PGHAP L1 2011 A5 19 Characteristics of the Monocots and Dicots MONOCOTYLEDONS DICOTYLEDONS 1. A single cotyledon in the seed 1. Cotyledons are in pairs (2 per seed) 2. The apex of the young stem is on a lateral shoot 2. The apex of the young stem is on a terminal shoot 3. The primary root dies and adventitious roots take its place. 3. The primary root often persists and forms a taproot. 4. The veins in the leaves are parallel (e.g. bog pond weed below) 4. The veins in the leaves are reticulate (netted) – see below 5. Leaves are usually strap-shaped with entire margins (e.g. Iris (below), daffodil, grasses, palms) 5. The leaves frequently have an interrupted margin and/or are narrow at the base 6. Flower parts are mostly in threes or multiples of three (e.g. snowdrop) 6. Flower parts are mostly in fours and fives, or multiples of two and five (rarely in threes). 7. Pollen has one furrow or pore. 7. Pollen has three furrows or pores. Reticulate (many plants) e.g. like an Iris stem leaf e.g. like bog pond weed Differences between Gymnosperms and Angiosperms Angiosperms differ in that their seeds (ovules) are protected in the ovary. This is what we know as a fruit. Gymnosperms have unprotected ovules and a different reproductive system, as in cones. Angiosperms also have vessels (specialised cells) in the xylem and companion cells in the phloem, whereas gymnosperms do not. [One of our members has mentioned the classification of plants document downloadable from: http:// theseedsite.co.uk] ©HCC RHS LVL3 PGHAP L1 2011 A5 20 1.2 Taxonomic Hierarchy Several levels exist within the structure for classifying plants. Horticulturally, the three most important are Family, Genus and Species. Other categories in the hierarchy are no less important but, from a practical point of view, the novice horticulturist should concentrate on these three. Others are also highlighted here since they are referred to in the syllabus. Some additional categories are also included. The definitions presented here are assessment criteria within the syllabus. Let us now consider the Order and further classifications beyond. The following is an example of a naming hierarchy for the Weeping Beech (Fagus sylvatica ‘Pendula’ or Fagus sylvatica cv. Pendula) (Plants) Kingdom: Plantae Division: Spermatophyta (Seed Plants) Subdivision: Angiospermae (Flowering Plants) Class: Dicotyledones (Dicotyledons) Order: Fagales (Beeches, Chestnuts, Oaks, Birches, Alders, Hazels & Hornbeams) Family: Fagaceae (Beeches, Chestnuts & Oaks) Genus: Fagus (The Latin name for a Beech tree) Specific epithet: Sylvatica (Latin, meaning ‘coming from the woods’) Cultivar: The Weeping Beech ‘Pendula’ Order Within Classes there are Subclasses which contain groups of Orders e.g. Class: Subclass: Order: Magnoliopsida Dilleniidae Ericales (including the heathers) A plant order contains groups of families. The names assigned to orders often end with ‘ales’. For example, the order of Urticales - includes Ulmaceae (Elms), Moraceae (Mulberry) and Urticaceae (e.g. Nettle). It is interesting to note that plants that we may perceive to be so different have a common ancestry. ©HCC RHS LVL3 PGHAP L1 2011 A5 21 Family Families of plants are composed of a number of closely related genera. Often the members of a family resemble each other, either in general appearance or in terms of some technical quality (e.g. they share similar characteristics of fruit, flower or seed). Family sizes vary considerably – some families contain only a single genus e.g. Ginkgoaceae consists of only Ginkgo biloba, while others consist of a large number of genera e.g. the Rosaceae family includes a wide range of herbaceous plants, shrubs and trees including Silverweed, Roses, Apples, Pears, Firethorn and Mountain Ash. The larger families are sometimes divided into Subfamilies, especially if the family is extremely large or diverse. Internationally, there are disputes about the placing of specific genera into families. The family name is written with a capital initial letter and italicised. Most family names end with the letters ‘aceae’, but there are some exceptions to this rule. The family name is not part of the plant name, although it may look similar e.g. the family of Primulaceae - includes Primula, Cyclamen and Dionysia. Family names are often based on the names of the genus they contain. The exceptions to this are the very first family names to be recognised. They were grouped because of their appearance and economic, medicinal, or culinary values e.g. Labiatae and Umbelliferae. Within the Family is the Genus (plural = genera). Genus The genus is generally considered to be groups of closely related species. The genus can be defined as a taxonomic sub-category within a family and is composed of one or more species with similar botanical characteristics. Often similar flowers and fruits are used, but roots, buds, stems and leaves may also be the shared characteristic. The species within a genus must have more characters in common with each other than with species within other genera in the same family. Some genera are monotypic (containing only one species e.g. Ginkgo). Most, however, contain a number of species - some over a thousand. For example, the genus Primula includes, among others, Primroses (Primula vulgaris), Auriculas (P. auricula), Cowslips (P. veris) and Polyanthus (P. elatior). ©HCC RHS LVL3 PGHAP L1 2011 A5 22 The written generic name begins with a capital letter and should be in italics, or underlined if handwritten. It is often formed by describing an important character of one or more of its species, for exmaple Rhododendron, from the Greek words rodon (rose) and dendron (tree). Alternatively the genus name may be derived from that of the plantsman or botanist responsible for its introduction, such as Hoya, named after Thomas Hoy. Some common names have come to resemble the genus name, such as Lily, derived from Lilium. When the same genus