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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.
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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.
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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.
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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
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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)
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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.
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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’
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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.
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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.
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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).
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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
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