Angiosperm Reproduction

Reproduction, Growth and Development

February 6, 2023
Melissa Robinson
 Intended Learning Outcomes
• To discuss the general plant life cycle: alternation
ofgeneration
• To describe the parts of a flower (complete flowers)
• To discuss the development of the gametophytes and the
differences between microgametophyte and
megagametophyte.
• Describe Post Fertilization changes: the development of the
endosperm, embryo, ovule into a seed and ovary into the fruit
and features of the pericarp that aid in dispersal
• To discuss and describe pollination which include but not
limited to: cross-pollination, self-pollination, stigma and
pollen incompatibility, and monoecious and dioecious species.
 Angiosperms
• Angiosperms are vascular plants with flowers that produce seeds
enclosed in an ovule.

• Also known as flowering plants; flowers are the defining feature

and is the reproductive structure.

• All make seeds are enclosed within the fruit or ovary.

• The most abundant plants on Earth with over 300,000 different

species.
Angiosperm Examples
 Angiosperm Reproduction
• Plants have a life cycle that is referred to as the alternation of
generations. Their life cycle has two stages: haploid and diploid
stage.

• The life cycle of angiosperms is dominated by the sporophyte

generation (2n); the typical plant body seen.

• Sporophyte generation has sex organs (flowers) that are capable

of undergoing meiosis leading to the formation of haploid spores
(n).

• Multicellular male and female gametophytes are produced within

the flowers of the sporophyte.
 Gametes vs Spores
Spores DO NOT
Gametes can fuse fuse. They
with other gametes undergo mitosis
in a process called and grow into an
syngamy or entire new
fertilization. haploid plant
called the
gametophyte.

Gamete
production.
 Structure of the flower
Flowers are actually modified leaves that contain complex
reproductive organs and cells within.
There are four main parts of a flower:
• Petals
Perianth
• Sepals
• Stamen- male organ
• Carpel/pistil- female organ

• A flower is considered a “complete flower” if it has all of these

components. Ex. Hibiscus
• If any one of these elements is missing it is referred to as an
“incomplete flower”.Ex. Corn and most grasses
 Structure of the flower
There are structures that hold the flower up including:

• Pedicel: this is the flower

stalk, this structure holds
the flower and is
connected to the rest of
the plant.

• Receptacle: where the

other flowering parts are
attached.
 Structure of the flower
• Sepals: lowermost & outermost of
the 4. Modified leaves that enclose
and protect the flower parts as
they mature. All the sepals
together are referred to as the
calyx.

• Petals: are above the sepals &

together make up the corolla.
-They contain pigments, a few
have no fibers and tend to be
thinner.
-Important in attracting pollinators.
Each plant species have a distinctive
size, shape, colour, and arrangement
to allow pollinators to recognize
specific species.
 Structure of the flower
• Sepals and petals together
form the perianth.

• Perianth is quite diverse

owing to its ability to
attract potential
pollinators or their
pollination style (eg: by
water, wind, animals, or
insects).
 Reproductive organs of flowers : Stamen
The stamen is the male reproductive organ of the flower.
Stamens are collectively known
as the androecium.
Consists of the:
• anther- site of pollen
development
• Filament- stalk-like structure,
which transmits water &
nutrients to the anther and
positions it to aid pollen
dispersal.
 The Stamen
• The process of microspore production is known as microsporogenesis.
• Pollen development occurs in a structure called the microsporangium
located within the anthers.
• Microsporangia (singular microsporangium) are called pollen sacs.

• The microsporangia contain diploid pollen

mother cells (microsporocyte) that divide
through meiosis.

• At the end of meiosis, each mother cells gives

rise to 4 haploid (n) microspores.

• These four microspores are arranged in a

tetrad. Each microspore will form a pollen
grain.
 The Stamen
• Neighboring anther cells are in a layer called tapetum and act as
nurse cells that contribute to microspore maturation. Provides
nutrition to the developing microspores.
 The Stamen
• Note: Haploid gametes in plants are
produced by mitosis from a haploid
gametophyte.

