Lesson Overview
Seedless Plants
Lesson Overview
22.2 Seedless Plants
Lesson Overview
Seedless Plants
THINK ABOUT IT
We generally think of plants as growing from seeds, but there are plenty of plants
that don’t produce seeds at all. How do they manage to reproduce and grow
without them?
Lesson Overview
Seedless Plants
Green Algae
What are the characteristics of green algae?
Lesson Overview
Seedless Plants
Green Algae
What are the characteristics of green algae?
Green algae are mostly aquatic. They are found in fresh and salt water, and in
some moist areas on land.
Lesson Overview
Seedless Plants
Green Algae
Algae are not a single group of organisms. “Algae” applies to any photosynthetic
eukaryote other than a land plant.
“Green algae” are classified with 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.
Ancient green algae shared the ocean floor with corals and sponges..
Lesson Overview
Seedless Plants
The First Plants
Green algae absorb moisture and nutrients directly from their surroundings
and do not contain the specialized tissues found in other plants.
Lesson Overview
Seedless Plants
Life Cycle
Many green algae switch back and forth between haploid and diploid phases.
However, some may not alternate with each and every generation.
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.
Lesson Overview
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.
Lesson Overview
Seedless Plants
Multicellularity
Green algae can form colonies.
Spirogyra forms long threadlike colonies called filaments.
Lesson Overview
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.
Lesson Overview
Seedless Plants
Mosses and Other Bryophytes
What factor limits the size of bryophytes?
Lesson Overview
Seedless Plants
Mosses and Other Bryophytes
What factor limits the size of bryophytes?
Bryophytes are small because they lack vascular tissue.
Lesson Overview
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.
Lesson Overview
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, nonreproductive 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.
Lesson Overview
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.
Lesson Overview
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.
Lesson Overview
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.
Lesson Overview
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.
Lesson Overview
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.
Lesson Overview
Seedless Plants
Gametophyte
Sperm and egg cells fuse to produce a diploid zygote.
Lesson Overview
Seedless Plants
Sporophyte
The zygote marks the beginning of the sporophyte stage of the life cycle.
Lesson Overview
Seedless Plants
Sporophyte
A sporophyte grows within the body of the gametophyte, depending on it
for water and nutrients.
Lesson Overview
Seedless Plants
Sporophyte
Eventually the sporophyte grows out of the gametophyte and develops a
long stalk ending in a capsule called the sporangium.
Lesson Overview
Seedless Plants
Haploid spores are produced inside the capsule by meiosis and are
released when the capsule ripens and opens.
Lesson Overview
Seedless Plants
Vascular Plants
How is vascular tissue important?
Lesson Overview
Seedless Plants
Vascular Plants
How is vascular tissue important?
Vascular tissue—xylem and phloem—make it possible for vascular plants
to move fluids through their bodies against the force of gravity.
Lesson Overview
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.
Lesson Overview
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.
Lesson Overview
Seedless Plants
Evolution of a Transport System
Tracheids are connected end to end like a series of tin cans, as shown in the
figure. Openings between tracheids, known as pits, allow water to move
through a plant more efficiently than by diffusion alone.
Lesson Overview
Seedless Plants
Evolution of a Transport System
Vascular plants also have a second transport tissue called phloem that
transports solutions of nutrients and carbohydrates produced by
photosynthesis.
Lesson Overview
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.
Lesson Overview
Seedless Plants
Seedless Vascular Plants
Ferns have true vascular tissues, strong roots, creeping or underground
stems called rhizomes, and large leaves called fronds, shown in the figure.
Ferns can thrive in areas with little light and are most abundant in wet
habitats.
Lesson Overview
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.
Lesson Overview
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.
Lesson Overview
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