Download Plants - NIU Department of Biological Sciences

Plants
• Plants are autotrophs: they get
their energy from sunlight, and
they synthesize their food from
carbon dioxide.
• Plants are multicellular.
• Plants live on land, or are
derived from land-dwelling
plants. The algae and
seaweeds that live in the sea
are multicellular protists.
• Plants alternate between a
haploid phase (the
gametophyte) and a diploid
phase (the sporophyte).
Evolutionary Trends
• Plants are thought to have evolved from the green algae.
• By moving onto the land, plants had to deal with 2 big issues: gravity
( or lack of buoyancy) and dryness.
• Major trends:
– 1. development of roots, shoots, vascular system. Roots needs to
absorb nutrients, not just hold onto the surface. Shoots need to support
photosynthetic system off the ground. Vascular system to transport
materials between parts of the plant. Waxy cuticle on the leaves to
prevent desiccation.
– 2. increasing the diploid phase of the life cycle, and decreasing the
haploid phase. Diploid gives a backup copy of each gene, as a defense
against random mutations. Allows a larger, more complex body.
– 3. Seed and pollen protection and dispersal. Development of very
different male and female gametes, so only 1 type needs to be
dispersed in the environment. The pollen (male gametes) needs to be
protected from desiccation, and needs to find the female gametes
successfully. Seeds also need to be protected from harsh conditions
and to disperse to new locations.
Major Plant Groups
• We are going to examine
several groups that show
these trends:
– 1. bryophytes: nonvascular plants including
liverworts and mosses
– 2. seedless vascular plants
such as ferns and
horsetails
– 3. gymnosperms, which
have seeds and a vascular
system, such as the
conifers
– 4. angiosperms, the
flowering plants that
dominate the world today.
Bryophytes
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The bryophytes include the mosses,
liverworts, and hornworts. They are short
plants mostly growing in wet environments.
Bryophytes have a waxy cuticle on their
leaves to prevent desiccation.
Bryophytes have no internal vascular
system.
Bryophytes spend most of their lives as
haploids: the body of the moss plant is
haploid.
The only diploid structure is a stalk and
spore capsule, which grow out of the haploid
plant body.
Peat moss is used to help soil hold water. It
can also be used as fireplace fuel when it is
dried. Peat bogs are very acidic, which
allows plants like cranberries and
blueberries to grow.
Also, the acidic conditions preserve animal
bodies—several humans who lived up to
5000 years ago have been dug out of peat
bogs.
Bryophyte Life Cycle
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The haploid gametophyte plant
bodies are either male or female.
Each produces a different kind of
gamete (eggs or sperm).
The sperm are motile: they swim
through drops of water (rain or
dew) to reach the eggs. The eggs
are encased within the female
gametophyte’s body.
After fertilization, the diploid
sporophyte grows as a stalk out of
the female gametophyte’s body.
After the diploid sporophyte
matures, the cells in it undergo
meiosis, forming haploid spores.
The haploid spores disperse in the
wind, and go on to form new
gametophyte plants.
Seedless Vascular Plants
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The seedless vascular plants include
ferns and horsetails.
A vascular system to distribute
nutrients throughout the plant allows
them to grow tall. Some ferns grow up
to 80 feet tall, and some extinct
horsetails were also tree-sized.
Being seedless means that the diploid
sporophyte grows out of the fertilized
egg, attached to the gametophyte.
The diploid sporophyte is much larger
than the haploid gametophyte stage:
most of what you see in these plants is
the sporophyte.
The sperm have flagella and swim to
the eggs through drops of water (just
like the bryophytes).
Fern Life Cycle
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The main plant body in the diploid
sporophyte. Specialized
structures on the underside of the
leaves develop, and inside them
meiosis occurs.
The haploid meiotic products are
released as spores, which are
dispersed to new locations and
germinate into gametophytes.
The haploid gametophytes are
quite small, a few millimeters in
diameter. They contain structures
that produce sperm and eggs.
