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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
Trichome on Silverleaf
Nightshade - Solonaceae
Snapdragons - Scrophulariaceae
Snapdragon stomata 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 (subfamily of
Convolvulaceae)
Broomrape – Orobanche fasciculata in
Orbanchaceae – parasitizes Asteraceae
Indian pipe - Monotropa uniflora Ericaceae - 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
Rhubarb leaves contain calcium
oxalate - Polygonaceae
Dieffenbachia – dumb cane – also
contains calcium oxalate - Araceae
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 - Asteraceae
Bracken fern – Pteridium aquilinum
- Dennstaedtiaceae
Annual bluegrass – Poa annua
Poaceae
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 – Liliaceae
from 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
A note of caution - it is possible that disjunct
distributions are due to human intervention
- native distribution of many weed and crop
plants is unknown because they were so
widely spread for many centuries
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
• Genus Cedrus is disjunct but species within
the genus are vicariant
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
Endemics and degree of isolation
• Degree of isolation for endemics can be measured
in terms of distance from other similar areas or
length of time that the area has been isolated often both factors apply
• British Isles only 35 km from France and
separated for 7500 years are 1% endemic
• Canary Islands always isolated from Europe, but
only 600 km from mainland have 47% endemics
• Hawaii, most isolated island chain in world, has
80 to 90% endemics
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
- genetic difference length of flower stalk
Willow herb – Epilobium – Ongraceae
phenotypic difference in leaves and height