• Each microspore then divides mitotically

forming the pollen grain (the multicellular,
haploid male gametophyte).

• The formation of pollen grains is known as

microgametogenesis and the pollen grains
are called microgametophyte (male
gametophyte) .

• A mature pollen grain contains 2 cells: a

generative cell and pollen tube cell. The
generative cell is found inside the tube
cell.
• The pollen wall is a cell wall;
however, it is quite complex. It has
an inner layer called the intine
(composed of cellulose) and an outer
layer called the exine that consist of
the polymer sporopollenin.

• It has one or several "weak spots",

germination pores, where the pollen
tube emerges after it has been
carried to the stigma of another
flower.

• Because sporopollenin is so resistant,

pollen grains and their characteristic
patterns fossilize well. Botanist can
determine exactly which plants grew
in an area at a particular time in the
ancient past!
Each pollen grain contains two cells: Pollen grain
• tube cell- develops in the pollen
tube.
• generative cell- divides into two
sperm by mitosis.

When a pollen reaches the stigma,

germination occurs where the tube cell
forms a pollen tube to reach the ovary.

During the transit inside the pollen

tube, the generative cell divides to
form 2 male gametes (sperm cells).
 The Carpel
• Carpels constitute the gynoecium,
located at the highest level on the
receptacle.

The carpel consists of 3 main parts:

• Stigma- landing spot for the pollen
grains.
• Style- elevate the stigma to a useful
position and used to transfer sperm to
ovary.
• Ovary- receptacle that contains ovules
with eggs.
Carpel and
Stamen
evolution
Sporophyll

Stamens are thought to

represent modified sporophylls
(leaves with sporangia on their
upper surface).
 The Carpel
• In the ovary are placentae (sing.: placenta), regions of tissue that
bear ovules.
• Ovules have a short stalk (funiculus) that carries water and
nutrient from the placenta to the ovule.
• Ovule also has a central mass of parenchyma called nucellus.
• Around the nucellus are 2 thin sheet of cells (integuments) that
cover the nucellus surface, leaving only a small hole (micropyle) at
the top.
The Carpel
• Some nucellus cells, usually 1 in each
ovule, enlarge in preparation for
meiosis.
• These are the megaspore mother cells
or megasporocytes.
• After meiosis, usually 3 of the 4
megaspores degenerate and only 1
survives, becoming very large by
absorbing the protoplasm of the other
3.
 The Carpel
• Within the ovule, the surviving megaspore
develops into a megagametophyte.
In one type of development:
• Nucleus undergoes 3 mitotic divisions,
producing 2, 4, and then 8 haploid nuclei.
• The nuclei migrate through the cytoplasm,
presumably pulled by microtubules, until 3
nuclei lie at each end and 2 in the center.
• Walls form around the nuclei, and the large,
eight nucleate megaspore becomes a
megagametophyte with 7 cells, one of which
is binucleate.
• The 7 cells are: one large central cell with 2
polar nuclei, 3 small antipodal cells, and an
egg apparatus consisting of 2 synergids and an
egg (the megagamete).
 Angiosperm Reproduction
• Angiosperms are heterosporous (producing two different kinds of spores).
• They generate microspores, which will produce pollen grains as the male
gametophytes, and megaspores, which will form an ovule that contains
female gametophytes.
 Angiosperm Reproduction
• Flowers with both male and female reproductive organs are
perfect flowers. They are bisexual.
• Flowers that have only male or only female reproductive
organs are imperfect flowers. They are unisexual.
 Angiosperm Reproduction
• In dioecious species, plants are either male or female (e.g.
nutmeg, marijuana).
• In monoecious species, there are separate male and female
flowers but on the same plant (e.g. castor oil, breadfruit, corn,
pumpkin).
 Pollination
• Pollination is the transfer of pollen from the anther to the stigma
of the same or a different flower.
• Pollen transfer is affected by wind, water, and animals, primarily
insects and birds.
• self-pollination: occurs when the pollen
from the anther is deposited on the stigma
of the same flower (autogamy) or another
flower on the same plant (geitonogamy).

