Plant Taxonomy

Basics of Characters

• A taxonomic character is any expressed attribute of an organism that can be evaluated and that has two or more discontinuous states or conditions - for example the number of petals on a flower - can be in 3's, 4's or 5's - thus distinct states and they are discontinuous

• The taxonomic value of a characteristic is increased if the biological significance of the characteristic has been determined

Anatomy (Internal)

• Anatomical characteristics are of most importance in studies for classification – especially when trying to determine relationships among large groups of plants, often at higher taxonomic levels - they are less often used for identification and aren't often mentioned in floras

Trichomes on stinging nettles - Urticaceae

Trichome Structure and Growth

Snapdragons - Scrophulariaceae

Acanthus - Acanthaceae

Pollen Grains – Many Species

Physiology

• Physiology is less important in classification, but can be of use for classifying parasitic and saprophytic plants which lack chlorophyll and thus don't do photosynthesis - this separates the dodders (genus Cuscuta) from morning glories (Convolvulaceae)

Dodder – Cuscutaceae (once in Convolvulaceae)

Broomrape – Orobanche fasciculata – parasitizes Asteraceae

Indian pipe - Monotropa uniflora- parasitizes mycorrhizal fungi

Chemistry

• Chemistry can be important because some plants produce distinctly different chemical compounds - aromatic mints (Lamiaceae) and the parsleys (Apiaceae) were separated into unique groups by Dioscorides in the 1st century A.D. due to aroma "chemistry"

Lamiaceae

Apiaceae

Starch grains from Sorghum (left) and Fescue (right) - Poaceae

Calcium oxalate crystals

Calcium oxalate in plant cells

Ecology

• Ecology and geography of plants provide data of much relevance to plant taxonomy because each taxon exhibits a certain pattern of distribution which is one aspect of its definition

• Coincidence or not of areas occupied by related taxa has bearing on classification of a group - especially when considering its evolutionary history

Patterns of Geographic Distribution

• Geographic differentiation exists between taxa at all levels and in all degrees of spatial separation - effective spatial separation varies greatly in absolute terms because different taxa may possess quite different abilities to migrate, either as pollen or as seeds

• Phytogeography is the study of patterns of distribution by plants

Patterns of Geographic Distribution

• Allopatric taxa occupy mutually exclusive geographic areas - i.e. they do not overlap in their distributions

• Sympatric taxa occupy overlapping geographic areas

• Sympatric taxa usually show different types of genetic, ecological and structural differentiation from those shown by closely related allopatric taxa - this is because geographic separation is a reproductive isolating mechanism and is effective in maintaining species differences

Silphium hybrids – compass plant and prairie dock

Bracken fern – Pteridium aquilinum - Dennstaedtiaceae

Annual bluegrass – Poa annuaPoaceae

Haleakala Silversword- Asteraceae

Giant Saguaro and Organ Pipe Cacti - Cactaceae

Blue Gum Eucalyptus - Myrtaceae

Floristic Elements• If the distribution of a large number of taxa is analyzed,

certain geographic patterns are found to recur consistently - such recurring patterns and the taxa which exhibit them are called "floristic elements"

• In Illinois the tall grass prairie could probably be divided into 3 floristic elements - wet prairies dominated by sedges, bluejoint reedgrass, New England aster, marsh muhly grass, cowbane, prairie cordgrass

• mesic prairies dominated by big bluestem, prairie dropseed, sky-blue aster, showy sunflower, false sunflower, Indian grass

• dry prairies dominated by little bluestem, big bluestem, needlegrass, heath aster, prairie coreopsis, prairie goldenrod

Mesic Prairie

Geographic vs. Ecological Distribution Patterns

• Geographic distribution – based on region/area of occurrence

• Ecological distribution – based on habitat of occurrence

• Sometimes may be hard to distinguish

Disjunct Distributions

• Most taxa are fairly continuous throughout their region of distribution, but some have distribution patterns which are interrupted by considerable areas from which the taxa is absent - such patterns are said to be disjunct distributions

• Disjunct distributions can arise in several ways – • 1) long range dispersal of a taxon from one area to

another • 2) current distributions represent relics of former,

wide continuous distribution patterns where the intervening areas have lost the members of the taxa

Coconut Palm - Palmaceae

Glacier Lily – Liliaceaefrom Basin and Range

Basin and Range Topography

Disjunct Distributions

• 3) it is possible that the taxon arose independently in the disjunct areas by convergent or parallel evolution - usually this is not thought to be likely, especially if potential ancestor species are not present

Spartina maritima - Poaceae

Spartina maritima

• Spartina maritima (Small Cordgrass) is native to the coasts of western and southern Europe and western Africa, from the Netherlands west across southern England to southern Ireland, and south along the Atlantic coast to Morocco and also on the Mediterranean Sea coasts. There is also a disjunct population on the Atlantic coasts of Namibia and South Africa - Wikipedia

Vicariance

• Another geographic distribution pattern is vicariance - when two similar taxa occupy separate geographic (or ecological) areas –

• the distinction between disjunct and vicariant taxa is subtle

Cedar of Lebanon - Pinaceae

Vicariant Distribution of Cedrus

Distribution: Cedrus atlantica in red, C. brevifolia in blue and C. libani in purple

Endemism

• Taxa which occur in single restricted geographical areas are known as endemics

• Endemism is a relative concept, but is normally applied only where there is considerable restriction in area of distribution

• Often the term is used in the same way as indigenous which means native to an area, but to a taxonomist, it should just mean geographically restricted

Senecio cambrensis – groundsel - Asteraceae

S. vulgaris

Gingko biloba - Gingkoaceae

Giant Sequoia- Taxodiaceae

Centers of Diversity

• If the distribution for every species in a genus is drawn on a map, it is usually found that there are one or more areas with a concentration of species - these areas are called centers of genetic diversity for that genus

• You can usually draw similar maps for higher levels of taxa as well

• Usually there is just a single center of diversity for a particular taxa and you find progressively fewer species as you move away from the center of diversity

Vavilov’s centers for agricultural diversity

Ecotypes

• Many species of plants form ecotypes - genetically distinct races when grown in different habitats - especially common are dwarf races in alpine or low rainfall areas

• It is important to note that similar ecotypes may arise due to different factors in different areas - thus a species may form dwarf ecotypes due to very different constraints - possibly water availability, or soil type, or temperature

Cupressus pygmea – pygmy cypress

Pygmy Forest

Plantago maritima - Plantaginaceae

Willow herb – Epilobium - Ongraceae