Plant Diversity: The Evolution of Seed Plants
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A seed consists of a sporophyte embryo
packaged along with a food supply within a
protective coat
All seed plants are heterosporous (two types
of spores)
Layers of tissues called integuments wrap
around the ovule, which contains all the
female reproductive parts
◦ Integument turns into the seed coat
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Microspores develop into pollen grains, which
mature to become the male gametophytes of
seed plants
Pollen can be carried away by wind or animals
Pollination is the transfer of pollen to the
ovule
◦ It will elongate a tube that discharges sperm into
the female gametophyte
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In seed plants, the use of resistant, fartravelling, airborne pollen to bring gametes
together is a great adaptation
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Angiosperms
Gymnosperms
Evolved from a common ancestor called
progymnosperms
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Most familiar are pine trees
Gymnosperms lack the enclosed chambers
(ovaries) in which angiosperm ovules and
seeds develop
Ovules and seeds develop on the surfaces of
specialized leaves called sporophylls
These are the plants the dinosaurs ate!
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Ginkgo
◦ Ginkgo biloba is the only one left
 Supposedly increases memory
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Cycads
◦ Resemble palms
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Gnetophytes
◦ Three genera that are very different
 One has giant strap-like leaves, some are trees or
vines, another is a desert shrub
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Conifers
◦ Largest of the four
◦ Term literally means to carry cones
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Most are evergreen
◦ Do not lose leaves or needles during winter
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Most of our lumber comes from conifer trees
Are among the oldest and largest organisms
on Earth
◦ Redwoods can grow more than 110 meters high
◦ One bristlecone, named Methuselah, is more than
4,600 years old
 Was a young tree when humans invented writing
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The pine tree is a sporophyte
Its sporangia are located on sporophylls located
in cones
From the time a young cone appears on a tree, it
takes nearly three years to produce the male and
female gametophytes, bring them together via
pollination, and form mature seeds from the
fertilized ovules
Scales then separate and the seed travels on the
wind
If it lands in a habitable place, it germinates and
the embryo emerges as a pine seedling
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Flowering plants
By far the most diverse and widespread of all
plants
◦ 250,000 species
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Used to be divided into two clades
◦ Monocots
◦ Dicots
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Today, there are three: monocots, dicots, and
eudicots
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Recently discovered plant in the New
Caledonia, a South Pacific Island
Thought to be the most primitive known
angiosperm
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Refinements in vascular tissue, especially
xylem, played a role in the spread of
angiosperms into diverse habitats
They have tracheid cells that function in both
mechanical support and water transport
There are also vessel element cells that are
shorter and wider and more efficient in
transporting water
Reproductive changes were the biggest help!
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The flower is a specialized reproductive organ
in angiosperms
Insects and other animals transfer pollen
from one flower to female sex organs on
another, which makes pollination less random
than the wind-dependent pollination of
gymnosperms
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A flower is a specialized shoot with four
circles of modified leaves; sepals, petals,
stamens, and carpels
Starting at the bottom of the flower are the
sepals ,which are usually green. They are
modified to hold the flower before it opens
Petals are above the sepals and aid in
attracting insects and other pollinators
◦ Flowers that are wind-pollinated generally lack
bright flowers
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Stamens are the male reproductive organs
◦ Consists of a stalk called a filament and a terminal
sac, the anther, where pollen is produced
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Carpels (or pistils) are the organs that make
the female gametophytes
◦ At the tip of the carpel is a sticky stigma that
receives the pollen
◦ A style leads to the ovary at the base of the carpel
◦ Within the ovary are the ovules, which develop into
seeds after fertilization
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A fruit is a mature ovary
As seeds develop from ovules after
fertilization, the wall of the ovary thickens
Fruits protect dormant seeds and aid in their
dispersal
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Some have seeds within fruits that function
like kites or propellers that enhance dispersal
by wind
◦ Dandelions
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Some have fruits modified as burrs that cling
to animal fur
◦ Cockleburs
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Others produce edible fruits that are
nutritious, sweet, and vividly colored that
advertises their ripeness to certain animals
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Begins to develop after pollination triggers
hormonal changes that cause the ovary to
grow
The wall of the ovary becomes the pericarp,
the thickened wall of the fruit
As the ovary grows, the other parts of the
flower wither away
If a flower has not been pollinated, fruit
usually does not develop, and the entire
flower withers and falls away
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Simple fruit are derived from a single ovary
◦ May be fleshy, such as a cherry, or dry such as a
soybean
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Aggregate fruit result from a single flower
that has many carpels
◦ Blackberries
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Multiple fruits develop from a group of
flowers tightly clustered together
◦ Pineapple
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Heterosporous
The immature male gametophytes are
contained within pollen grains, which develop
within the anthers of stamens
Each pollen grain has two haploid cells
Ovules, which develop in the ovary, contain
the embryo sac
◦ Consists of a few cells, one of which is the egg
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Pollen is carried to the sticky stigma at the
top of the carpel
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Most mechanisms ensure cross-pollination
Pollen grain germinates after it adheres to the
stigma
Pollen grain extends a tube that grows down
within the carpel
Reaches the ovary and penetrates through a pore
in the integument of the ovule, and discharges
two sperm cells
One nucleus unites with the egg, forming a
diploid zygote
The other sperm nucleus fuses with two nuclei in
the large center of the female gametophyte
◦ Double fertilization
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The ovule matures into a seed, with
cotyledons (seed leaves)
The triploid nucleus in the center of the sac
divides repeatedly and gives rise to the
endosperm, rich in starch and other food
reserves
◦ Corn store most of their food as endosperm
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Coevolution is the mutual evolutionary
influence between two species
Pollinator=plant relationships are partly
responsible for the diversity of flowers
◦ Flowers of a particular plant species may be
adapted for attracting insects rather than birds
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Flower color, fragrance, and structure reflect
specialization for a particular group of
pollinators
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Has a tripping mechanism that arches the
stamens over the bee and dusts it with pollen
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Hummingbird’s long, thin beak and tongues
enable it to probe flowers that secrete nectar
deep within the flower tubes
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This baobab tree depends on nocturnal
pollinators; include large light-colored, highly
fragrant flowers that nighttime pollinators
can locate
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Agriculture
◦ Flowering plants provide nearly all our food
◦ Fruit and vegetables are angiosperms
◦ Corn, rice, wheat, and other grains are grass fruit
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Plant diversity is nonrenewable
◦ We are extinguishing plant species at a fast rate
 Slash and burn clearing of forests for agriculture use
 50 million acres (size of Washington) are cleared yearly
 No more rainforests in 25 years
 100’s of species each year are becoming extinct in rain
forests
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More than 25% of prescription drugs are
extracted from plants
Only researched less than 5,000 plant species
◦ Pharmaceutical companies looked at these after
being led to them by native people that used them
in traditional medicine
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Pupil Dilator in eye exams
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Heart medication
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Malaria preventative
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Ovarian Cancer Drug
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Muscle Relaxant During Surgery
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Leukemia drug
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Earliest Aspirin