• cross-pollination- is the transfer of pollen

from the anther of one flower to the stigma
of another flower on a different individual
of the same species.
 Pollination and Fertilization
• Fertilization (fusion of male and female gametes) in plants starts
with the pollen landing on the stigma (pollination).

Fertilization/ syngamy of sperm and egg involves both :

1. Plasmogamy- the fusion of the cytoplasm of the gametes.
2. Karyogamy- the fusion of the nuclei.

• In angiosperms, a process of double fertilization occurs.

• Double fertilization is the fusion of the egg and sperm and the
simultaneous fusion of a second sperm and two polar nuclei that
ultimately results in the formation of the endosperm (the food-
storage tissue) of the seed.
 Double fertilization
1. Pollen lands on the stigma of a flower of the correct species.

-The pollen grain germinates by producing a tiny pollen tube

through the style of the ovary.
 Double fertilization
2. The tube cell enlarges and
comes out of the pollen grain
through one of the germ pores to
form a pollen tube.
-The tube nucleus descends to the
tip of the pollen tube.
-The generative cell also passes
into it. It divides by mitosis
producing two male gametes/
sperm cells.

3. It is guided to the ovule's

micropyle.
 Double fertilization
4. The pollen tube completes growth
toward the egg by passing through
the micropyle.

5. The pollen tube penetrates the nucellus

and enters one of the paired synergids,
where it bursts almost instantaneously,
discharging the two sperm cells.
 Double fertilization

6. One of the sperm cells migrate

towards the egg cell and as it does so
the plasma membrane breaks down.
-As the sperm fuses with the egg
plasma membrane, only the sperm
nucleus enters the egg.
-This fusion establishes the diploid
zygote nucleus.
 Double fertilization

(7) The other sperm’s nucleus

fertilizes the polar nuclei both
haploid (n), generating a 3N
triploid endosperm, which
provides nutrients to the
developing embryo.
 Endosperm Development
• The endosperm is the surrounding nutritive tissue that develops from
the primary endosperm nucleus.
• The endosperm provides nutrition to the developing embryo.
• It stores oil, starch and other nutrients.
• The endosperm tissue is formed by the rapid mitotic division of the
main endosperm nucleus.
• The endosperm develops before the embryo.
• Based on the development, the endosperm is of three (3) types:
-nuclear endosperm
-cellular endosperm
-helobial endosperm
 Endosperm Development
Nuclear endosperm: the primary endosperm nucleus divides
repeatedly without the formation of cell walls. Many free nuclei
are formed. These may become separated by walls in later
stages. Ex. coconuts, maize, rice, wheat, corn, sunflower
 Endosperm Development
• Cellular endosperm: each nuclear division of the primary
endosperm nucleus is consecutively followed by cell wall
formation. Ex. Petunia and datura
 Endosperm Development
• Helobial endosperm: a combination of both cellular and nuclear
types of development. The primary endosperm nucleus divides into
two nuclei. These 2 nuclei are separated by a wall to form a large
micropylar chamber and a small chalazal chamber. In the micropylar
chamber, the nucleus undergoes several free nuclear divisions. In the
chalazal chamber, the nucleus may or may not divide.
 Endosperm Development
• Ruminate endosperm: The endosperm with irregularity and
unevenness in its surface. It is formed when the seed coat
intrudes inward into the young endosperm via meristematic
growth.
 Endosperm development
• Endosperm varies between plants.
-in coconuts it is liquid
-in corn it is solid
 Embryo development
• Review: sperm + egg = zygote embryo
• The development of the embryo occurs through embryogenesis.
• After fertilization in dicots, the zygote divides to form two cells: the
upper apical cell and the lower basal cell.
• The division of the basal cell gives rise to the suspensor. The suspensor
attaches the embryo to the micropyle and provide a route for nutrition
to be transported from the mother plant to the growing embryo.
• The apical cell divides producing the proembryo.
 Embryo development
• As the proembryo divides, it
takes on a spherical form-the
globular stage.
• The three basic tissue
systems (dermal, ground,
and vascular) can be
recognized at this point
based on characteristic cell
division patterns.
• The globular shape of the
embryo is then lost as the
cotyledons (embryonic
leaves) begin to form.
 Embryo development
• Next, cotyledons arise forming the heart stage. The cotyledons function
in food storage, food absorption and/or photosynthesis.

