Plant Classification
Historical Perspective:
 Preliterate & primitive
societies
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grouped plants in terms of
uses, properties,
appearance: “folk
taxonomies”.
Often useful classifications,
but only for local area.
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Plant knowledge important
for (agriculture & medicine)
Greek & Roman Civilization
Theophrastus “father of Botany”: Follower of
Aristotle
Recognized 4 plant groups: trees, shrubs,
undershrubs, herbs
Described 500 species. Best work until after the
Middle ages
Middle Ages
Mostly copied from Theophrastus & other classical authors.
Little original work
Renaissance (15th & 16th
centuries)
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New technologies:
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Type and printing of
books. etc.
herbarium (Luca Ghini,
1490-1556, Italy)
Return to field observations
Herbals: illustrated
descriptions of medicinal
plants.
The 17’th Century: Age of
Exploration.
Botany expands beyond Europe
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Expeditions to the New World,
Java, Africa, India etc.
Many new species collected &
described:
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500 known in 1542
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18,000 in 1682.
Need for a comprehensive plant
classification system felt.
Carl Linnaeus (1707-1778)
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Swedish physician/ botanist:
 Devised classification system based
on simple flower characters: Classes:
stamen number, Orders: number of
styles, etc.
 Published “Species plantarum”
First consistent use of binomial
names for species (Main contribution
to modern taxonomy).
Named a large number of species including
many from America.
Later began effort to devise a more
“natural” classification.
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Carl Linnaeus
Natural Classification Systems
18th & 19th centuries:
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By end of 18th Century, botanists dissatisfied with
artificial systems (classified unlike plants together)
Quest for classification that recognized 'natural
affinities' between plants.
Darwin: “Origin of Species” 1859
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Theory of evolution by natural selection:
Implies that plant populations are variable, change over
time, & form lineages of variously related groups.
Justified concept of natural groupings.
Idea that classification should reflect evolution emerged.
Charles Darwin
Adolf Engler (1844-1936)
“Die Natürlichen
Pflanzenfamilien”.
First classification influenced
by evolutionary theory.
Some herbaria & flora manuals
still arranged by this
obsolete system.
Adolf Engler
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Twentieth Century Traditional
Classification Systems:
Takhtajan (former USSR, 1980).
Cronquist (USA, New York Botanical Garden
1981).
Dominant US classification of late 20th C.
Flora N.C.T. uses Cronquist’s classification--See
FNCT p. 1353-1356; Flora E.Tx p. 1158-1163.
Armen Takhtajan
Flora E.Tx. retains monocot-dicot grouping but
circumscribes families largely like a 21st C.
system called APG.
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Flowering plants classified into
2 main groups:
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Monocots-5 subclasses
Dicots-6 subclasses
Arthur Cronquist
Recent Approaches to
Classification:
Angiosperm Phylogeny Group (APG), p. 1163.
Bremer, Chase, & Stevens (1998, 2003, 2009)
Tries to reclassify flowering plants into monophyletic
(single-lineage) groups at the family & order-level)
based on recent research.
Did not attempt to formally name ranks above order.
Basal
families
Magnoliids
Monocotyledons
Eudicotyledons
The APG Classification
(p. 1163-1165)
Flowering plant groups:
3 early branches
Magnoliids
Monocots
Eudicots
Eudicots
Early
Magnoliids Monocots
branches
APG & the angiosperm
phylogenetic tree
Modern Trends in Plant
Classification:
See p. 1169-1177 (Apx. 5); 1180-1192 (Apx. 6).
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Increased emphasis on phylogeny.
New sources of taxonomic information (DNA).
Emphasis on quantitative methods.
Debate between Cladistic & traditional
(Phenetic) schools (Apx. 6).
Increased emphasis on phylgenetic trees:
they express evolutionary relationships
better than classification.
Current dominance of the APG system for
flowering plant classification.
Methods to Develop Plant
Classifications (p. 1169-1172)
Cladistics: Find shared derived characters (synapomorphies).
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Only these indicate evolutionary relationships & have
taxonomic value.
Rationale: groups of organisms that share derived (vs.
ancestral) characters will be closely related.
Goal: find branching pattern of evolution (p. 1169-1172)
Traditional Approaches ('Phenetics', p. 1171): Quantify total
difference between plant groups based on as many features as
possible.
 Rational: not only phylogenetic relationships but also
amount of evolutionary change important for
classification (p. 1169-1172).
More on Cladistics
Main goal: Reconstruct branching pattern of evolution.
Product: Branching diagram (phylogenetic tree) called a cladogram.
Some advocate eliminating hierarchical classification altogether! (p.