name is used more than once within a sentence or paragraph (and there is no possibility of ambiguity about which genus name us involved), then only the first letter is used. For example, ‘Acer palmatum and A. japonicum’ implies ‘Acer palmatum and Acer japonicum’. Species The species is the fundamental unit of classification. It is an assemblage of individuals with morphological (structural) features in common and separable from other such assemblages by morphological discontinuities (differences). Fertile offspring can be produced through interbreeding within a species; the resulting offspring will show typical qualities of the species. Usually, but not always, there is a breeding barrier between species. For example, Primula vulgaris, our native Primrose and Primula veris, the Cowslip, are both Species of the Genus Primula. The species are written in lower case, in italics or underlined, and are often abbreviated as sp. or as a plural, spp. Sometimes, when the precise specific epithet is not required or not known, the word ‘sp.’ is substituted. For example, when making a comment about a pine in general terms, the plant will be referred to as Pinus sp. (or in the case where many different Pine species are being referred to, Pinus spp.). Variation within the species is subdivided into subspecies (subsp.), varietas (var.) and forma (f.) for wild plants. These are written in lower case italics/ underlined and follow the species to which they belong e.g. Acer glabrum var. douglasii. Take note on the use of italicisation and Capitalisation when stating botanical names! In the absence of an italic font or in handwritten texts, the family name, genus name, specific epithet etc. should be underlined. ©HCC RHS LVL3 PGHAP L1 2011 A5 23 Subspecies (subsp.) The subspecies is a category used to designate variants of a species, which are morphologically different and geographically separate from each other in the wild. In other words, a subspecies is used for plants with slight differences of some quality (like appearance). The subspecies should be written in lower case and italicised. Plants within one subspecies may breed with plants within another subspecies (that makes the plants the same species). However, in nature they are found in geographically distinct areas (meaning they do not naturally interbreed with each other). Examples include: Cedrus libani subsp. libani (type subsp.) - from Lebanon, Turkey. Cedrus libani subsp. brevifolia (syn. C. brevifolia) - from Cyprus. Cedrus libani subsp. atlantica (syn. C. atlantica) - from N.W. Africa. Varietas (var.) A botanical variety is an individual type, noticeably different from the parent species, which breeds true and will pollinate other plants in the same specific group. This category is used for lesser variants of a species - only for wild plants and usually local. An example would be Salix repens var. argentea (Salix repens is the Creeping Willow, and ‘argentea’ meaning silver, so this is the creeping willow with distinctly silver foliage). The variety name should be written in lower case and italicised. Unlike sub-species, the plants within different varieties are rarely found in geographically distinct areas. Forma (f.) (Forma specialis) This term refers to a point of minor variation by which plants possessing the characteristic may be grouped together. Form is the lowest category reserved for single character variants. For example, if a plant has white flowers and this is the only variation, it may be known as Form alba (Forma alba, usually written f. alba). This name is italicised and written in lower case following the specific epithet. Examples would include Clematis montana f. grandiflora (grandiflora = ‘large flowered’) - this form has larger flowers than the type species, or Gleditsia triacanthos f. inermis (inermis = ‘unarmed’) - this form is thornless, the type ©HCC RHS LVL3 PGHAP L1 2011 A5 24 species having thorns on its trunk and shoots, or Cyclamen purpurascens f. album, which is a white-flowered form of this hardy Cyclamen. Up to this point we have been dealing with distinct forms that occur in the wild as covered by the International Code of Botanical Nomenclature. Plants of garden origin are covered by the International Code of Nomenclature for Cultivated Plants. There are three types: the cultivar, the hybrid and the group. Cultivar (cv.) A cultivar is a cultivated variety. It is maintained by humans and is named on three levels; genus, specific epithet and cultivar e.g. Cucurbita pepo ‘Table Queen’. A cultivar may be defined as “an internationally agreed term for a cultivated variety” and was a term introduced in the late 1950’s for cultivated varieties. A cultivar is an assemblage of plants clearly distinguished by any character (morphological or otherwise) and which, when reproduced, retains its distinguishing character; it corresponds exactly to the old horticultural term - the variety. Such a plant has been selected by man and is perpetuated by man using primarily vegetative propagation techniques, although there are some cultivars that can be raised by sowing seed. Plants propagated from a single plant are called ‘clones’. They are identical to the original plant in all characteristics with regard to flower colour, form, vigour etc. Most cultivars are, in fact, clonal in origin and some plants have many clones. The spontaneous bigeneric hybrid x Cuprocyparis leylandii is a good example: x Cuprocyparis leylandii ‘Haggerston Grey’ x Cuprocyparis leylandii ‘Naylors Blue’ x Cuprocyparis leylandii ‘Leighton Green’ x Cuprocyparis leylandii ‘Stapehill 20’ Cultivar is often abbreviated to cv. The distinctive trait for which the plant is grown may be flower type, habit, leaf colour/shape, fruit size/colour etc. The cultivar name starts with a capital letter, is NOT written in italics, but is enclosed by single quotation marks e.g. Acer pseudoplatanus ‘Leopoldii’. ©HCC RHS LVL3 PGHAP L1 2011 A5 25