• As the cotyledons elongate, the base of

the embryo thickens and results in the
torpedo stage.

• Cell division is concentrated at the shoot

apical meristem, located at the shoot tip
in between the cotyledons and the root
apical meristem at the most basal part of
the embryo.

• Most of the suspensor deteriorates during

the torpedo stage.
 Embryo development
• The mature embryo includes an
embryonic root called radicle, the
hypocotyl (stem-like axis below the
cotyledons) and the epicotyl (stem-
like axis above the cotyledons).
• At this point, the embryo becomes
dormant,, halting metabolic activity
and cell division.
• The seed is ready for dispersal and
growth only resumes after the seed
germinates. The embryo develops into
a seedling.
• The integument developed into the
seed coat.
Embryo development
 Embryo development
• In monocots, the process of embryogenesis is similar. However,
only a single cotyledon is present and no heart stage occurs.
• The shoot apical meristem, while still present at the shoot tip,
is not in between cotyledons in the monocot (because there is
only a single cotyledon).
 Embryo development
The seed is the fertilized ovule and
possesses an embryo, endosperm and a
protective coat.

Structures of the dicot seed include:

• Plumule- develops into the shoot.

• Epicotyl- below the plumule. Function:

allows the embryonic shoot to break
through the soil.

• Hypocotyl- develops into stem.

• Radicle- forms roots.

 Embryo development
• Hilum- scar left from the funiculus
disappearing.

• Micropyle- small pore facilitating

entry of oxygen and water.

• Seed coat- developed from the

integuments of the ovule. Outer coat
called testa which is hard; inner coat
called tegmen, which is thin.
 Types of seed- Dicot
Dicots- seeds containing two
cotyledons.
• cotyledons store nutrients that are
used during and after germination
• cotyledons become thick and filled
with starch, oils, or protein in
embryo development
• endosperm that supply nutrient,
shrinks
• when the seed matures, the
cotyledons are large and endosperm
may be completely used up.
 Types of seed-Monocot
Monocot- embryo has a single cotyledon.
• the one cotyledon generally does not become
thick
• endosperm usually remain and is present in
the mature seed
• During germination, the cotyledon acts as
digestive/absorptive tissue, transferring
endosperm nutrients to the embryo
• Coleoptile: protective sheath enclosing the
shoot tip and embryonic leaves of grasses
• may also have coleorhiza: a protective sheath
enclosing the embryonic root of grasses
 Types of seed
• A mature seed in which endosperm is rather abundant: albuminous
seed (eg: corn)
• If endosperm is sparse or absent at maturity: exalbuminous (eg:
peas and beans)
• Some dicots are intermediate: the cotyledons store some starch
and protein, but a considerable amount of endosperm remains in
the seed.
 Fruit Development
The ovules develop into the seed and the
ovary develops into the fruit.
Fxn: to spread seeds.
The fruit wall or pericarp is divided into 3
regions/layers:
• Exocarp: outer layer- the skin or peel
• Mesocarp: middle layer- the flesh
• Endocarp: innermost layer- may be
tough like the stones or pit of a cherry or
may be thin.
• The relative thickness and fleshiness of
these layers varies with fruit type, and
often 1 or 2 layers are absent.
 Fruit Types and Seed Dispersal
• The term pericarp refers to the tissue of the fruit regardless of their
origin.
• In most cases, the pericarp comes from the ovary wall.
• However, there are instances where the receptacle tissues, or sepal, petal,
and stamen tissue become involved in the fruit.
• True fruit: fruits containing only ovarian tissue.
• Accessory fruit (false fruit): fruits that contain other tissues as well.
 Fruit Types and Seed Dispersal
Fruits can be classified based on their carpels. Some fruits fit into more
than one type.