1173, also the debate in Apx. 6).
Bryophytes
Lycophytes
Ferns
Gymnosperms
Green algae
A Cladogram
Flowering plants
Cladistics Terms
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Phylogenetic Systematics: = Cladistics
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Synapomorphy: Shared derived character (p. 1171).
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Evolved in ancestor of a group & thus present
in the group but not outside it.
Pleisiomorphy: Ancestral character (p. 1171).
Inherited from a distant ancestor thus found both in
the group and in other related groups.
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Clade: monophyletic group (p. 1170).
Synapomorphies & Pleisiomorphies
of Flowering Plants
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Synapomorphies: Unique to flowering plants
 Double fertilization.
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Seeds enclosed in fruits.
Unique arrangement of sex parts (flower).
Pleisiomorphies: Found in both flowering plants &
related groups.
 Seeds.
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Lignin in cell walls.
A certain chloroplast
DNA mutation.
Synapomorphies &
Pleisiomorphies of Plants
Lycophytes
Ferns Gymnosperms
Seeds
Lignin
Chloroplast DNA
inversion
Flowering
plants
Double fertilization,
Fruits; flowers
Critique of Cladistics
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(p. 1174-1175)
Choice of what characters are important/derived, ancestral
can be subjective.
Difficulties with convergent evolution: Parallelisms
(=Homeoplasies) can be confused with shared derived
characters.
“Evolutionary distance” not considered.
Classification in form of a branching tree impractical (but
good trees basis for changes to traditional classification).
Loss of stability in classification.
Cladistics quite successful in generating believable
phylogenetic trees:
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Significant changes in understanding of plant evolution.
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Resulted in substantial revisions to classification.
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Classification Groups &
Phylogeny (p.1170):
Monophyletic group: One complete evolutionary lineageancestor & all descendants; = 'clade'.
Polyphyletic group: Several lineages- members don’t share a
single recent ancestor.
 Example: 'Liliaceae'; “dramatically polyphyletic”, p. 726.
All taxonomists recognize these as inappropriate.
Paraphyletic group: One ancestor but not all descendants.
Examples: Reptiles, dogbanes, dicots) pp. 1171-1173).
Cladistsic purists don't recognise these groups.
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Monophyletic
Paraphyletic
Polyphyletic
Producing Classifications:
Characters for Taxonomic Studies
Pollination biology
Anatomy
Morphology
Secondary
compounds
Molecular (DNA & RNA)
Palynology
(Pollen study)
Embryology
Proteins
Chromosome
Structure & number
Molecular Systematics: the
Plant Genomes
Nucleus (1.1 x106 - 110 x109 kbp)
Mitochondrion (200-2500 kbp)
Chloroplast (135-160 kbp)
Map of Corn
Chloroplast Genome
Critique of Molecular
Taxonomy
Same basic problems as morphological data
(convergence, parallelism, etc.)
All genes don’t evolve at same rate.
Hybridization / introgression & random
mutation may result in a gene’s tree not
matching the species’ tree.
Big advantage: large number of molecular
characters available.
Best trees & classifications come from
studies integrating morphological,
molecular & other characters.
A
T
G
C
T
A
C
G
Classification of Organisms:
3 Domains
Archaea: (Non-bacterial
prokaryotes)
Bacteria: (true
bacteria)
Eukarya:
Traditional Kingdoms of Eukarya
“Protists”: (Eukaryotic cells, often single-celled organisms.
 Euglenoids, Red Algae, Dinoflagelates, Diatoms,
Brown algae, Green algae…
 “Catch-all” group: not monophyletic.
 Revisions in progress:
 'Supergroups' of Eukarya (next slide)
Fungi: (Heterotrophic, external digestion).
Plants: (multicellular autotrophs).
Plants in strict sense: 'embryophytes'
Animals: (Heterotrophs, internal digestion).
'Supergroups' of Eukarya
Alveolata
Stramenopila
Plants
Plants &
Relatives
Rhizaria
Early
Eukaryotes
Animals
Opisthokonta
Fungi
Excavata
Selected Autotrophic“Protista”
Diatoms
Unicellular or small-colonial.
Important phytoplankton, may account for up to 25%
of global photosynthesis!
Critical for many aquatic ecosystems.
Unique 2-part polymerized silica cell walls--Frustules.
“Protista” : Brown Algae
Large, conspicuous marine
seaweeds (kelps).
Basic body structure: thallus—
multcellular, but simple.
Stramenopile clade (also
diatoms).
Not related to plants, etc.
Plstids captured via secondary
endosymbiosis.
