Transcript Seed plants - 549online.org

Overview of Green Plants
Chapter 30
Defining Plants
The kingdom Viridiplantae includes land
plants and green algae
-Red and brown algae are excluded
All green plants arose from a single species
of freshwater algae
The green algae split into two major clades
-Chlorophytes – Never made it to land
-Charophytes – Did!
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Defining Plants
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Defining Plants
Land plants have two major features
1. Protected embryos
2. Multicellular haploid and diploid phases
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Defining Plants
Adaptations to terrestrial life
1. Protection from desiccation by a waxy
cuticle and stomata
2. Evolution of leaves which increase
photosynthetic surface area
3. Shift to a dominant vertical diploid
generation
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Plant Life Cycles
Humans have a diplontic life cycle
-Only the diploid stage is multicellular
Plants have a haplodiplontic life cycle
-Multicellular diploid stage = Sporophyte
-Multicellular haploid stage = Gametophyte
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Plant Life Cycles
Sporophyte produces haploid spores by
meiosis
Spores divide mitotically, producing the
gametophyte
Gametophyte produces gametes by mitosis
Gametes fuse to form the diploid sporophyte
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Plant Life Cycles
As more complex plants evolved:
1. Diploid stage became the dominant
portion of the life cycle
2. Gametophyte became more limited in
size
3. Sporophyte became nutritionally
independent
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Chlorophytes
Green algae have two distinct lineages
-Chlorophytes – Gave rise to aquatic algae
-Streptophytes – Gave rise to land plants
Chlamydomonas
-Unicellular chlorophyte with two flagella
-Have eyespots to direct swimming
-Reproduces asexually as well as sexually
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Chlorophytes
Volvox
-Colonial chlorophyte
-Hollow sphere of
a single layer of
500-60,000 cells
-A few cells are
specialized for
reproduction
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Chlorophytes
Ulva
-Multicellular chlorophyte
-Haplodiplontic life cycle
-Gametophyte and
sporophyte have
identical appearance
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Charophytes
Charophytes are green algae related to land
plants
There are two
candidate clades
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Charophytes
Charales (300 species)
-Macroscopic
-Plant-like plasmodesmata
-Sister clade to land plants
Choleocaetales (30 species)
-Microscopic
-Plant-like mitosis
-Next closest plant relatives
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Charophytes (Cont.)
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Bryophytes
Bryophytes are the closest living
descendants of the first land plants
-Called nontracheophytes because they
lack tracheids (specialized transport cells)
Simple, but highly adapted to diverse
terrestrial environments
Non-photosynthetic sporophyte is nutritionally
dependent on the gametophyte
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Bryophytes
Liverworts (phylum Hepaticophyta)
-Have flattened gametophytes with liver-like
lobes
-Form gametangia
in umbrellashaped structures
-Also undergo
asexual
reproduction
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Bryophytes
Hornworts (phylum Anthocerotophyta)
-Sporophyte has stomata
-Sporophyte is
photosynthetic
-Cells have a
single large
chloroplast
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Bryophytes
Mosses (phylum Bryophyta)
-Gametophytes consist of small, leaflike
structures around a stemlike axis
-Anchored to substrate by rhizoids
-Multicellular gametangia form at the tips of
gametophytes
-Archegonia – Female gametangia
-Antheridia – Male gametangia
Mosses withstand drought, but not air pollution
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Features of Tracheophyte Plants
Cooksonia, the first vascular land plant,
appeared about 420 MYA
-Only a few
centimeters tall
-No roots or
leaves
-Homosporous
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Features of Tracheophyte Plants
Vascular tissues are of two types
-Xylem – Conducts water and dissolved
minerals upward from the roots
-Phloem – Conducts sucrose and
hormones throughout the plant
-These enable enhanced height and size in
the tracheophytes
Tracheophytes are also characterized by the
presence of a cuticle and stomata
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Features of Tracheophyte Plants
Vascular plants have gametophytes reduced in
size and complexity relative to sporophytes
Seeds
-Highly-resistant structures that protect the
plant embryo
-Occur only in heterosporous plants
Fruits in flowering plants add a layer of
protection to seeds
-Also attract animals that disperse seeds 25
Features of Tracheophyte Plants
Vascular plants include seven extant phyla
grouped in three clades
1. Lycophytes (club mosses)
2. Pterophytes (ferns and their relatives)
3. Seed plants
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Lycophytes
Club mosses are the earliest vascular plants
-They lack seeds
-Superficially
resemble true
mosses but they
are not related
-Homosporous
or heterosporous
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Pterophytes
The phylogenetic relationships among ferns
and their relatives is still being sorted out
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Pterophytes
Whisk ferns
-Saprophyte consists of evenly forking
green stems without leaves or roots
-Some gametophytes
develop elements of
vascular tissue
-Only one known
to do so
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Pterophytes
Horsetails
-All 15 living species are homosporous
-Constitute a single species, Equisetum
-Consist of ribbed,
jointed photosynthetic
stems that arise from
branching rhizomes
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Pterophytes
Ferns
-The most abundant group of seedless
vascular plants with about 11,000 species
-The conspicuous
sporophyte and much
smaller gametophyte
are both photosynthetic
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Pterophytes
The fern life cycle differs from that of a moss
-Much greater development, independence
and dominance of the fern’s sporophyte
Fern morphology
-Sporophytes have rhizomes
-Fronds (leaves) develop at the tip of the
rhizome as tightly rolled-up coils
-They unroll and expand
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Pterophytes
