Download Reproduction of Seed Plants

Reproduction of Seed Plants
Reproduction of Seed Plants
• Remember that seed plants are very well
adapted to life on land – they don’t need
water for fertilization as ferns and mosses and
other seedless plants do. Having seeds and
either cones or flowers allows gymnosperms
and angiosperms to reproduce almost
anywhere.
Alternation of Generation
• In seed plants, the dominant and recognizable
form of the plant is the diploid sporophyte.
The gametophyte in a seed plant is found
inside the cones of gymnosperms and inside
the flowers of angiosperms.
Life Cycle of Gymnosperms
• Reproduction in Gymnosperms takes place in
cones produced by the mature sporophyte.
Pollen Cones
• Male gametes are produced in pollen cones
(also called male cones). These pollen cones
produce the male gametophytes, which are
called pollen grains – this is the entire male
gametophyte generation.
Seed Cones
• Female gametophytes are produced in seed
cones; which are the larger cones we are
familiar with. 2 ovules are produces at the
bottom of each scale of the cone, and the
female gametophytes develop inside these
ovules. When mature, each female
gametophyte contains a few large egg cells,
each of which can be fertilized by a sperm
nucleus from the pollen.
Pollination
• In spring, male pollen cones produce huge
numbers of pollen grains.
• Pollen is carried by the wind, and some will
reach female cones, where they get stuck to a
sticky secretion (called pollination drop) on
the scales. Pollination drop helps to ensure
that the pollen stays on the female cone so
fertilization can occur.
Fertilization
• Pollen grain lands near an ovule, the pollen
grain splits open and produces a pollen tube
– Pollen tube contains 2 haploid sperm nuclei
– When it reaches the female gametophyte, 1
sperm nuclei fertilizes it, and the other sperm
nuclei disintegrates
• If a sperm from a different pollen tube reaches the
female gametophyte, more than 1 egg can be fertilized,
but only 1 will develop into an embryo.
Fertilization
• At fertilization, a diploid zygote is formed –
this is the first cell of a new sporophyte plant
– Zygote divides by mitosis to develop into an
embryo
Development
• The seed starts to form; it has made up of
tissues from a combination of 3 generations
– The outer seed coat is from the old sporophyte
generation
– The embryo is the new sporophyte generation
– Between these is a layer of haploid cells from the
female gametophyte
Structure of a Flower
• We think of flowers as decoration, but really,
they are the reproductive organs of
angiosperms.
Sepals and Petals
• Sepals are the small, usually green, leaf-like
structures that cover the bud before it opens
and protect the flower when it is developing.
• Petals are found just inside the sepals, and
they are modified leaves which are usually
brightly coloured to attract pollinators.
Stamens and Carpels
• Moving inward from the petals, we come to the
structures that produce the gametophytes.
• The male structure is the stamen, which is made
up of two parts:
– Filament – a long thin stalk that supports the anther
– Anther – an oval sac at the top of the filament, this is
where meiosis occurs to produce the haploid male
gametophytes (pollen grains).
• If you touch a flower, you may get yellow-gold dust on your
fingers – this is pollen, made up of thousands of individual
pollen grains.
– Most flowers contain several stamens
Reproduction Song – Grease 2
• https://www.youtube.com/watch?v=x3OQaa
mLsSo
Stamens and Carpels
• The female part is found in the middle of the
flower, and is called the carpel (or the pistil).
Parts of the carpel:
– Ovary – the broad base of the carpel, it contains the
ovules, where the female gametophytes are
produced.
– Style – the narrow stalk that forms the upper portion
of the carpel
– Stigma – the sticky portion at the top of the style,
where pollen grains land
• Some flowers have multiple carpels fused
together into a compound carpel.
Stamens and Carpels
• Most plants have flowers that produce both
the male and female gametophytes.
• However, in some plants the two types of
gametophytes are produced in separate
flowers on the same individual
• In some plants many flowers grow together
into a composite flower
– Example: sunflowers
Composite Flower
Life Cycle of Angiosperms
• Reproduction in angiosperms occurs within
the flower. After pollination and fertilization,
the seeds develop inside protective
structures.
