Lecture 24: Plant Reproduction &
Covers Chapter 44
• AS mentioned in Lecture 22, what separates
plants from their ancestors is the multicellular
• Think of it as being very similar to
humans….sperm meets egg, fertilization,
development into a new organism AND,
importantly, the new plant embryo develops
ON the parent plant (attached to and
dependent upon…just like us!)
Plant Sexual Life Cycle is complex
• Called alternation of generations: sexual life
cycle alternates between diploid and haploid
stages that give rise to each other
– The diploid form is called a sporophyte (this is the
plant body that you see)
– The haploid form is called a gametophyte (not a
separate plant, but a part of the cycle that
involves only the reproductive parts of the plant)
Sporophyte/gametophyte are not names for the plants themselves, just
the STAGE of the life cycle!
• Before we go over the process, let’s take a
close look at the flower
Closeup on the flower*
• Sexual reproductive structures of angiosperms
(they are actually modified leaves):
– Sepals: green leaves at base of flower
– Petals: usually colorful (ATTRACT POLLINATORS)
– Sporangia: sex organs
• Stamen: male reproductive structure
– Anther: area where pollen is produced
– Filament: stalk that holds anther
• Carpel: female reproductive structure
– Stigma: sticky structure at top of carpel
– Style: tube connecting stigma and ovary
– Ovary: bulb-like structure at bottom of style that contains one or
more eggs/ovules (Fruit is a mature ovary with seed inside!)
• Male gamete: SPERM/POLLEN/POLLEN GRAIN
• Hundreds of thousands in each anther
• Each pollen grain is TWO sperm and a tube cell surrounded by a tough,
• One sperm will fertilize female egg, other will contribute to endosperm:
food storage tissue that nourishes the embryo.
• Female gamete: egg/ovule
– Egg is the actual gamete
– Egg ENCLOSED in a nutritive food layer (for future embryo) is called an OVULE
– Each ovary can hold one or many ovules, depending on type of plant
– Each ovule has numerous eggs in it.
How are gametes formed?
• Each diploid cell in ovule & anther will do
meiosis FIRST, producing haploid spores.
– Male: Each spore will then do one round of
mitosis, resulting in two gametes enclosed in a
– Female: Three of four eggs from meiotic division
will degenerate, leaving one egg. This egg will do 3
rounds of mitosis, and result will be one egg that
will be fertilized and all others will contribute to
Pollinators & Plants
• Animals and insects help plants to reproduce
• Pollinators will transport pollen from one plant to another.
• Flowers have evolved to attract pollinators and pollinators
have co-evolved to be more effective at getting pollen!
– Some flowers look like insects (to draw insects in)
– Some flowers provide a place for insects to lay eggs
– Some flowers have evolved structures to make it easier for
pollinators to access pollen
– Pollinators have evolved body structures that allow best
access to pollen
Sexual Deception Promotes Pollination
• Sporophyte (diploid) plant grows and matures, developing
male AND female sporangia ON THE SAME PLANT.
• Cells in anther and ovary perform meiosis (making spores)
and then spores do mitosis, resulting in pollen grains and
ovule(s) with eggs inside.
– THIS IS THE GAMETOPHYTE STAGE!
• Pollen grain (from same plant or perhaps another) lands on
• Tube cell in pollen grain burrows down the style (making a
pollen tube) and merges with egg in the ovule.
• Result is diploid zygote.
• Zygote develops into an embryo: a seed!
Pollination and Fertilization of a Flower
tube cell nucleus
a pollen grain
lands on the
stigma of a
the style of the
carpel to the
ovary; the tube
travels at the
tip of the tube,
and the two
fuses with the
fuses with the
• Seed contains embryo (diploid organism that is
developing-but only so far) and endosperm- nutritive
layer made of unfertilized eggs and extra sperm from
• Seed will be dropped from plant, blown by the wind,
picked up by an animal, or become encased in the
ovary (fruit) depending on plant type.
• Once seed “lands” in an area appropriate for growth,
it will germinate.
• We have a new sporophyte!!
• Germination: embryo breaks out of seed and forms a seedling. This
happens after a dormant period
– Dormant period keeps seeds from breaking out of ripe fruit (dormant
period ends after fruit rots/dries out)
– Three criteria determine period of dormancy:*
• *Drying: some seeds must be dried out before they will germinate
• *Cold: some seeds require exposure to cold for a period of time,
then warmer temps before germination
• *Seed Coat Disruption: some seeds require the coat be disrupted
• Fruit is a ripe ovary with seeds inside.
• Packaging the embryo in fruit allows embryo to get far away from the
mother plant…..better to spread DNA! Genetic diversity!
• Only ripe fruit containing mature seeds are sweet and brightly colored
• Some fruits:
– Explode and disperse seeds
– Float in water to other areas
– Float in the wind (really lightweight fruit)
– Have evolved to be edible and attract herbivores (eat fruit, travel,
poop out seeds)