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Binomials in Use The Genus (capital first letter) and species (small letter even for proper names) are typed in italics or alternatively underlined – if you are using a pen (or were using a traditional typewriter). Magnolia grandiflora6 Magnolia grandiflora type script The name of the genus can come from any source and may be: Commemorative – Lobelia is named after M. Matthias de Lobel, a plant collector who in the 16th century first drew attention to the leaf characteristics which distinguish monocots from dicots. Descriptive of the plant – Chionanthus, from the Greek words chion (meaning ‘snow’) and anthos (meaning ‘flower’) describes the white fleece of flowers on this small tree or shrub. From mythology – Hebe, from the name of the Greek goddess of youth Hebe. Figure 9 Hebe - Goddess of Youth All plants within a genus share some common characteristic (although this characteristic may not be obvious to all). 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 the first letter only is used. For example, ‘Acer palmatum and A. Japonica’ implies ‘Acer palmatum and Acer japonica’. 6 Magnolia grandiflora: a large evergreen shrub for a sheltered location or warm wall. Luxurious leathery leaves and wonderfully lemon-fragrant flowers much later than other magnolias. Worth seeking named cultivars as the species can take a long time to get into flowering stride. ©HCC RHS LVL2 POH L1 2011 A5 30 The Specific Epithet The specific epithet (the second part of the name) is written completely in lower case. Names do not have to be informative although they often are and this can be a great help to a gardener. For example: Origin or location – for example, occidentalis (meaning ‘of the west’) and monensis (meaning ‘from the Isle of Man’). Descriptive – hirsutus (meaning ‘hairy’), pubescens (meaning ‘covered with downy, soft hairs’), tuberosum (meaning ‘tuberous’ – which could be a reference to the root shape). Habitat – montanus (meaning ‘of mountains’), sylvaticus (meaning ‘of woods’), pratensis (meaning ‘of meadows’), paludosus (meaning ‘of marshes’). The ICBN code requires specificity rather than description and there is nothing to stop names being quite arbitrary or superficially confusing as when a colour description doesn’t refer to the most visible characteristic. (Helleborus niger: the ‘black refers to the root not the flower). The two words together (Genus and specific epithet) are called the species. For example, Quercus robur is the species name – Quercus is the name of the genus and robur is the specific epithet. In spite of the logic of the proper use of the terms ‘Genus’ and ‘species’, very commonly gardeners refer to the ‘species’ when they are discussing plants. It is a word in common use and like the use of the word ‘variety’ – its use is rarely precise. Exercise Find examples of plants with the following specific epithets; officinalis, arvensis, striata, glabra, praecox, palustris, humile. Do the names give you helpful information about the plant? Learn ones that you already know and are thus easier to recall, maybe, in the exam. We have now looked at the names which gardeners encounter most frequently: Genus and species. We have also looked at angiosperms, monocots and dicots and noted that modern taxonomists distinguish three classes. ©HCC RHS LVL2 POH L1 2011 A5 31 Between these categories are other taxons7 which make it easier to relate and categorise plants and below the rank of species are finer divisions which relate to more precise differences among a species, both occurring naturally and in cultivated plants. Taxons below angiosperm and gymnosperms traditionally include: Class Order Family e.g. Dicotyledons e.g. Fagales ending -ales e.g. Fagaceae ending -aceae The classes ‘dicot’ and ‘monocot’ were taxonomically displaced in the late 20th century by Arthur Cronquist, one of the most influential botanists of the 20th century. He replaced them with the classes ‘Magnoliopsida’ and ‘Liliopsida’. This in turn is duly being replaced by the APG system mentioned above where formal scientific names are not used above the level of order in favour of recognising named clades (e.g. eudicots, monocots and rosids. Orders and families remain however, with some individual reorganisation. Families consist of one or several similar or closely related genera. Similar families are grouped into an order. Family names end in -aceae. The names are usually derived from a type genus that is characteristic of the whole family. The immutable ending -aceae and the type genus represents a change for some older gardeners who are still used to descriptive family names such as the Cruciferae and the Umbelliferiae which have been in use for centuries. On the whole, families (although generally not orders) can provide useful information to gardeners about likenesses. Many gardeners will recognize that apples, pears and thorns bear a family likeness to roses although fewer might extend that likeness to Alchemilla mollis and Aceana (the New Zealand Burr), which are also in the rose family. Genera are by and large easy to distinguish for the gardener without the use of specialist equipment although facility in the use of keys (see below) makes for more accurate results. The divisions below genus and species are the everyday currency of horticulturalists because in these more finely divided categories lie the variety which is the spice of life for gardeners and shoppers alike… Whilst on the subject of variety… this is a term you will have already have come across in gardening. For botanists, horticulturalists and the RHS examiners, it has a specific and limited meaning, so pay attention below! 