Download Plant Evolutionary Trends

Plant Evolutionary Trends
• Plants are thought to have evolved from the green algae, which live in the
water.
• 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 one 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.
– 4. Flowers and fruits used to attract animals to help spread pollen and
offspring.
Major Plant Groups
• We are going to briefly
examine several groups that
show these trends:
– 1. bryophytes: non-vascular
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
• 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) at the tip of
the plant body.
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.
They are multicellular: small
plants that need very little
growth to live independently.
Gymnosperms
• 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.
– Cycads were the dominant plant
type in the Mesozoic era
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 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.
Angiosperm Groups
• Flowering plants used to be split
into 2 groups: monocots and
dicots.
• More recently it has become
clear that several groups split
off from the main evolutionary
lineage before the monocots
did.
• Now, we can divide the
angiosperms into 3 main
groups: the basal angiosperms,
the monocots, and the
eudicots.
--basal angiosperms are not a single
unified group. We are just
throwing them together for
convenience.
Basal Angiosperms
• Most basal, meaning the earliest to split off from
the main lineage: Amborella. A group of shrubs
growing on the island of New Caledonia in the
Pacific Ocean east of Australia.
• Magnolia and relatives is the largest group of basal
angiosperms. Several useful ones: nutmeg, bay
laurel, cinnamon, avocado, black pepper.
• Water lilies are another group of basal
angiosperms.
Monocots
• Monocots are a very large group.
– One cotyledon leaf. The cotyledons are the leaves
found in the seeds that push up above the soil
when the seed imbibes water and starts to grow.
– Parallel leaf veins
– Flower parts in groups of 3
– Scattered vascular bundles. Means there are no
woody monocots.
• Main groups: grasses, lilies, orchids, palms,
onions.
Eudicots
• The largest group of plants today.
• Many groups, mostly of interest only to botanists.
• We will often speak of plant families. A few
examples:
– Nightshade family: tomato, potato, tobacco,
capsicum pepper
– Rose family: apples, cherries, strawberries
– Legume family: peas, beans