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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.
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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.
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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
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Seedless, vascular plants e.g. ferns and
horsetails
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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’.
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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.
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(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’.
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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.
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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]
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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.
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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).
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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.
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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
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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’.
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