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Seed Plants
Chapter 31
1
The Evolution of Seed Plants
• Seed plants first appeared 305–465 MYA
– Evolved from spore-bearing plants known as
progymnosperms
2
http://www.devoniantimes.org/who/pages/archaeopteris.html
The Evolution of Seed Plants
• Success attributed to evolution of seed
– Protects and provides food for embryo
– Dormancy allows the “clock to be stopped” to
survive harsh periods before germinating
– Later development of fruits enhanced seed
dispersal
3
http://www.sweetacrebirdfarm.com/fruits.htm
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display.
Stored food
Integument
(seed coat)
Embryo
• Seed
right: © Biology Media/Photo Researchers, Inc.
300 m
– Embryo protected by integument
• An extra layer or two of sporophyte tissue
• Hardens into seed coat
– Megasporangium divides meiotically inside ovule to
produce haploid megaspore
– Megaspore produces egg that combines with sperm
to form zygote
– Also contains food supply for embryo
4
Seed plants produce 2 kinds of gametophytes
• Male (micro-) gametophytes
– Pollen grains
– Dispersed by wind or a pollinator
– No need for water
• Female (mega-) gametophytes
– Develop within an ovule
– Enclosed within diploid sporophyte tissue in
angiosperms
5
Five Phyla of Extant Seed Plants
•
•
•
•
Coniferophyta
Cycadophyta
Gnetophyta
Ginkophyta
• Anthophyta
Gymnosperms
Angiosperms
6
7
–
–
–
–
Coniferophytes
Cycadophytes
Gnetophytes
Ginkgophytes
• All lack flowers & fruits of the
angiosperms
• All have ovule exposed on a scale
8
http://www.gc.maricopa.edu/biology/lsola/bio182/plantselftests/nonflwr/conif105.htm
Angiosperms
• Plants with “naked seeds”
• There are four living groups
Gymnosperms
Gymnosperms
Ferns and Allies
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
http://en.wikipedia.org/wiki/Sequoia_sempervirens
Conifers
(phylum Coniferophyta)
• Most familiar gymnosperm
phylum
• Pines, spruces, firs,
cedars, and others
– Coastal redwood – Tallest
living vascular plant
– Bristlecone pine – Oldest
living tree
9
http://www.arhomeandgarden.org/plantoftheweek/articles/Bristlecone_Pine.htm
http://borealforestinnamerica.blogspot.com/
Conifers
(phylum Coniferophyta)
• Found in colder and
sometimes drier regions
of the world
• Conifers are sources of
important products
– Timber, paper, resin,
and taxol (anti-cancer)
10
Boreal forest in
Saskatchewan,
Canada
Taxol was first isolated from
the bark of the Pacific yew
tree, from paclitaxel produced
by endophytic fungi in the bark
http://www.projectnoah.org/spottings/17406090
• Pines
– More than 100 species, all in
the Northern hemisphere
– Produce tough needlelike
leaves in clusters
– Leaves have thick cuticle and
recessed stomata to retard
water loss
– Leaves have canals with resin
to deter insect and fungal
attacks
Florida slash pine
(Pinus elliottii var. densa)
11
http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/Secretion/Secretion-2.htm
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Pollen
Microspores
Air bladder
Pollination
Microspore
mother cell
Pollen tube
Scale
Sperm
Pollen tube
Megaspore
mother cell
Megaspore
n
2n
Pollenbearing
cone
Ovulate
(seed-bearing)
cone
(15 months
after pollination)
Zygote
Sporophyte
MITOSIS
Seedling
MITOSIS
Pine
seed
Section of seed (second year),
showing embryo embedded
in megagametophyte
14
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display.
Cycads
(phylum Cycadophyta)
• Slow-growing
gymnosperms of tropical
and subtropical regions
• Sporophytes resemble
palm trees
• Female cones can weigh
45 kg
• Have largest sperm cells
of all organisms!
a.
© Luca Invernizzi Tetto/agefotostock
Female cone
17
http://plantnet.rbgsyd.nsw.gov.au/cgi-bin/cycadpg?taxname=Encephalartos+manikensis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or
display.
Gnetophytes
(phylum Gnetophyta)
• Only gymnosperms with
vessels in their xylem
• Contain three (unusual)
genera
Welwitschia of Africa
b.
© Juan Carlos Muñoz/agefotostock
– Welwitschia
– Ephedra
– Gnetum
18
Ephedra viridis (Mormon Tea)
of American Southwest
http://www.swcoloradowildflowers.com/Yellow%20Enlarged%20Photo%20Pages/ephedra.htm
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ginkgophytes
(phylum Ginkgophyta)
• Only one living species
remains
– Ginkgo biloba
• Flagellated sperm
• Dioecious
– Male and female
reproductive structures form
on different trees
c.