• The mature sporophyte produces flowers. In
most cases, each flower contains both anthers
and an ovary
Producing Pollen
• In the anthers, the haploid spores are
produced meiosis, which each develop into a
male gametophyte (pollen grain)
– The pollen grain develops a hard wall to protect it
from drying out
– The nucleus of the pollen grain goes through
mitosis once to produce two haploid sperm nuclei
– The pollen grain stops growing until it is deposited
on a stigma
Producing The Egg
• The female gametophyte is produced in the
ovules, which are contained in the ovary.
– A diploid cell undergoes meiosis to produce 4
haploid cells. Only 1 survives, and it then goes
through mitosis to produce eight nuclei in a single
membrane, called the embryo sac.
• This embryo sac is the female gametophyte.
• One of the nuclei becomes the egg nucleus – the
female gamete
Zygotes
• If fertilized, this egg will form part of the
zygote that will grow into the new sporophyte
plant
– The embryo cells begin to differentiate,
developing into specialized cells as part of the
new sporophyte or seedling.
Pollination
• Now that the male and female gametophytes
have been produced, it is time for pollination
to occur.
• Where most gymnosperms are pollinated by
the wind, most angiosperms are pollinated by
animals, and only a few angiosperms use wind
pollination.
Animal Pollination
• Pollinators are animals that carry pollen from
one flower to another; most pollinators are
insects, birds, and bats
Animal Pollination
• Animal pollination is much more efficient
than wind pollination (A great example of
mutualism)
– Wind pollinated plants must produce large
quantities of pollen, and depend on weather
conditions for pollination to occur  lots of
wasted pollen
– Animal pollinated plants have many adaptations
to improve the effectiveness of pollination
• Brightly coloured petals to advertise to pollinators
• Sweet nectar to reward pollinators
Attracting Pollinators
• Some have specific shapes or behaviours
depending on their main pollinator species
– A “landing pad” for insects
– Long corollas for hummingbirds
– Blooming at night for bats
 Insect pollination may be the factor that is
mostly responsible for angiosperms outcompeting gymnosperms over the last 100
million years!
Benefit for Pollinators
• What’s in it for the pollinator?
– Provides a dependable source of food for the
pollinator  Nectar, pollen, or both
Fertilization in Angiosperms
• A pollen grain lands of
the stigma of a flower of
the same species, and
begins to grow a pollen
tube.
• The pollen tube grows
down into the style, and
all the way to the ovary
and into the ovule.
Two Fertilizations
• The pollen tube contains two sperm nuclei,
and then two distinct fertilizations take place
– 1 sperm nucleus fuses with the egg nucleus to
produce the diploid zygote
– The other sperm nucleus fuses with two polar
nuclei (extra cells that are produced by meiosis
when the egg nucleus is formed), to form a
triploid (3N) cell
• This triploid cell will grow into the endosperm, a foodrich storage tissue which provides nourishment to the
embryo and seedling as it grows
Double Fertilization
• This process is known as double fertilization
• Thought to be one of the reasons that
angiosperms have been so successful
– In gymnosperms the food reserve is built up
before fertilization
 if fertilization doesn’t occur, the food in that
ovule is wasted!
– Angiosperms are more efficient, since food isn’t
stored for the seed until fertilization has occurred
 no wasted food.
Crash Course: Plant Reproduction
• https://www.khanacademy.org/partnercontent/crash-course1/crash-coursebiology/v/crash-course-biology-137 (9:46)
Seed & Fruit Development
• Once fertilization has occurred, the parent plant
sends nutrients to the flower to support
development of the seed.
• In Angiosperms, the ovary walls thicken and form
a fruit around the developing seeds.
• Parts of the ovule become tougher and form the
seed coat (outer protective layer of the seed,
enclosing the embryo and the food supply).
• The ovary wall thickens and may join with parts
of the stem, to form the fleshy part of the fruit.
Fruits
• Biologically speaking a fruit is any seed that is
enclosed within the ovary wall.