“Protista”: Red Algae
Structurally complex seaweeds;
Mostly marine; Most abundant in tropics/ warm water.
Phycobilin pigments capturegreen light in deep water.
Distantly related to plants.
Plastids aquired via primary endosymbiosis.
Protista: Green Algae
Together with higher plants, form a monophyletic group
(= “Viridophytes”;
G. algae minus plants are paraphyletic.
Share basic cellular characters with plants.
Variable habit: Unicellular, filamentous, colonial multicellular,
etc.
Primarily fresh water but terrestrial & marine (Ulva)
examples.
Plastids via primary endosymbiosis.
Two phylogenetic groups (clades):
Chlorophyte lineage
Other green algae +
“higher” plants = “streptophytes”.
Green Algae
Spyrogyra & Oedogonium: Filamentous green algae
Green Algae
Volvox: a colonial green alga
Green Algae
Ulvophycea: a family of marine green algae (p. 334)
Green Algae
Chara & relatives: “Sister
group” to land plants.
Multicellular with branched
apical growth.
Chara: Pinkston Lake, Texas
Phylogeny of Viridophytes
Chara &
Chlorophytes relatives
Other
Green algae
Lycophytes
Ferns Seed plants
Mosses &
Hornworts
Liverworts
Seedless
Vascular Plants
Bryophytes
Gymnosperms &
Angiosperms
Green Algae
(Protista)
•Streptophytes
Embryophytes: Plants in the
Strict Sense
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All non-green algal groups of the viridophyte clade.
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Monophyletic.
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Multicellular
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Terrestrial
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Alternation-of-generations-life cycle:
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1n gametophyte alternates with 2n sporophyte.
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Generations unequal.
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Resting embryo stage at start of sporophyte generation.
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Multicelled meiosis organ = sporangium.
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Water proof wax covers surfaces: cuticle
Embryophyte Plant Lifecycles
Sporophyte (2N)
Sporangium
Mitosis
Zygote (2N)
Fertilization
2N
1N
Meiosis
♀
Spores
egg
♂
sperm
Mitosis
Gametophyte (1N)
Cladogram of Major Embryophyte
Groups
Mosses,
Clubmosses
Group:
liverworts,
Hornworts
(lycophytes)
Ferns
Pines
Oaks
(conifers) (angiosperms)
Flowers.
Seeds
in fruits.
Seeds.
30bp Cl-DNA inversion.
Over-topping branches.
Selected
Derived
Charactertraits
Xylem & phloem.
Lignin.
Branched, independent
sporophyte.
Embryo.
Sporangium.
Cuticle.
Multicelled sporophyte.
Bryophytes: Mosses, Liverworts &
Hornworts:
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First land plants: Ordovician (510-439) M.Y.A. had some
bryophyte-like characteristics.
Said to be “transitional from green algae to vascular plants”.
 But: diverse, successful modern plants; NOT
'throwbacks'!
 Worldwide in most habitats.
 2nd (mosses:12,000 sp.) & 4th (liverworts: 9,000 sp.)
largest plant groups.
Evolutionary strategies:
 Stay small & remain close to water source.
 Survive (rather than avoid) dehydration.
 Produce many spores via many small sporophytes (rather
than 1 large one).
 Gametophyte retains vegetative role.
Features of Bryophytes
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Non-vascular plants: lack xylem & phloem.
Often typical of moist areas (but dry-site species also
exist)!
Rely on immediate contact with moisture (but many
can survive long periods of dehydration)!
3 clades
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Mosses
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liverworts
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hornworts
Hornworts: Anthocerophyta
Sporophyte
Gametophyte
An East Texas Hornwort!
(Sabine N.F.)
Sporophytes
Gametophyte
Liverworts: Hepatophyta
A leafy liverwort
Gemma cups for
asexual propagation
A thallose liverwort
Liverworts: Hepatophyta
Sporophytes
Gametophyte
Mosses: Bryophyta
Capsule
(Sporangium)
Sporophyte
Gametophyte
Bryophytes on Ravine Wall: Sabine
National Forest TX
Bryophyte Life Cycle
(example: moss)
Sporophyte (2N)
Mitosis
Zygote (2N)
Fertilization
2N
Meiosis
1N
♀
Archegonium
& egg
antheridium
& sperm
Spores
♂
Mitosis
Gametophyte (1N)
Vascular Plants (Tracheophytes)
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All non-bryophyte embryophytes: monophyletic.
Specialized water /food-conducting vascular tissues
Lignin to strangthen cells in support tissues.
3 clades:
Seed plants
Ferns & allies
Lycophytes
The End!
Version 15.09