© Robert Gustafson/Visuals Unlimited
Flagellated sperm of
Ginkgo
19
http://kwanten.home.xs4all.nl/ovule.htm
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Angiosperms
Ferns and Allies
• Flowering plants
• Ovules are enclosed/
protected in diploid
tissue at the time of
pollination
• Carpel, a modified leaf
that covers seeds,
develops into fruit
(modified ovary wall)
Gymnosperms
Angiosperms
Cross-section of pea pod (carpel) showing
ovule (where seed develops) inside the fruit
20
http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/Angiosperm/placentation.htm
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fruits
Angiosperm origins are a
mystery
– Origins as early as
145–208 MYA
– Oldest known
angiosperm in the fossil
record is Archaefructus
Recent evidence from fossil
pollen pushes angiosperm
appearance back 100 million
years to ~245 mya, alongside
the earliest dinosaurs
http://www.livescience.com/40088-flowers-existed-with-dinosaurs.html
Paired
stamens
© David Dilcher and Ge Sun
22
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Gymnosperms
Ginkgo
Gnetophytes
Conifers
Angiosperms
Cycads
Archaefructus Amborella Water lillies Staranis
(extinct)
Eudicots Magnoliids Monocots
Archaefructus may be the sister clade to all other angiosperms
23
Closest living relative to the
original angiosperm is Amborella
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• Horizontal gene transfer explains
the presence of moss
mitochondrial genes in the
genome of Amborella
– Mosses may have been epiphytes
on Amborella ancestor, increasing
likelihood of gene transfer
Carpel
Petal
Sepal
Courtesy of Sandra Floyd
Moss epiphyte growing on
leaf of Amborella trichopoda
in New Caledonia
24
http://www.pnas.org/content/101/51/17747.figures-only
a.
Mitochondrial
Genes A–C
b.
Moss species 4 – gene C
Amborella – gene C (HGT)
Moss species 3 – gene B
Moss species 2 – gene B
Amborella – gene B (HGT)
Moss species 1 – gene A
Amborella gene A (HGT)
Charales
Mosses
Ferns
Pines
Flowering
plants
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http://fruitandnuteducation.ucdavis.edu/generaltopics/AnatomyPollination/VegetativeFloral_Development/
Flowers house the gametophyte generation
• Flower morphology
– Modified stems bearing modified leaves
– Primordium develops into a bud at the end of a stalk
called the pedicel
– Pedicel expands at the tip to form a receptacle, to which
other parts attach
– Flower parts are organized in circles called whorls
Stigma
Style
Carpel
Ovule
Ovary wall
a.
Anther
Stamen
Filament
Petal
Sepal
Ovary
Receptacle
Pedicel
26
http://ww2.biol.sc.edu/~krizeklab/krizek.html
• Flower whorls
– Outermost whorl – sepals
– Second whorl – petals
– Third whorl – stamens
(androecium)
• Pollen is the male gametophyte
• Each stamen has a pollenbearing anther and a filament
(stalk)
– Innermost whorl – gynoecium
• Consists of one or more carpels
• House the female gametophyte
27
https://s10.lite.msu.edu/res/msu/botonl/b_online/library/knee/hcs300/angio1.htm
Almond flower development
Click on image for YouTube video
http://www.youtube.com/watch?v=DijGvPMuHhI
28
http://homemadenaturalsoaps.blogspot.com/2011/0
6/sweet-almond-oil-as-name-suggests-is.html
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Anther
Filament
Stigma
Petal
Sepal
Style
Carpel
Ovule
Ovary wall
Stamen
Ovary
Receptacle
Pedicel
a.
• Carpel has 3 major regions
– Ovary – swollen base containing ovules
• Later develops into a fruit
– Stigma – tip where pollen lands
– Style – neck or stalk
29
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MITOSIS
Megaspore (n)
Generative
cell
Tube
nucleus
Polar
nuclei
Egg
Ovule
Megaspore
mother cell (2n)
Pollen tube
Sperm
Pollen (n)
Tube
nucleus
Formation of
pollen tube (n)
Anther
Stigma
Style
Anther (2n)
Microspore
mother cells (2n)
2n
Ovary
Adult sporophyte
with flower (2n)
Young
sporophyte (2n)
Cotyledons
Polar
nuclei
Endosperm
Embryo (2n)
Endosperm (3n)
Seed
coat
Zygote
DOUBLE
FERTILIZATION
Seed (2n)
Sperm
Egg
32
http://leavingbio.net/the%20structure%20and%20functions%20of%20flowers.htm
• Pollination
– Mechanical transfer of pollen from anther to stigma
– May or may not be followed by fertilization
– Pollen grain grows pollen tube
• Guided to embryo sac by pheromones from synergids
• Pollen tube enter embryo sac via micropyle
– One of the two pollen grain cells lags behind
• This generative cell divides to produce two sperm cells
• No flagella on sperm
See YouTube video
of pollen tube
development
34
• Double fertilization
– One sperm unites with egg to form diploid zygote
• New sporophyte
– Other sperm unites with the two polar nuclei to form
the triploid endosperm
• Provides nutrients to embryo
35
http://vitae-scientia.tumblr.com/
Seeds
• In many angiosperms, embryo development
is arrested soon
after meristems &
cotyledons (embryonic leaves) differentiate
• Integuments develop into a relatively
impermeable seed coat
– Encloses the seed with its dormant
embryo and stored food
• May remain dormant for many years
– Germinate when conditions are favorable
Shoot apical
meristem
Seed coat
(integument)
Procambiu
m
Root
apical
meristem
Root cap
Endosper
m
Cotyledo
ns
36
http://www.cactus-art.biz/note-book/Dictionary/Dictionary_S/dictionary_seed.htm
http://www.allposters.com/-sp/Sectioned-Seed-of-a-Runner-Bean-Phaseolus-Coccineus-Showingthe-Seed-Coat-Embryo-and-Cotyledons-Posters_i6010357_.htm
• Seeds are an important
adaptation
1. Maintain dormancy under
unfavorable conditions
2. Protect young plant when it
is most vulnerable
3. Provide food for the embryo
until it can produce its own
food
4. Facilitate dispersal of the
embryo
Runner bean seed
(Phaseolus coccineus)
Seed dispersal in white
fir (Abies concolor)
37
http://commons.wikimedia.org/wiki/File:Abies_concolor_seed-dispersal.jpg
• Once a seed coat forms, most of the embryo’s
metabolic activities cease
• Germination cannot take place until water and
oxygen reach the embryo
• Seeds of some plants have been known to
remain viable for thousands of years
38
http://www.ask.com/explore/what-seed-germination
• Specific adaptations ensure that seeds will
germinate only under appropriate
conditions
– Some seeds lie within tough cones that do not
open until exposed to fire
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
b.