The sperm swim to the eggs and
fertilize them
The fertilized eggs are diploid, and
they grow into the sporophyte
plant body.
Seeds and Pollen
• A major development in plant
evolution was the development
of pollen grains and seeds.
• Pollen grains are the male
gametophyte packaged in a
hard coat that allows it to reach
the female without having to
swim through water. This is a
large advantage on dry land.
• Seeds are diploid sporophyte
embryos, packaged to survive
a period of dormancy and bad
environmental conditions.
Seeds develop from the
fertilized egg.
Gymnosperms
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Gymnosperms were the first
plants to have pollen grains and
seeds.
Gymnosperm means “naked
seed”: their seeds develop on the
outside of the plant, instead of
inside an ovary as in the flowering
plants.
The most important gymnosperms
today are the conifers: pines,
redwoods, cedars, etc. All are
woody plants with needles or
scales as leaves.
Conifers are our main source of
wood and paper.
Ginkos and cycads are other
gymnosperms.
Angiosperms
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Angiosperms are flowering plants.
Most of the plants we see are
angiosperms.
Unlike the other plant groups,
angiosperms are often fertilized with
the aid of animals: insects, birds, bats,
that carry the pollen from one plant to
another. The plants and their
pollinators have co-evolved in a
symbiotic relationship.
Flowers produce the visual signals
and the scents that pollinators use to
find the plants. Flowers secrete nectar
which is eaten by the pollinators. The
pollen is carried from flower to flower
on the body of the pollinator, as a
consequence of its going into the
flower in search of nectar.
Some angiosperms have winddispersed pollen. Flowers on these
plants are usually small and
inconspicuous.
Angiosperm Gametophyte Stage
• The haploid gametophyte
stage in angiosperms is very
short and small.
• The male gametophyte is the
pollen grain. Pollen is
produced by the anthers. Each
pollen grain contains 3 haploid
cells, one to run the
metabolism of the pollen grain
and the other 2 (sperm cells) to
fertilize the egg.
• The female gametophyte is the
ovule. It is buried in an ovary,
inside the plant body. It has 8
haploid cells. Three of them
are used in fertilization.
Angiosperm Fertilization
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The plant body is the diploid sporophyte.
Meiosis occurs in separate organs to produce male and female gametophytes.
The male gametophyte, the pollen, lands on the stigma of the flower. The pollen
grain then grows a tube down to the ovule, the female gametophyte.
The 2 sperm nuclei migrate down the pollen tube and enter the ovule.
One sperm fertilizes one of the ovule cells, producing a diploid embryo.
The other sperm fertilizes 2 ovule cells, producing the triploid endosperm.
Endosperm acts as a nutritive tissue for the developing seed, much like yolk does for
animal eggs.
Seed Development
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After fertilization, the egg and
endosperm grow and develop into a
very small plant surrounded by
protective layers.
The seed then stops development and
dries out: it becomes dormant. At this
point it is released from the parent
plant and dispersed to a new location.
Seeds can remain dormant for a long
time: the current record is about 2000
years for some lotus seeds.
When the dormant seed finds proper
conditions, it breaks out of dormancy,
takes on water, and germinates.
The embryonic plant grows its shoot
downward, way from the light, and its
shoot upward, towards the light.
When light reaches the shoot,
chlorophyll synthesis starts and the
plant turns green, starting
photosynthesis.
Angiosperm Life Cycle
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Most of the angiosperm’s life is
the diploid sporophyte stage.
The male gametophyte is the
pollen grain; the female
gametophyte is the ovule.
Angiosperms have double
fertilization: 2 sperm fertilize
different cells in the ovule,
producing the diploid embryo and
the triploid endosperm.
The embryo develops into a seed,
a small immature plant, which
goes into a dormant phase.
The seed germinates, putting our
a root and a shoot. The shoot
turns green and starts
photosynthesis when light hits it.