1. Simple- the fruit develops from a single ovary of one flower. Ex. apple

2. Aggregate- the fruit develops from the several ovaries of one flower
that fuse during development. Ex. Raspberry

3. Multiple-fruit develops from a cluster of flowers. Ex. Pineapple

4. Accessory fruit-fruit comes from both ovary and non-ovary tissue. Ex.
Strawberry
 Classification of fruit types
There are two types of simple fruits:
Dry- the pericarp is hard and paper-like. Not typically eaten by seed-
distributing animals. Seed dispersal takes place via water or wind.
No differentiation of layers.

• Fleshy- pericarp is soft and full of pulp. Eaten by animals and seeds
dispersed by animals. Has three different layers.
 Classification of fruit types
The main types of fleshy fruits are:

• Berry: a fleshy fruit in which all three layers are soft. Ex. Grape,
tomato

• Pome: similar to berry except that the endocarp is paper and leathery.
Ex apple

• Drupe: similar to berry except that the endocarp is hard and

sclerenchymatous. Ex. Peach, plum

• Pepo: exocarp is tough, hard rind; the inner soft tissues may not be
differentiated into two distinct layers Ex. Cantelope, pumpkin

• Hesperidium: exocarp is leathery. Ex citrus

 Classification of fruit types
There are two categories of dry fruits based on fruit opening:
Dehiscent- fruits that break open and release seeds.
1. follicle: develop from 1 carpel & split along a single side of the
fruit. Ex. Milkweeds,magnolias.
2. legumes: develop from 1 carpel & split along both sides. Ex. Beans &
peas
3. capsule: develop from several carpels & split open in various ways.
 Capsules

Silicle-a special Silique - a Poricidal capsule - one

Loculicidal capsule - Septicidal capsule - one short broad special long which opens with round
one which splits along which splits along the septa capsule of 2 slender holes. (poppies).
the outer median line. and opens at the top. carpels. capsule of 2
(lilies). (yucca, agave). (mustards). carpels.
(mustards).
 Classification of fruit types
Indehiscent-fruits remain closed retaining the seed. Many are
one- seeded fruits.
• Achene - the one seed is attached to the fruit wall at a single point.
(buttercups, dandelion, sunflower).
• Nut - similar to an achene but with the wall greatly thickened and
hardened. (beech, chestnut, oak, hazel)
• Samara - A one- or two-seeded fruit in which part of the fruit wall
grows out into a wing. (elm, maple, ash).
• Caryopsis/grain - fruit in which the fruit wall and the seed coat are
fused. (wheat, corn, grasses).
• Schizocarp - A fruit formed from several carpels.At maturity, the
carpels separate as separate indehiscent fruits. (mallow, wild carrot,
dill).
Indehiscent fruits

samara
 Pollination

Self-pollination has no possibility of Cross-pollination can result in new

brining in new genes to provide more combinations of genes, at least a few of
fitness; however, it allows plant that is which can be better adapted than either
isolated by distance or lack of pollinators parent.
to still set seed and propagate.
 Pollination
Many mechanisms have evolved that decreases the probability of self-
pollination and increases the changes of cross-pollination which
include:

• Stamen and style maturation times: anthers and styles mature at

different times. In many species, anther will begin to release the
pollen when the stigma is still immature.

