Transcript Plant Reproduction - mvhs

Plant Reproduction
AP Biology
Unit 5
Alternation of Generations
• All land plants can exist in
two different multicellular
forms
– Sporophyte (2n)
– Gametophyte (n)
• In a life cycle, plant
generations alternate between
sporophyte and
gametophyte
• Each form gives rise to the
other form
Sporophyte vs. Gametophyte
• Sporophyte (2n)
– Diploid, multicellular form
of a plant
– Forms haploid spores
through meiosis
• Gametophyte (n)
– Haploid, multicellular form
of a plant
– Forms gametes through
mitosis
Alternation of Generation
How is this different from other
organisms?
• In other organisms (like humans) the
haploids are unicellular (sperm and egg)
• In plants both forms (sporophyte and
gametophyte) can be multicellular
Evolution of Plant Reproduction
• Each type of plant has a
dominant form
(sporophyte or
gametophyte)
• Dominant form = the
“plant” we think of
• As plants have evolved,
there have been changes
to relationship between
sporophyte and
gametophyte
Gametophyte/Sporophyte
Relationships
• 3 different possibilities
– Larger gametophyte,
sporophyte is dependent on it
– Larger sporophyte,
gametophyte is independent
– Larger sporophyte,
gametophyte is dependent
Question…
• What trend do you see between the
evolution of plants (from nonvascular 
vascular; nonseed  seed plants) and the
alternation of generations?
– As plants have evolved, the gametophyte has
become more reduced, sporophyte has become
more dominant
Reproductive Organs
• Archegonium
– Female sex organ  produces egg
• Antheridium
– Male sex organ  produces sperm
Vascular Plants: Homosporous vs.
Heterosporous
• Vascular plants have
sporophylls
– modified leaves that produce
sporangia (which become
spores)
• Homosporous plants
– Most ferns
• Heterosporous plants
– All seed plants (angiosperms
and gymnosperms) and some
seedless plants
Water vs. Wind
• Mosses and Ferns require water to help the
sperm reach the egg.
• Seed plants no longer require water to
reproduce
– Pollen (male gametophyte) can be carried by
wind, animals, etc.
Gymnosperm Life Cycle
• Sporophyte is the dominant
form
• Heterosporous
• Have “naked” seeds –
nothing around the seed
• Cones are either male
(contain pollen) or female
(contain ovule)
Flower Anatomy
• The flower is the
reproductive organ of an
angiosperm
• Female part = Carpel
– Consists of the stigma,
style, and ovary
• Male part = Stamen
– Consists of anther and
filament
Angiosperms
• Dominant form is
the sporophyte
• Heterosporous
– Male gametophyte
= pollen grain
– Female
gametophyte =
embryo sac
Female Gametophyte
• The megaspore will go
through MITOSIS to
create a series of
different cells in the
female gametophyte
• Cells involved in
fertilization are
– Egg (n)– haploid
– Polar nuclei (2n) – 2
nuclei in one cell
When the pollen grain
lands on the stigma, it
will extend a pollen
tube down the style to
get to the egg
Specific cells in the
pollen tube go
through mitosis to
form 2 sperm
Sperm will enter
Double Fertilization
• Angiosperm fertilization
actually consists of 2 separate
fertilization events = double
fertilization
• Both sperm in the pollen tube
are used
– One sperm fertilizes the egg 
diploid zygote (2n) forms
– The other sperm fertilizes the
polar nuclei (2n)  forms
triploid endosperm (3n)
Double Fertilization
• Double fertilization is unique to
angiosperms
• Through this process, resources are used
most efficiently since endosperm can only
develop in a fertilized ovule.
• Endosperm = starchy material that serves as
nutrition for the growing embryo
Other Post-Fertilization events
• Embryo develops (diploid)
• Seed coat forms
• Ovary develops into fruit around the seed
Germination and Seedling Formation
• Seeds are dormant until they
begin germination
• When conditions are right,
seeds will begin to germinate
to form a seedling.
• Germination starts with
imbibition—when water enters
the seed and triggers enzymes
that are needed for growth.
Seed Benefits
• What are the evolutionary benefits of seeds?
– Dispersal- embryo can travel far from parent
– Protection- seed coat protects embryo
– Dormancy- can wait until conditions are
favorable to germinate
– Storage- food for the embryo can be stored
inside seed (ex. endosperm)
Monocots vs. Eudicots
• Most angiosperms can be classified as
either Monocots or Eudicots