a: © Ed Reschke; b: © David Sieren/Visuals Unlimited
39
http://biology-pictures.blogspot.com/
Fruits
• Most simply defined as
mature ovaries (carpels)
• During seed formation,
the flower ovary begins
to develop into fruit
• It is possible for fruits to
develop without seed
development
Sterile cultivated bananas
with reduced seeds
– Bananas are propagated
asexually
40
http://b4fa.org/biosciences-and-agriculture/plantbreeding/where-do-our-crops-come-from-banana/
• The ovary wall is termed the pericarp
– With 3 layers:
• Exocarp – skin or rind
• Mesocarp – flesh or pulp
• Endocarp – surrounds seeds (pit)
– Their fate determines fruit type
41
http://vitae-scientia.tumblr.com/
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
True Berries
Outer pericarp
The entire pericarp is
fleshy, although there
may be a thin skin.
Berries have multiple
seeds in either one or
more ovaries. The
tomato flower had four
carpels that fused.
Each carpel contains
multiple ovules that
develop into seeds.
Fused
carpels
Seed
© Kingsley Stern
43
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Legumes
Split along two carpel edges (sutures) with seeds
attached to edges; peas, beans. Unlike fleshy
fruits, the three tissue layers of the ovary do not
thicken extensively. The entire pericarp is dry
at maturity.
Stigma
Pericarp
Seed
Style
© Kingsley Stern
44
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Drupes
Pericarp
Single seed
Exocarp (skin)
enclosed
Mesocarp
in a hard pit;
Endocarp (pit)
peaches, plums,
cherries. Each
layer of the
pericarp has
a different structure
and function, with
Seed
the endocarp
forming the pit
Samaras
Not split and
with a wing
formed from the
outer tissues;
maples, elms,
ashes.
Pericarp
Seed
(left): © Kingsley Stern; (right): Courtesy of Robert A. Schisling
45
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Aggregate Fruits
Derived from many
Sepals of a
ovaries of a single
single flower
flower; strawberries,
blackberries.
Unlike tomato,
Ovary
these ovaries
Seed
are not fused
and covered
by a continuous
pericarp.
Multiple Fruits
Individual flowers form fruits
around a single stem. The fruits
fuse as seen with pineapple.
Main stem
Pericarp of
individual flower
(left): Courtesy of Robert A. Schisling; (right): © Charles D. Winters/Photo Researchers, Inc.
46
Fruit Dispersal
• Occurs through a wide
array of methods
a. Ingestion and transportation
by birds or other vertebrates
b. Hitching a ride with hooked
spines on birds and
mammals
c. Blowing in the wind
d. Floating and drifting on
water
e. Burial in caches by
herbivores
Cedar Waxwing
(Bombycilla cedrorum)
47
http://blog.duncraft.com/2011/08/26/birds-migration-and-berries/
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
b.
a: © Edward S. Ross; b: © Nigel Cattlin/Visuals Unlimited
b. Hitching a ride with hooked spines on
birds and mammals
48
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
c.
d.
c: © Phil Ashley/Getty Images; d: © John Kaprielian/Photo Researchers, Inc.
c. Blowing in the wind
d. Floating and drifting on water
49
http://en.wikipedia.org/wiki/Wikipedia:Wikipedia_Signpost/2011-12-26/WikiProject_report
Eastern chipmunk (Tamias striatus)
carrying nuts for caching
e. Burial in caches by
herbivores
50
Western scrub-jays (Aphelocoma
californica) caching seed
http://www.courierpress.com/news/2012/dec/29/blue-jays-cashing-in-on-caches-to-survive/