• Self-incompatibility (SI): inability of a pollen to fertilize the stigma.

Pollen tube fails to initiate or mature fully. Recognition of “self”
pollen is based on genes, S-genes, with many different alleles.
-gametophytic self-incompatibility
-sporophytic self-incompatibility
 Pollination
• Gametophytic self-incompatibility: incompatibility due to the
genotype (genetic makeup) of pollen. Determined by the single S
allele in the haploid pollen grain.

• Pollen is haploid containing a single

allele (eg. S1)

• Stigma diploid contains 2 alleles (eg.

S1S2)

• The pollen tube growth will be

arrested if the stigma contains the
same allele as that of the haploid
pollen grain.
 Pollination
• Sporophytic self-incompatibility: incompatibility determined
by genotype of sporophyte (2n). Both alleles of the stigma
function together.

Any pollen from an S1S2 plant

cannot fertilize an S1 or S2
plant.

Ex. S1 and S2 pollen

germination is arrested at the
stigma surface with allele
S1S3.
 Animal-pollinated flowers
• The evolution of animal-mediated pollination had a dramatic impact on the
evolution of flowering plants.

• Pollen carried by insects had a higher probability of landing on a stigma

compared to those carried by wind.

• About 120 million years ago, insects would pollinate a variety of flowers and this
led to many pollen grains landing on the wrong stigma. Mutations which
increased a plant's distinctiveness, its recognizability by an insect (color, size,
shape, fragrance, etc) became selectively advantageous.

• As a result, many lines of animals and flowers underwent coevolution (the

influence of two groups of organisms on the evolution of each other).
 Shapes of flowers
Most flowers are radially symmetrical-capable of being divided,
by more than one line passing through the middle of the flower,
into two equal parts that are mirror images of one another.
Flowers are termed as actinomorphic or regular.
 Shapes of flowers
• All insect and most animal pollinators are
bilaterally symmetrical-- only one
longitudinal plane produce two halves that are
mirror image.

• Many flowers now also evolved to be

bilaterally symmetrical and are termed as
zygomorphic.

• When a pollinator approaches zygomorphic

flower, only one orientation is comfortable as
a result, when the pollinator feed, the pollen
is placed on a predictable part of its body.
 Wind-pollinated flowers & anemophily
• Wind-pollinated flowers may be small, have
no petals, and do not produce scent or nectar.

• The anthers may produce a large number of

pollen grains, while the stamens are generally
long and protrude out of flower.

• Large feathery stigmas are adaptive by

increasing the area that can catch the pollen
grain.

• Wind-pollinated plants include grasses and

cereal crops. Flower of wheat showing feathery stigmas.
 Ovary Position
• Ovaries and ovules must be protected from pollinators and plants
do so by burying the ovaries deep within the flower.Flowers can
be described based on the position of the ovary in the flower.

Superior ovary- the ovary lies above the attachment of the petals
and sepals. (hypogynous flower)
 Ovary Position
• Inferior ovary – the ovary lies below the
attachment of the other floral parts. The
other parts are attached at the top of the
ovary. (Epigynous flower)
 Ovary Position
• Half-inferior or perigynous ovary: a hypanthium (a floral tube
formed from the fusion of the stamens, petals, and sepals) is
attached below the gynoecium and surrounds the ovary; the ovary
is said to be half inferior as the free parts of the petals, sepals,
and stamens are attached to the rim of the hypanthium.
 Inflorescences
• When many flowers group together they are called an inflorescence.
• Inflorescence: a cluster of flowers on a branch or system of branches.
• Can be categorized into two groups based on the arrangement of flowers
on the main axis (peduncle).

1. Determinate-have the central flower maturing first with the arrest of

the elongation of the central axis.

2. Indeterminate-have the lateral or lower flowers maturing first without

the arrest of the elongation of the central axis.
-raceme, panicle, spike, catkin,corymb, umbel, spadix, head