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Plant Taxonomy
Alien Species
The flora of almost every region now contains alien or
introduced plants - it is important to recognize them for 4
reasons
1. if their origin remains undiscovered, they will lead to
false results in floristic studies
2. if their presence is undetected they will be mis-identified
and can lead to wrongly documented experimental and
observational results
3. alien species may hybridize with native species and can
create successful new species - cordgrass in England
4. alien species can have serious (though not always
harmful) ecological consequences in either natural or
cultivated vegetation
Japanese Maple and Norway Maple
More on Aliens
• The British Isles have about 2500 native species;
over 5000 alien species have been recorded
• aliens may be casual - must be maintained or they
will die out
• aliens may become naturalized - able to maintain
themselves on their own after introduction
• British Isles have about 800 naturalized aliens
Japanese Cedar in Kew Garden,
London, England
Plant Breeding Systems
• The breeding system of a plant may be defined
broadly as the mode, pattern and extent to which it
interbreeds with other plants of the same or of
different taxa
• Inbreeders are plants which predominantly or
wholly produce seed from self-fertilization
• Outbreeders are plants which produce seed from
cross-fertilization
• In nature every situation from one extreme to the
other exists
Hog peanut – Amphicarpaea bracteata
Plant Breeding Systems
Breeding system is taxonomically important for
three reasons:
1. the extent of interbreeding largely defines the
pattern of variation and hence the delimitation of
taxa
2. a knowledge of the breeding system frequently
helps to understand taxonomic complexity,
although often it does not solve problems
associated with it
3. a study of the breeding system is often vital in
unraveling evolutionary pathways
Ideal vs. Hybridizing Species
• An ideal species is a taxonomically distinct species - it
poses no problems and always is recognizable as a distinct
entity - it does not merge with other species
• This is true of many species of peas and parsleys
• Species which pose taxonomic problems either hybridize
with other taxa so that genetic limits are wider than
morphological limits in the species
• Or they have breeding barriers between members of what
appears to be a morphologically recognizable taxon - these
are called semi-cryptic species
Astragalus canadensis – milkvetch Fabaceae
Conium maculatum - poison
hemlock - Apiaceae
Plant Hybridization
• British flora probably contains 780 interspecific
hybrids among vascular flora of 2500 species –
about 31% of all native British species
• Because 2500 is 1% of world's total flora of
250,000 flowering plants; it is possible that 78,000
species of interspecific hybrids exist in the world or 31% of all flowering plants may be the result of
interspecific hybrids
Orchidaceae – Cymbidium astronaut
Consequences of Hybridization
• The existence of hybrids between two
species can cause practical taxonomic
problems because such plants are not
readily identifiable with either species they may either have intermediate
characteristics or some characteristics
typical of one parent and other
characteristics typical of the other parent
Quercus agrifolia – coast live oak Fagaceae
Betula occidentalis - Water birch Betulaceae
Ulmus americana - American Elm Ulmaceae
Crataegus douglasii – black
hawthorn - Rosaceae
Hybrid Swarms
• Fertile hybrids can lead to hybrid swarms - where
by backcrossing with the parents and hybrid
interbreeding, the parental species become
connected phenetically with every possible
intermediate type, so that the species can grade
imperceptibly into each other
• The existence of hybrid swarms indicates that
there is a spectrum of ecological niches available
to satisfy the requirements of a wide range of
hybrid offspring or it may indicate that the hybrids
do not differ in an ecologically significant trait
White Oak – Quercus alba
Oak hybrid swarm
• ‘Any man who claims
he can reliably identify
oaks in southern
Wisconsin is either a
liar or a fool.’
- John Curtis
Author of the Flora of
Wisconsin
Stabilization of a hybrid - Senecio
cambrensis – groundsel – Asteraceae
S. vulgaris
Polyploidy
Spartina alterniflora marsh – North
Carolina - Poaceae
Spartina alterniflora – Poaceae
Spartina maritima – Poaceae
Spartina x townsendii - Poaceae
Spartina anglica – Poaceae
Spartina anglica – invasive in New
Zealand - Poaceae
Semi-cryptic Species
• The other major taxonomic problem comes from
semi-cryptic species - so-called because their
differences are marked in anatomical, chemical,
cytological or (frequently) genetic characters
rather than morphological characters - often they
are long established species which simply do not
differ greatly in gross morphology
• Because of their semi-cryptic nature, there is often
considerable taxonomic argument about the
correct taxonomic status of these plants - should
they be considered species or subspecies or
something else
Eleocharis – spike rush – Cyperaceae
Senecio vulgaris – groundsel Asteraceae
Taraxacum officinale – dandelion
- Asteraceae
Apomixis
Apomixis is reproduction without sexual
fertilization. It can happen via:
• Producing seeds by non-sexual means
(usually forming embryos directly without
pollen)
• Vegetative growth – plant spreads clonally
Plant Classification
Process of Classification
Classification has two desirable goals:
1. The arrangement of groups into a pattern
that accurately reflects their evolutionary
relationships
2. The placement of groups into a reference
system so their major features are easily and
efficiently described and identified
(information storage and retrieval)
Identification and Classification
• Plant identification usually requires the use of
only a few of a plant's more obvious
morphological characteristics, and only one or two
characteristics are usually considered at any one
time as the plant is worked through the key
• In plant classification (developing a logical system
of organizing plants), it is necessary to consider
and to evaluate all pertinent information more or
less simultaneously
Conservative Characters
• Flowers provide the bulk of the taxonomic
information in classifying flowering plants this is because they tend to be conservative
characters - characters which tend to
remain relatively unchanged over a long
period of evolutionary development and
hence vary little between closely related
taxa
Conservative Characters
• Conservative characters are most useful in
delimiting the higher taxa - genera and above - in
developing higher taxa, the emphasis is usually on
recognition of similarity between members of the
taxon
• At lower levels of classification (species and
below) the emphasis is more often on distinction
between taxa than on clustering taxa - thus at
lower levels, non-conservative characters - those
which show greatest diversification, are most
valuable
Flower Structure for Brassicaceae
Flower Structure in Rosaceae
Good vs. Bad Characters
• Taxonomists sometimes refer to characters
as being "good characters" or "bad
characters" - these are relative terms
• In general a good character is one which is
constant in a taxa and allows positive
identification of a taxa - good characters are
therefore usually conservative
• A bad character is one that is highly
variable and does not allow positive
identification
Flower Structure in Liliacea (left)
and Iridiaceae (right)
Ovary
Traditional Classification
• Most traditional classification systems derived
from Linneaus and depend not on evolutionary
relationships, but rather on similarity in form or
organization – taxonomic groups are based on
organisms having a particular “grade”
• For example the grass family Poaceae is made up
of a grade of organisms having jointed stems,
leaves with sheathing bases, and greatly reduced
flower parts
Poaceae
Downy Hawthorn – Crataegus mollis
White Hawthorn – Crataegus monogyna –
flowers, and fruits of 4 species of Hawthorn