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  Section: Principles of Horticulture » Classification and naming
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Further classifications of plants

Classification and naming
  The classification of plants
  Kingdom Plantae
  Further classifications of plants
  Non-plant kingdoms

Plants can be grouped into other useful categories. A classification based on their lifecycle (ephemerals, annuals, biennials and perennials) has long been used by growers, who also distinguish between the different woody plants such as trees and shrubs. Growers distinguish between those plants that are able to withstand a frost (hardy) and those that cannot (tender); plants can be grouped according to their degree of hardiness. Table 4.4 brings together these useful terms, provides some definitions and gives some plant examples.

Identifying plants
Floral diagram of wallflower
Figure 4.6 Floral diagram of wallflower
A flora is a text written for the identification of flowering plant species. Some flora use only pictures to classify plants. More detailed texts use a more systematic approach where reference is made to a key of features that, by elimination, will lead to the name of a plant. Species are described in terms of their flowers, inflorescences, stems, leaves and fruit. This description will often include details of shape, size and colour of these plant parts. Flowers: The number and arrangement of flower parts (see Figure 4.6) is the most important feature for classification and is a primary feature in plant identification. It can be described in shorthand using a floral formula or a floral diagram. For example, the flora formula, with the interpretation, for Wallflower (Cheiranthus cheiri), a member of the Cruciferae family is as follows:

4 sepals
in calyx
4 petals in
A2 + 4
6 anthers
2 ovaries joined

Other examples of floral formulae include:

Sweet pea
./. K(5) C5A(9) + 1 G1
Dead nettle
./. K(5) C(5)A4 G(2)
K C(5)A(5) G(2)

Wallflower flower (a) from above, (b) the side and (c) LS, illustrating the floral diagram above
Figure 4.7 Wallflower flower (a) from above, (b) the side and (c) LS, illustrating the floral diagram above

Figure 4.8 Leaf forms : (a) linear e.g. Agapanths; (b)
lanceolate e.g. Viburnum; (c) oval e.g. Garrya elliptica;
(d) peltate e.g. nasturtium; (e) hastate e.g. Zantedeschi ;
(f) lobed e.g. Geranium; (g) palmate e.g. lupin; (h) pinnate
e.g. rose
The way that flowers are arranged on the plant is also distinctive in different families, e.g. raceme, common in the Fabaceae; corymb and capitulum found in the Asteraceae and umbel, very much associated with Apiaceae.

Leaf form (see Figure 4.8) is a useful indicator when attempting to identify a plant and descriptions often include specific terms, a few are described below but many more are used in flora.
  • Simple leaves have a continuous leaf blade, for example: linear, lanceolate, ovate, obovate, orbicular, oval.
  • Margins of leaves can be described: entire, sinuous, serrate, and crenate.
  • Leaf vein arrangement also characterizes the plant: reticulate, parallel, pinnate and palmate.
  • Compound leaves, such as compound palmate and compound pinnate, have separate leaflets each with an individual base on one leaf stalk, but only the axillary bud is at the base of the main leaf stalk.
Most horticulturists yearn for stability in the naming of plants. Changes in names confuse many people who do not have access to up-to-date literature. On the other hand, the reasons for change are justifiable. New scientific findings may have shown that a genus or species belongs in a different section of a plant family, and that a new name is the correct way of acknowledging this fact. Alternatively, a plant introduced from abroad, maybe many years ago, may have mistakenly been given the incorrect name, along with all the cultivars derived from it.

Evidence from biochemistry, microscopy and DNA analysis is proving increasingly important in adding to the more conventional plant structural evidence for plant naming. There may be differing views whether a genus or species should be 'split' into smaller units, or several species be 'lumped' into an existing species or genus, or left unchanged. It seems likely that changes in plant names will continue to be a fact of horticultural life.

There has been a massive increase in communication across the world, especially as a result of the Internet. The level of information about plant names has improved. The International Code of Botanical Nomenclature (ICBN) has laid down an international system. Within Britain, the Royal Horticultural Society (RHS) has an advisory panel to help resolve problems in this area. An invaluable reference document 'Index Kewensis' is maintained by Kew Gardens listing the first publication of the name for each plant species not having specific horticultural importance. Cultivated species are listed in the 'RHS Plant Finder', which also indicates where they can be sourced, is updated annually and can be viewed on the Internet. Further cooperation across Europe has led to the compilation of The International Plant Names Index with associated working parties formed from scientific institutions and the horticultural industry.

Geographical origins of plants
Gardens and horticultural units, from the tropics to more temperate climates, contain an astonishing variety of plant species from the different continents. Below is a brief selection of well-known plants, grown in Britain, illustrating this diversity of origin. It is salutary, when considering these far-flung places, to reflect on the sophisticated cultures, with skills in plant breeding and a passion for horticulture over the centuries that have taken wild plants and transformed them into the wonders that we now see in our gardens.
Oak Tree
Figure 4.9 Oak Tree
(Quercus robur) – a native of
the British Isles
  • British Isles; English Oak (Quercus robur), Geranium robertianum, foxglove, peppermint, Pinus sylvestris.
  • Far East (China and Japan); cherry, cucumber, peach, walnut, Clematis, Forsythia, hollyhock, Azalea, rose.
  • India and South-East Asia; mustard, radish.
  • Australasia; Acacia, Helichrysum, Hebe.
  • Africa; Phaseolus, pea, African violet, Strelitzia, Freesia, Gladiolus, Impatiens, Pelargonium, Plumbago.
  • Mediterranean; asparagus, celery, lettuce, onion, parsnip, rhubarb, carnation, hyacinth, Antirrhinum, sweet pea, Rosemarinus officinalis.
  • Middle East and Central Asia; apple, carrot, garlic, grape, leek, pear, spinach.
  • Northern Europe; cabbage, Campanula, Crocus, forget-me-not, foxglove, pansy, Primula, rose, wallflower, parsley.
  • North America; Aquilegia, Ceonothus, lupin, Aster, Penstemon, Phlox, sunflower.
  • Central and South America; capsicum, maize, potato, tomato, Fuchsia, nasturtium, Petunia, Verbena.


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