7 Taxon – a group of organisms of any taxonomic rank e.g. family, genus or species. ©HCC RHS LVL2 POH L1 2011 A5 32 Species and the ranks below If families consist of one or several similar or closely related genera and a genus is a group of similar or closely related species what is a species? Not an easy question, even though this is the basic unit of biological classification. To quote the RHS Plant Finder: The basic unit of plant classification - the species, is a group of interbreeding individuals producing more or less similar offspring and differing from other similar groups by a number of factors. Members of a species are isolated reproductively from other populations - this includes groups that could physically interbreed but are prevented from doing so by geographic separation. The keen gardener will immediately say ‘but species can interbreed to form hybrids’ and correctly so, but the results are usually infertile, or if they are fertile they are not practically viable in the wild. Fertilisation across species is very unusual. While exceptions prove the rule, some of our favourite plants are inter specific crosses. An example is Platanus x acerifolius - a hybrid of P. orientalis from Turkey and P. occidentalis from North America. This tree formed the arboricultural backbone of smoky London: it shrugged off pollution by shedding its bark. A further complication is that some interspecific crosses exhibit hybrid vigour - a characteristic which is much exploited in the world of bedding plants and vegetables. Although even less likely, hybrids can occur not only between species but also between genera. One such is Leyland cypress - it is a cross between the Nootka cypress, now in a new genus – Xanthocyparis and Cupressus macrocarpa and the hybrid is known as x Cuprocyparis leylandii. Species (which is singular and plural, there’s no ‘specie’) can occur naturally in different races with subtle or not-so subtle differences which are not so great as to form a new species. They can be spread over wide geographical areas and evolutionary forces may act on them in different ways. Where variation within a species calls for distinguishing between variants the term subspecies is used (abbreviated to ‘subsp.’ not ssp. as it is confusable with ‘spp’ which is the abbreviation for species plural.) An example of a subspecies is Anthemis punctata subsp. cupaniana - a lowgrowing but spreading daisy with silver filigree leaves. This naturally occurring ©HCC RHS LVL2 POH L1 2011 A5 33 variant therefore sports a trinomial, not a binomial, with the subspecies name also in lower case italics. This notation is the same for even more finely differentiated groups which exhibit individual or group differences and are termed varietas and forma (var. and f.). The precise designation depends on the degree of difference and also on the predilections of the naming botanist. The autumn flowering Cyclamen hederifolium has a white variant var. hederifolium f. Albiflorum. Hybrids We’ve already come across hybrids in unauthorised interbreeding between species which very occasionally also occurs between different (although usually closely related) genera. As in the other designations we’ve looked at, hybrids are written in a precise way. Hybrids between species in a single genus are joined with a cross (x). Mahonia lomariifolia (an elegant but tenderish winter flowering shrub) crossed with M. Japonica gives an ‘interspecific’ cross, a hybrid called M. X media. (M. X media Brickell to include the authority). The plant arose from a batch of M. lomariifolia seedlings and one of them was named ‘Charity’ at the Savill Gardens. The same cross has resulted in the clones ‘Winter Sun’ and ‘Buckland’. (To confound earlier comments Figure 10 Mahonia ‘Charity’: a plant with about the reproductive isolation sensitive stamens of species, M. X media is fertile. The (edible) fruit of these crosses produces viable seed, and several seedlings from ‘Charity’ have been given clonal names such as ‘Faith’ and ‘Hope’). Cultivars The last paragraph introduced the concept of hybrid names and clones. The hybrid name is given because the individual plant has precise characteristics which mark it out. A clone is a group of individuals which all derive from vegetative replication i.e. not from seed. Because sex (the mixing of genes) ©HCC RHS LVL2 POH L1 2011 A5 34 is not involved, there is no variation in the progeny – they are genetically identical. The hybrids that gardeners come across almost all occur