Lesson Overview
Seedless Plants
Lesson Overview
22.2 Seedless Plants
Lesson Overview
Seedless Plants
The First Plants
Green algae are mostly aquatic. They are found in fresh and
salt water, and in some moist areas on land.
Green algae absorb moisture and nutrients directly from their
surroundings and do not contain the specialized tissues found
in other plants.
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Seedless Plants
Life Cycle
Many green algae switch back and forth between haploid and
diploid phases.
In the life cycle of Chlamydomonas, as long as living conditions
are suitable, the haploid cell reproduces asexually by mitosis.
Lesson Overview
Seedless Plants
Life Cycle
If conditions become unfavorable, Chlamydomonas can
switch to a stage that reproduces sexually.
Its cells release gametes that fuse into a diploid zygote (a
sporophyte). The zygote has a thick protective wall,
permitting survival in harsh conditions.
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Seedless Plants
Life Cycle
The zygote begins to grow once conditions become
favorable.
It divides by meiosis to produce four haploid cells that
swim away, mature, and reproduce asexually.
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Seedless Plants
Multicellularity
Green algae can form colonies.
Spirogyra forms long threadlike colonies called filaments.
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Seedless Plants
Multicellularity
Volvox colonies consist of as few as 500 to as many as 50,000
cells arranged to form hollow spheres.
Volvox shows some cell specialization and straddles the fence
between colonial and multicellular life.
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Seedless Plants
Mosses and Other Bryophytes
Mosses have a waxy,
protective coating that
makes it possible for them to
resist drying, and thin
filaments known as rhizoids
that anchor them to the soil.
Rhizoids also absorb water
and minerals from the soil.
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Seedless Plants
Mosses and Other Bryophytes
Mosses, hornworts, and liverworts all belong to a
group of plants known as bryophytes.
Bryophytes have specialized reproductive organs
enclosed by other, non-reproductive cells.
Bryophytes show a higher degree of cell
specialization than do the green algae and were
among the first plants to become established on
land.
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Seedless Plants
Why Bryophytes Are Small
Bryophytes do not make lignin, a substance that hardens cell
walls, and do not contain true vascular tissue. Because of
this, bryophytes cannot support a tall plant body against the
pull of gravity.
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Seedless Plants
Life Cycle
Bryophytes display alternation of
generations.
The gametophyte is the dominant,
recognizable stage of the life cycle
and the stage that carries out most
of the photosynthesis.
The sporophyte is dependent on the
gametophyte for its supply of water
and nutrients.
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Seedless Plants
Life Cycle
Bryophytes produce sperm cells that swim using flagella.
For fertilization to occur, the sperm must swim to an egg.
Because of this, bryophytes must live in habitats where open
water is available at least part of the year.
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Seedless Plants
Gametophyte
When a moss spore lands in a moist place, it sprouts and
grows into a young gametophyte.
The gametophyte forms rhizoids that grow into the ground
and shoots that grow into the air.
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Seedless Plants
Gametophyte
Gametes are formed in reproductive structures at the tips of
the gametophytes.
Eggs are produced in archegonia.
Sperm are produced in antheridia.
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Seedless Plants
Gametophyte
Sperm and egg cells fuse to produce a diploid zygote.
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Seedless Plants
Sporophyte
The zygote marks the beginning of the sporophyte stage of
the life cycle.
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Seedless Plants
Sporophyte
A sporophyte grows within the body of the gametophyte,
depending on it for water and nutrients.
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Seedless Plants
Sporophyte
Eventually the sporophyte grows out of the gametophyte
and develops a long stalk ending in a capsule called the
sporangium.
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Seedless Plants
Haploid spores are produced inside the capsule by meiosis
and are released when the capsule ripens and opens.
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Seedless Plants
Vascular Plants
About 420 million years ago, plants for the first time were
able to grow high above the ground.
Fossil evidence shows these plants were the first to have a
transport system with true vascular tissue. Vascular tissue
carries water and nutrients much more efficiently than does
any tissue found in bryophytes.
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Seedless Plants
Evolution of a Transport System
Vascular plants are known as tracheophytes, after
a specialized type of water-conducting cell they
contain. These cells, called tracheids, are hollow
tubelike cells with thick cell walls strengthened by
lignin.
Tracheids are found in xylem, a tissue that carries
water upward from the roots to every part of a
plant. They also have a second transport tissue
called phloem that transports solutions of
nutrients and carbohydrates produced by
photosynthesis.
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Seedless Plants
Seedless Vascular Plants
Among the seedless vascular plants alive today are three
phyla commonly known as club mosses, horsetails, and ferns.
The most numerous of these are the ferns.
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Seedless Plants
Life Cycle
In the life cycle of a fern, spores
produced by the sporophyte
grow into thin, heart-shaped
haploid gametophytes. The
gametophytes grow
independently of the
sporophyte.
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Seedless Plants
Life Cycle
Sperm and eggs are produced
on the gametophytes in
antheridia and archegonia.
Fertilization requires at least a
thin film of water, so that the
sperm can swim to the eggs.
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Seedless Plants
Life Cycle
The diploid zygote produced by
fertilization develops into a new
sporophyte plant. This is the
dominant stage of the fern life
cycle.
Haploid spores develop on the
undersides of the fronds in
sporangia, and the cycle begins
again