Origins of Angiosperms
Spring 2013
Outline
• Origin of the angiosperms
• Characters of angiosperms
• Brief history of angiosperm
classification
• Major groups of angiosperms
• ANITA grade
Origin of the Angiosperms
Cycads
Ginkgo
Conifers
Gnetophytes
Angiosperms
Progymnosperms
(“seed ferns”)
Divergence estimated at
ca. 325 mybp
Geologic Time
Timing of Angiosperm
Divergences
• The timing of the origin of the
•
angiosperms still is uncertain, but
most would agree on a Triassic or
Jurassic initial divergence, although
there is no unequivocal fossil evidence
A demonstrable “burst” of
phylogenetic radiation is found in the
fossil record beginning in the mid- to
late Cretaceous, 140 - 100 mybp
Figure 7.1
Origin of the Angiosperms
•pollen grains from ca. 140
mya (early Cretaceous) but
already major radiation!
•earliest flowers 130 mya
•likely no extant group of
seed plants is very closely
related to the angiosperms!
Origin of Angiosperms
•Cycad-like plants: Bennettitales?
•large, flowerlike strobili:
•pollen-producing organs surrounding an axis
bearing naked ovules/seeds
Origin of Angiosperms
Modification of a “seed
fern” such as Caytonia?
Caytonia
fossil: ovule
Origin of Angiosperms
•Archaefructus
•ca. 130 mya
•ancestral flowering plant or
extinct off-shoot of an extinct
lineage?
•aquatic plant (dissected leaves)
•elongate reproductive axes:
-paired stamens below
-several-seeded carpels above
Characters of Angiosperms
Fig. 6.1
What makes a plant an angiosperm?
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Flower (usually with perianth)
Carpels with a stigmatic surface for pollen tube
germination; ovules enclosed within carpels; fruit
Ovules with two integuments
Reduced female gametophyte, usually 8 nuclei in 7 cells –
no archegonium
Double fertilization with the production of 3N endosperm
Stamens with two pairs of lateral pollen sacs
(microsporangia); pollen (male gametophyte) 3-nucleate
Xylem – most with vessels (evolved within angiosperms)
Phloem – sieve tube members with 1 or more companion
cells derived from the same mother cell
Figure 6.2
Flower
Magnolia
Spiral undifferentiated
perianth parts = tepals
(plesiomorphic)
Magnolia
Differentiated sepals and
petals (each in whorls)
(apomorphic)
Abutilon
Laminar stamens
in basal angiosperms
(plesiomorphic)
paired pollen
sacs
connective
microsporangium
filament
Early carpel with
stigmatic crest
(plesiomorphic)…
…to the derived carpel
with a style and an
apical stigma (apomorphic).
Figure 6.9 from the text
Female gametophyte in angiosperms
-no waiting time as in gymnosperms!
-note 2 integuments (bitegmic; some angiosperm lineages
have lost one integument)
-gymnosperms have
mature ovule
only 1 integument
(unitegmic)
ovule
And it’s off
to the races!
Indirect
pollination
(due to presence
of carpels)
Seed development in angiosperms
-no waiting time as in gymnosperms!
double fertilization
seed
Avocado
(Persea, Lauraceae)
Flower
pericarp
exocarp
mesocarp
endocarp
seed
fruit
Vessels in Angiosperms
•are the water (solute) conducting cells of the
xylem in most angiosperms
•ends of cells have openings (perforation
plate), cells shorter and wider
•more efficient, faster rate of flow but more
susceptible to air bubbles (embolisms) than
tracheids are
•may have arisen independently in two or more
angiosperm lineages but may have had a
single origin
Origin of vessels from tracheids
Figure 6.16B from the text
Angiosperm phloem
•sieve tube members +
companion cells
•stm = specialized sugar-conducting
cells of the phloem of angiosperms;
lack a nucleus at functional maturity
•cc = parenchyma cells associated
with stm
-function to load/unload sugars
into stm cavity
-derived from the same mother
cell as its stm
stm
Brief history of angiosperm
classification
Alternative ways of thinking about
early angiosperm characters…
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“Old” School (German) - Engler
- “Simple is primitive” (Few floral parts)
- Ancestors are conifers
- Pollination by wind
- Modern relicts = “Amentiferae” (catkins)
“New” School (American) – Bessey
- “Flowers with many parts are primitive”
- Ancestors are Cycad-like plants
- Pollination by primitive insects
- Modern relicts = Magnolias and allies
Heinrich Gustav Adolph Engler
(1844-1930)
-German Botanist at Berlin Botanical
Garden
-Was the primary European in
interpreting the grouping of
major angiosperm
assemblages
-“Few simple flower parts primitive”
-Small, unisexual flowers primitive
Englerian ‘Primitive Taxa’
Juglans sp.
Quercus sp.
“Amentiferae”
Betula sp.
Charles Edwin Bessey
(1845-1915)
-Botanist at Iowa State University
from 1869-1884 (left in 1884 to
teach in Nebraska)
-Was a “major player” in interpreting
and understanding angiosperm
evolution
-“Many flower parts primitive”
Bessey Hall
Iowa State University
Bessey’s “Cactus”
(1915)
Placed plant groups
with many floral
parts in a basal
position as the
‘ancestral’ forms.
Outlined ‘dicta’ for
the construction of
phylogenies using the
evolutionary trends in
character changes.
Polypetalous flowers,
insect pollination,
cycad-like ancestors
Besseyan ‘Primitive Taxa’
Nymphaeaceae
Magnoliaceae
Figure 6.1 from the text
Major Groups of Angiosperms
• Basal Angiosperms (ANITA grade)
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- Amborellaceae
- Nympheaceae
- Illiciaceae
Magnoliid Complex
- Magnoliales
- Piperales
- Winterales
MONOCOTS
EUDICOTS (tricolpates)
Major Groups of Eudicots
• Basal Eudicots
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- Ranunculales and allied families
Caryophyllales & Saxifragales
Rosid Clade
- Fabids
- Malvids
Asterid Clade
- Basal Asterids
- Lamiids
- Campanulids
ANITA grade
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Amborella (Amborellales)
Nymphaea (Nymphaeales)
Illicium (Austrobaileyales)
Trimenia (Austrobaileyales)
Austrobaileya (Austrobaileyales)
Fig. 6.1
ANITA Grade
or “basal”
angiosperms
ca. 125 mybp
Grade = a paraphyletic (or
Polyphyletic) group whose
members share a similar
level of morphological or
physiological complexity.
Major Angiosperm Clades
Amborellaceae
Nymphaeales
Austrobaileyales
MAGNOLIID
COMPLEX
MONOCOTS
Soltis et al. 2000,
APG II 2002,
Judd et al. 2002
EUDICOTS
[TRICOLPATES]
ANITA
grade
(basal
groups)
Basal Angiosperms:
Amborellaceae
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New Caledonia
Understory shrub; plants dioecious
1 species (monotypic): Amborella trichopoda
Leaves simple, evergreen
Flowers small, unisexual: ♀ apocarpous, with
stigmatic crests; ♂ with laminar stamens
Significant features: Most basal of all flowering
plants; no vessels in wood
Special uses: (none)
Basal Angiosperms:
Amborellaceae (Amborella Family)
Amborella trichopoda
As we venture through the
various major groups of
angiosperms…
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Identify the plesiomorphic characteristics
associated with particular groups and note their
apomorphies (if any) as well.
Try to associate “syndromes” of characteristics
with each group (make note of special characters
occurring together).
One good way to study is to write keys to the
groups we cover in any given unit.
Names of groups are important! Learn to spell and
say them!
Ask questions!!