in cultivation, either as a chance seedling as in the first Mahonia ‘Charity’ cross or by deliberate crossing as in M. X media ‘Buckland’ or by a lucky mutation in a group of vegetative propagated plants (a common example being golden sports of ornamental conifers). The cultivar (cultivated variety) retains its character from generation to generation (by propagating vegetatively) and because of this, it is a clone. Cuttings taken from M. X media ‘Charity’ produce another M. cv. Charity. Gardeners particularly value the variety brought by cultivars in flower and fruit, colour, variegation and habit, as well as eating qualities in fruit and vegetables. A look through the RHS Plant Finder will show that some groups have many hundreds of valid cultivars each with subtle or not so subtle differences. To distinguish them from the botanical variations, cultivar names are not written in italics and are put in single quotes or preceded by cv. Cultivar names are subject to the International Code of Nomenclature for Cultivated Plants (see Professor Stearn above); they should not be latinised and they should not be repeated within a genus. As many cultivars were named before the Code was introduced in 1959, some do not follow the rules. ? Correct the following erroneous plant names according to the rules of nomenclature. betula pendula Ranunculus bulbosa Urtica Dioica Laburnocytisus adamii Cedrus libani atlantica Cuprocyparis X leylandii Naylors Blue Hydrangea arborescens Grandiflora Geum intermedium Salix x rubra ‘abbeys harrison’ erica Carnea ‘Springwood White’ Check your answer at the end of this section. ©HCC RHS LVL2 POH L1 2011 A5 35 Trade designation and selling name (Note: this topic is not in the exam syllabus). Plant breeders often use code names when developing a large group of new cultivars. An example is Rosa ‘Ausbord’ bred by David Austin. This particular rose is subject to Plant Breeders’ Rights and it is a legal requirement to put the correct cultivar name on the label. However, the ICNCP allows the use of an additional name called a trade designation or selling name, which will be more effective for selling purposes. Sometimes different names are used in different countries as what sounds attractive in one language may not in another. The trade designation is written in a different typeface to the cultivar and is used without quotes. As the whole point of botanical nomenclature is to avoid confusion the ICNCP recommendation is to always include the cultivar name and trade designation together. Thus Rosa Gertrude Jekyll = ‘Ausbord’. Question: A cross between two genera is a ________ hybrid? What symbol denotes the hybrid? ? Answer to Self Check Question from page 127 Correct the following erroneous plant names according to the rules of nomenclature. Corrections are in bold and underlined. 1. Betula pendula 2. Ranunculus bulbosus 3. Urtica dioica 4. + Laburnocytisus adamii 5. Cedrus libani subsp. atlantica 6. X Cuprocyparis leylandii ‘Naylors Blue’ 7. Hydrangea arborescens cv. Grandiflora 8. Geum X intermedium 9. Salix X purpurea ‘Abbeys Harrison’’ 10. Erica carnea ‘Springwood White’ ©HCC RHS LVL2 POH L1 2011 A5 36 2 PLANT DEVELOPMENT AND LIFE CYCLES By the end of this section, you will be able to demonstrate a knowledge of plant development and life cycles. Describe the five stages of the life cycle of plants: seed, juvenile, adult, senescent, death. Define the terms annual, ephemeral, biennial and perennial. Define the terms tender, half-hardy and hardy in relation to annuals. Define the terms tender, half-hardy, hardy, herbaceous and woody in relation to perennials, including the process of lignification. State the meaning of the terms evergreen, semievergreen and deciduous. The Stages of Life All living things go through a life cycle which ends, inevitably, with the death of the organism. At different stages in the life cycle, plants exhibit special characteristics which should inform gardeners when choosing maintaining or propagating plants. The stages of a plant’s life cycle are: Seed Concerning potential, reproduction, storage, dormancy Juvenility Rapid expansion, vigour, growth Adulthood Maturity, reproductive period Senescence Retrenchment, reduction in function Death No longer alive, followed by breakdown, recycling Seed Starts the cycle from a practical point of view. The development of seed as a means of regeneration is one of the key reasons for the success of gymnosperms and angiosperms. The spores of the mosses and ferns are groups of relatively simple cells in a protective coat that are produced in very high numbers and are light enough to be easily carried by air or water. They are dependent on finding a favourable situation almost immediately on dispersal or they will die. ©HCC RHS LVL2 POH L1 2011 A5 37 Seed, on the other hand, forms a much more sophisticated package. In flowering plants, when the pollen fertilises the ovules, the embryo forms while still attached to the parent plant, gaining nutrients from it. When the seed is released it is accompanied by a high energy food store to nourish the plant in its