Plant Taxonomy

Alien SpeciesThe 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