– Examples: apples, grapes, peas, corn, beans, rice,
tomatoes
Types of Fruits
• There is a wide variety of types of fruits
– Fleshy tissues: grapes, tomatoes
– Tough tissues: bean pods
– Inner wall of ovary attached to the surface of the
seed: peaches, cherries
– Dry fruit in an aerodynamic shape: maples
Seed Dispersal Video
• https://www.youtube.com/watch?v=xY4JFOSu
qvY (12 min)
• 10 Things - for your logbook
– Write down at least 10 things that you found
interesting or important, from the video
• If time: Exploding Seeds!
https://www.youtube.com/watch?v=nHqHSpZ
hjeY (2:30)
Dispersal by Animals
• Seeds with (usually) sweet, fleshy fruits are
intended to be eaten by animals
– These seeds usually have a tough coating to
protect the seed from the harsh digestive
chemicals in the digestive tract of the animals
– Then the seeds are deposited in another distant
location, in a patch of the animal’s feces, which
provide a natural fertilizer
Dispersal by Animals
• The sweet, nutritious fruits provide an
important source of food for animals, and the
animals help disperse the seeds longer
distances than the plants could manage on
their own
 Another example of mutualism
Dispersal by Wind and Water
• Lightweight seeds are specialized to be
dispersed by wind or water
– Wind:
• Ash and maple seeds have wings
• Tumbleweeds break off at their roots and tumble along
the ground, scattering their seeds as they go.
Dispersal by Water
• Coconuts are a large seed filled with the liquid
endosperm (the coconut milk), which helps it
to be buoyant enough to float in seawater for
up to weeks
– This has allowed coconuts to spread to remote
islands effectively
Seed Dormancy
• Some seeds germinate almost immediately
when they land in a suitable location
• However many seeds enter a period of
dormancy (where the embryo is alive but not
growing), from a few days to weeks, years or
even decades.
– The oldest known seed that was still able to
germinate was over 2000 years old!
Seed Dormancy
• Seed Dormancy video
– https://www.youtube.com/watch?v=JRc7FAHiNOo
(1:23)
Benefits of Dormancy
• Long-term dormancy can allow for longdistance dispersal (such as coconuts floating
to a new island)
• Can allow seeds to germinate under ideal
conditions (such as spring instead of summer
or winter, or when competition is reduced)
Ending Dormancy
• Environmental factors such as air temperature
and moisture can trigger a seed to end
dormancy and germinate
• Some plants (such as bishop pines and
lodgepole pines) have a resin coating on their
seed cones, and require the high
temperatures of a severe forest fire to open –
this ensures that the seeds will have very little
competition when they germinate.
Seed Germination
• Germination is the early growth of a plant
embryo
• Seeds absorb water
– This water causes the endosperm tissues to swell,
cracking the seed coat open
• Young root emerges through this crack and
starts to grow
Shoot Development
• Development of the shoot varies
– In most monocots, the single cotyledon (seed
leaf) stays underground, and the new shoot
emerges, protected by a sheath
– In dicots, there are 2 main possibilities
• The two cotyledons can emerge above ground and
protect the stem and the first foliage leaves
– The cotyledons will then either wither and drop off, or
become photosynthetic (pumpkin)
• The cotyledons can stay underground, and provide a
food source for the new seedling
Germination
Mung Bean Germination (if time)
• https://www.youtube.com/watch?v=pB4ASdE
LBbQ (3 min)
What is Asexual Reproduction?
• Asexual reproduction is any type of
reproduction that involves only one parent.
• All offspring from asexual reproduction are
genetically identical to the parent and to each
other. These identical offspring are sometimes
called clones.
Advantages
• One advantage of asexual reproduction is that
many offspring can be produced very quickly,
creating a large population.
• When the environment is stable, genetic
variability is not very important, so this can be
a very successful strategy.
Disadvantages
• The main disadvantage is that any factor that
negatively affects the parent will also impact
all of the offspring in the same way.
• This means that a single environmental
change or a new drug or disease can wipe out
the whole population very easily.
Vegetative Reproduction
• The asexual production of genetically identical
offspring (clones) from the parent plant,
without the formation of a seed. Since it does
not require pollination or seed formation, it
allows plants to reproduce very quickly.
• Different plants have can use different
methods of vegetative reproduction, including
stolons, plantlets, and rhizomes.
Stolons
• Stolons are long, trailing horizontal stems
(commonly called runners), which produce
roots when they touch the ground. Once
these new roots are established, a new plant
starts to grow, and the stolon can be broken,
creating a completely independent new plant.