early stages. The seed is also adapted for survival: if dry, it barely metabolises; it’s alive, but barely ticking over and will resist desiccation, cold and generally unfavourable conditions in a way that is impossible for a spore. The length of time the seed remains ‘viable’ depends Metabolism on species and environmental conditions. Extreme conditions can The breakdown of complex be too adverse for survival, but the substances with the release of fact remains that for its size, the seed energy and waste products. is a remarkably resilient package. Given the right environmental conditions, and assuming there are no internal dormancy constraints, seeds germinate into seedlings. This part of the life cycle exhibits vigorous growth. However, because seedlings are made of newly divided plant cells full of sap and with a relatively soft external covering (the epidermis) they are very susceptible to grazing by organisms such as slugs, damage from environmental factors such as cold and wet, and by the numerous fungal organisms that exist in soil and water. The stages that follow are of varying lengths and broadly reflect the longevity of the plant – the juvenile phase of a woody plant may last several years, whereas an annual has to pack all of the stages into a single year. Figure 11 Adult [left] and juvenile [right] foliage of Ivy, Hedera helix Juvenility Assuming a seedling has survived the hazards of germination, it grows into a young plant which exhibits juvenile characteristics. These include a tendency to strong vegetative growth at the expense of flowering and fruiting. ©HCC RHS LVL2 POH L1 2011 A5 38 Examples of differences between juveniles and adult forms Figure 12 Adult Eucalyptus gunnii Leaves (Wouter Hagens Creative Commons) Leaf shape – for example, Hedera helix. Some Eucalyptus have bluish, circular foliage when in the juvenile phase but dark green, elliptic leaves in the mature phase. Differences in growth habit – Ivy (Hedera helix) has a creeping growth habit when in the juvenile phase, but an upward growth habit in the mature phase. The angle of branches to the main axis of spruce (Picea spp.) changes to drooping in the mature phase. Differences in leaf abscission – in autumn and winter the leaves persist withered until spring on juvenile growth, which often occurs near the base of trees, but are shed on the mature growth of, for example, beech. Presence or absence of thorns – young citrus trees tend to have thorns to deter grazing. New growth at the top of the tree (the mature portion) has no need of this mechanism and does not develop thorns. Young Gleditsia plants are similarly protected. Differences in rooting ability for example, Hedera helix (Ivy) produce adventitious roots on juvenile growth but not on mature growth (this difference is exploited during the propagation – more about this during the section on horticultural significance. The presence of flowers and fruit in the mature stage only (this is the most important difference). ©HCC RHS LVL2 POH L1 2011 A5 39 The Horticultural Significance of Juvenility One of the differences associated with the juvenile phase is exploited during vegetative plant propagation. Cuttings of some species of plants are very difficult to root when taken from mature growth (and may not root at all), while cuttings taken from juvenile growth root relatively easily (even cuttings taken from the same plant as the mature growth cutting). The mechanism for influencing rooting in cuttings is still not well understood. However, in many species, the ability of cuttings to produce adventitious roots declines as plant material ages. This is demonstrated by Juniperus species where many of the juvenile foliage forms root more readily than the adult foliage types. The juvenile phase can be retained in many plants by stimulating the growth of adventitious shoots and gibberellin sprays can stimulate juvenility in some plants. Growths from plant parts containing meristematic tissue can yield juvenile growth with good potential for rooting when cuttings have been taken. Growth in the juvenile condition is found in tissues… - originating from young seedlings arising from adventitious buds on stems grafted onto young wood A second horticultural significance of juvenility involves leaf retention. In some plants, withered leaves are retained by juvenile growth. In some situations, for example, hedging, this characteristic of juvenile growth is exploited. A hedge even with withered leaves still provides a visual screen throughout the year. As a consequence, pruning the hedge to maintain the shape has the added benefit of maintaining juvenility and the leaf-retention characteristic. ? Beech has this characteristic; can you think of any other woody plants which retain their leaves in this way, used for hedging? Check your answer on page 62. Adult/Mature We have seen that the adult or mature plant is marked by the ability to produce offspring. Seasonal, environmental or hormonal changes bring about the cessation of leafy growth (the juvenile stage) and the onset of flowering and ©HCC RHS LVL2 POH L1 2011 A5 40