Transcript Reproduction in Angiospermophytes

Reproduction in
Angiospermophytes
Topic 9.3
Assessment Statements
9.3.1 Draw and label a diagram showing the structure of a
dicotyledonous animal-pollinated flower.
9.3.2 Distinguish between pollination, fertilization and seed
dispersal.
9.3.3 Draw and label a diagram showing the external and
internal structure of a named dicotyledonous seed.
9.3.4 Explain the conditions needed for the germination of
a typical seed.
9.3.5 Outline the metabolic processes during germination
of a starchy seed.
9.3.6 Explain how flowering is controlled in long-day and
short-day plants, including the role of phytochrome.
Flowers
• Reproductive structures of
angiospermophytes
• Dependent upon animals for
pollination
• Can grow as large as 3 feet in
diameter and weigh as much
as 14.5 lbs.
• Rafflesia arnoldii
• When mature, this flower
smells like rotting meat thus
attracting flies that transfer
pollen from the male
reproductive structures to the
female structures
Flower structure and function
Flower part
Function
Sepals
Protect the developing flower while in
the bud
Petals
Often are colorful to attract pollinators
Anther
Part of stamen which produces the
male sex cells, pollen
Filament
Stalk of stamen that holds up the
anther
Stigma
Sticky top of carpel on which pollen
lands
Style
Structure of the carpel that supports
the stigma
Ovary
Base of carpel in which the female sex
cells develop
• Carpel (entire female
part)
• Stamen (entire male part)
• Complete – contain
sepals, petals, stamen,
and carpal
• Incomplete – lack at
least one part
• Staminate – have only
stamens
• Carpellate – have only
carpels
Alternation of generations
• All plants show two different generations in their life
cycle:
• Gametophyte generation which is haploid
• Sporophyte generation which is diploid
• Gametophyte generation produces plant gametes by
mitosis
• Sporophyte generation produces spores by meiosis
• For example, a cherry tree is in the sporophyte form (it
grew from a zygote and produces new cells by mitosis).
When the cherry tree produces flowers, haploid spores
are formed and develop into haploid bodies referred to
as gametophytes. Sperm form within the male
gametophytes, and eggs form within the female
gametophytes.
Pollination
• Process by which pollen
(containing male sex
cells) is placed on a
female stigma
• First step towards
fertilization and the
production of seeds
• Common vectors: wind,
insects, birds, water, and
other animals
• Means of attraction
• Red flowers are
conspicuous to birds
• Yellow and orange flowers
are noticed by bees
• Heavily scented flowers are
easily found by nocturnal
animals
• Plants that rely on wind
have inconspicuous,
odorless flowers
Types of pollination
Self-pollination
• Pollen from the anther of
the same plant falls onto
its own stigma
• Form of inbreeding and
results in less genetic
variation within a species
Cross-pollination
• Pollen is carried from the
anther of one plant to the
stigma of a different plant
• Increases variation and
may result in offspring
with better fitness
• Problem: distance
Fertilization
• Pollen grain adheres to the stigma, which is covered by a sticky,
sugary substance
• Pollen germinates to produce a pollen tube
• Pollen tube grows down the style of the carpel
• Within the growing pollen tube is the nucleus that will produce the
sperm
• Pollen tube completes its growth by entering an opening at the
bottom of the ovary
• Sperm moves from the tube to combine with the egg of the ovule to
form a zygote
• Zygote develops with the surrounding tissue into the seed
• As the seed is developing, the ovary around the ovule matures into
a fruit
• The fruit encloses and helps to protect the seed
The seed (means by which an embryo can
be dispersed to distant locations)
Seed part
Function
Testa
Tough, protective
outer coat
Cotyledons
Seed leaves that
function as
nutrient storage
structures
Micropyle
Scar of the
opening where
the pollen tube
entered the ovule
Embryo root
(radicle) and
embryo shoot
(epicotyl)
Become the new
plant when
germination
occurs
Plumule will become first leaves.
Hilum is where the seed was
attached to the ovary.
Endosperm provides nutrition for
growing embryo.
Maturation
• Dehydration until water
content of the seed is
about 10% - 15% of its
weight
• Seed enters dormancy
(low metabolism, not
growth or development)
• Adaptation to overcome
harsh environmental
conditions
• Conditions needed for
germination:
• Water
• Oxygen for aerobic
respiration
• Appropriate temperature
for enzyme action
• Other (testa disrupted, fire
exposure)
• Most will not become a
functional plant so plants
produce large numbers of
seeds
Seed metabolism during germination
1. Uptake of water
2. Gibberellin is released
3. Gibberellin (growth hormone) triggers the production of
the enzyme amylase
4. Amylase causes the hydrolysis of starch into maltose.
The starch is present in the seed’s endosperm
5. Maltose is further hydrolyzed into glucose that can be
used for cellular respiration or may be converted into
cellulose by condensation reactions.
6. Cellulose is then used to produce the cell walls of new
cells being produced
Control of flowering in angiosperms
• Photoperiodism – plant’s response to light involving the
relative lengths of day and night (a very important factor
in the control of flowering)
• A plant must flower when pollinators are available and
when necessary resources are plentiful.
Plant-type
Flowering and light
Examples
Long-day plants
Bloom when days are
Radishes, spinach, lettuce
longest and nights shortest
(midsummer)
Short-day plants
Bloom in spring, late
Poinsettias,
summer, and autumn when chrysanthemums, asters
days are shorter
Day-neutral plants
Flower without regard to
day length
Roses, dandelions,
tomatoes
Phytochrome
• Phytochrome is a photoreceptor, a pigment that plants
use to detect light.
• There are two forms: Inactive (Pr) and active (Pfr)
• When red light (wavelength of 660 nm) is present in
available light, the inactive form Pr is converted into the
active form Pfr which has the ability to absorb far-red
light (wavelength of 730 nm).
• This Pfr is rapidly converted back to the inactive form in
daylight.
• However, in darkness, the conversion is very slow.
• It is thought that this slow conversion of Pfr back to Pr
allows the plant to time the dark period.
• In long-day plants, the remaining Pfr at the end of a short
night stimulates the plant to flower. In this case, Pfr acts
as a promoter
• In short-day plants Pfr appears to act as an inhibitor of
flowering. For these short-day plants, enough Pfr has
been converted to Pr to allow flowering to occur.
• Even though the names refer to day length, it is actually
the length of night that controls the flowering process.