– Strawberry plants are a great example of this.
Plantlets
• Plantlets are tiny new plants that form at the
ends of elongated stems, without needing to
touch the ground. If these plantlets break off
and fall to the ground, or something happens
to the parent plant, they can grow roots and
form a whole new plant.
– Spider plants are very well known for producing
multiple plantlets
Spider Plants
Rhizomes
• Rhizomes are
underground stems
that can send up
multiple new shoots
from beneath the soil.
– Bamboo forests are a
good example of this –
the whole forest can be
a group of clones of the
same original bamboo
plant.
Bamboo Rhizomes
Bamboo growth time lapse video: https://www.youtube.com/watch?v=-aARFhjJ7EA
(2 min)
Plant Propagation for Human Use
• When a plant has traits that are particularly
desirable, often gardeners and
horticulturalists will want to make more plants
with the same traits.
• However simply breeding the plant normally
(sexual reproduction through pollination)
produces additional genetic variation; which
may interfere with the desired trait.
Plant Propagation for Human Use
• Additionally, some food crops have been
specifically bred to produce seedless varieties.
These plants cannot reproduce sexually
through pollination and seed production, so
gardeners must intervene.
• In both of these cases, propagation
techniques are used to produce new plants
through asexual reproduction.
Cuttings
• The simplest way to propagate a plant is using
cuttings.
• The gardener cuts a length of stem including
several buds (with their meristematic tissue),
and then “plants” the cutting into soil.
• Some plants produce new roots easily in this
setting, while others (particularly woody
plants) often require the addition of plant
hormones called rooting powders.
Grafting
• Generally used to reproduce
seedless varieties of plants,
and woody plants that have a
harder time producing roots.
• A piece of stem (or a lateral
bud) is cut from the parent
plant and attached to another
plant. The vascular cambiums
of the two plants must be
firmly connected to each
other, or it will not work.
Grafting
• This procedure is generally most effective if
performed when plants are dormant – wounds
can heal before growth begins.
• The cut piece is called the scion and the plant it is
attached to is called the stock. When the scion is
a stem, this procedure is known as grafting.
When the scion is a bud, it is known as budding.
• This technique can be used to grow more than
one flower or fruit on the same tree or bush
– Fruit salad trees
– Can help preserve endangered species
Fruit Salad Tree
Fruit Salad Tree company: https://www.fruitsaladtrees.com/Home
Tree of 40 Fruit: http://time.com/3086552/tree-of-40-fruit-sam-van-aken/
Plant Tissue Cultures
• Plant cells can be placed in a
petri dish with nutrients and
grow into new individuals.
• Once they have developed
into a seedling, they can be
transplanted into soil and
then continue to grow
normally.
• This can be used to preserve
endangered species of
plants, or to clone plants for
use in experiments.
Agriculture
• When humans started cultivating plants for
their own use, it caused major changes to
human society.
• Later, as explorers visited other parts of the
world, food crops were shared and
transported back home.
• In fact, many of the foods we associate with a
particular country, were actually introduced
from somewhere else relatively recently.
Imported Plants
• Great plains are the “breadbasket” of North
America
– wheat originated in the Middle East
• Tomato sauce is a key part of Italian food
– tomatoes originated in North America
• Potatoes are key to German and Irish food
– potatoes are also from North America
• Rice is now included in Mexican foods such a
burritos
– rice originated in China, spread to Europe, and then
was introduced to Mexico by the Spanish
Crops
• Today thousands of plants (mostly
angiosperms) are used as food crops around
the world. However relatively few crops, are
used to provide most of the food supply (for
humans and livestock) around the world. For
example, in the US over 80% of the cropland is
used for growing wheat, corn, soybeans and
hay.
Crops from Grass
• Interestingly 3 of these (wheat, corn, and hay)
are modified types of grasses. This is an
example of how selective breeding (even by
accident) can have major effects on plants and
on humans.
Homework
• Remember to finish reading all sections from
this chapter, and do the assigned questions
• Logbook Due tomorrow
– Don’t forget the glossary, the 10 things from
yesterday’s video (posted on my website)
• Notebook Quiz tomorrow – Chapter 22