Fern reproduction
-Most fern are homosporous
-Produce distinctive sporangia in clusters
called sori on the back of the fronds
-Diploid spore mother cells in sporangia
produce haploid spores by meiosis
-At maturity, the spores are catapulted by
snapping action
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The Evolution of Seed Plants
Seed plants first appeared 305-465 MYA
-Evolved from spore-bearing plants known
as progymnosperms
The seed represents an important advance
1. Protects the embryo
2. Easily dispersed
3. Introduces a dormant phase
in the life cycle
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The Evolution of Seed Plants
Seed plants produce 2 kinds of gametophytes
-Male gametophytes
-Pollen grains
-Dispersed by wind or a pollinator
-Female gametophytes
-Develop within an ovule
-Enclosed within diploid sporophyte tissue
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Gymnosperms
Gymnosperms are plants with “naked seeds”
-Ovule is exposed on a scale at pollination
There are four living groups
-Coniferophytes
-Cycadophytes
-Gnetophytes
-Ginkgophytes
All lack flowers and fruits of angiosperms
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Gymnosperms
Conifers (phylum Coniferophyta) are the
largest gymnosperm phylum
-Include:
-Pines, spruces, firs, cedars and others
-Coastal redwood – Tallest tree
-Bristlecone pine – Oldest living tree
Conifers are sources of important products
-Timber, paper, resin and taxol (anti-cancer)
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Gymnosperms
Pines
-More than 100 species, all in
the Northern hemisphere
-Produce tough needlelike
leaves in clusters
-Leaves have:
1. Thick cuticle and
recessed stomata
2. Canals into which
cells secrete resin
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Gymnosperms
Pine reproduction
-Male gametophytes (pollen grains) develop
from microspores in male cones by meiosis
-Female pine cones form on the upper
branches of the same tree
-Female cones are larger, and have
woody scales
-Two ovules develop on each scale
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Gymnosperms
Pine reproduction
-Each ovule contains a megasporangium
called the nucellus
-Surrounded by the integument
-Opening – Micropyle
-One layer becomes the seed coat
-While scales of female cone are open,
pollen grains drift down between them
-Are drawn to top of nucellus
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Gymnosperms
Pine reproduction
-While female gametophyte is developing, a
pollen tube emerges from the pollen grain
-It digests its way to the archegonium
-Fifteen months after pollination, pollen tube
reaches archegonium and delivers its sperm
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Gymnosperms
Cycads (phylum Cycadophyta)
-Slow-growing gymnosperms of tropical and
subtropical regions
-Sporophytes
resemble palm trees
-Have largest sperm
cells of all organisms!
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Gymnosperms
Gnetophytes (phylum Gnetophyta)
-Only gymnosperms with vessels in their
xylem
-Contain three
(unusual) genera
-Welwitschia
-Ephedra
-Gnetum
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Gymnosperms
Ginkgophytes (phylum Ginkgophyta)
-Only one living species remains
-Ginkgo biloba
-Dioecious
-Male and female
reproductive
structures form
on different trees
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Angiosperms
Angiosperms are the flowering plants
-Ovules are enclosed in diploid tissue at the
time of pollination
-The carpel, a
modified leaf that
covers seeds,
develops into fruit
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Angiosperms
Angiosperm origins are a mystery
-The oldest known angiosperm in the fossil
record is Archaefructus
-The closest living relative to the original
angiosperm is Amborella
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Angiosperms
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Angiosperms
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Angiosperms
Flower morphology
-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
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Angiosperms
Flower morphology
-Outermost whorl = Sepals
-Second whorl = Petals
-Third whorl = Stamens (androecium)
-Each stamen has a pollen-bearing
anther and a filament (stalk)
-Innermost whorl = Gynoecium
-Consists of one or more carpels that
house the female gametophyte
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Angiosperms
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Angiosperms
Carpel structure
-Three major regions
-Ovary = Swollen base containing
ovules
-Later develops into a fruit
-Stigma = Tip
-Style = Neck or stalk
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Angiosperm Life Cycle
The female gametophyte (embryo sac) has 8
haploid nuclei arranged in two groups of four
A nucleus from each group migrate toward the
ovule’s center and become polar nuclei
Cell walls form round remaining three nuclei
-At the micropyle end, one cell functions as
the egg, and the other two are synergids
-At the other end, three cells are antipodals
-They eventually break down
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Angiosperm Life Cycle
Pollen production occurs in the anthers
-It is similar but less complex than female
gametophyte formation
-Diploid microspore mother cells undergo
meiosis to produce four haploid microspores
-Binucleate microspores become pollen
grains
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Angiosperm Life Cycle
Pollination is the mechanical transfer of pollen
from anther to stigma
-Pollen grains develop a pollen tube that is
guided to the embryo sac
-One of the two pollen grain cells lags behind
-This generative cell divides to produce
two sperm cells
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Angiosperm Life Cycle
As the pollen tube enters the embryo sac, a
double fertilization occurs
-One sperm unites with egg to form the
diploid zygote
-Other sperm unites with the two polar nuclei
to form the triploid endosperm
-Provides nutrients to embryo
When the seed germinates, a young
sporophyte plant emerges
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Angiosperm Life Cycle
Angiosperms include:
-Eudicots (about 175,000 species)
-Trees, shrubs, snapdragons, peas, other
-Use flowers to attract insect pollinators
-Monocots (about 65,000 species)
-Grasses, lilies, palms, irises, others
-Some rely on wind for pollination
Note: Self-pollination may also occur
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