Angiosperm Reproduction

Concept 38.1: Pollination enables gametes to come together within a flower

In angiosperms, the dominant sporophyte ◦ Produces spores that develop within flowers into male

gametophytes (pollen grains) ◦ Produces female gametophytes (embryo sacs)

An overview of angiosperm reproduction

Figure 38.2a, b

Anther at tip of stamen

Filament

Anther Stamen

Pollen tube

Germinated pollen grain (n) (male gametophyte) on stigma of carpel

Ovary (base of carpel)

Ovule

Embryo sac (n) (female gametophyte)

FERTILIZATION Egg (n)

Sperm (n)

Petal Receptacle

Sepal

Style

Ovary

Key

Haploid (n) Diploid (2n)

(a) An idealized flower.

(b) Simplified angiosperm life cycle. See Figure 30.10 for a more detailed version of the life cycle, including meiosis.

Mature sporophyte plant (2n) with flowers

Seed (develops from ovule)

Zygote (2n)

Embryo (2n) (sporophyte)

Simple fruit (develops from ovary)

Germinating seed

Seed

Carpel Stigma

Three “Fs” ◦ Flowers ◦ Double fertilization ◦ Fruits

Flowers ◦ Are the reproductive shoots of the angiosperm

sporophyte ◦ Are composed of four floral organs: -Sepals and petals (sterile) -Stamens and carpels (reproductive)

Stamen (male) ◦ Filament ◦ Anther (microsporangia

or pollen sacs) makes pollen

Carpal/Pistil (female) ◦ Ovary (contains ovules) ◦ Style ◦ Stigma

Complete flowers ◦ Have all FOUR flower

organs

Incomplete flowers ◦ Lack one or more flower

organs Ex. Grass lacks petals

In angiosperms ◦ The anther’s development (of pollen) and the ovary’s

development (of the ovule) are crucial to plant success

Pollen ◦ Develops from microspores within

the sporangia of anthers

3 A pollen grain becomes a mature male gametophyte when its generative nucleus divides and forms two sperm. This usually occurs after a pollen grain lands on the stigma of a carpel and the pollen tube begins to grow. (See Figure 38.2b.)

Development of a male gametophyte (pollen grain)

(a)

2 Each microsporo- cyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain.

Pollen sac (microsporangium)

Micro- sporocyte

Micro- spores (4)

Each of 4 microspores

Generative cell (will form 2 sperm)

Male Gametophyte (pollen grain)

Nucleus of tube cell

Each one of the microsporangia contains diploid microsporocytes (microspore mother cells).

1

75 µm

20 µm

Ragweed pollen grain

Figure 38.4a

MEIOSIS

MITOSIS

KEY to labels

Haploid (2n) Diploid (2n)

Key to labels

MITOSIS

MEIOSIS

Ovule

Ovule

Integuments

Embryo sac

Mega- sporangium

Mega- sporocyte

Integuments Micropyle

Surviving megaspore

Antipodel Cells (3)

Polar Nuclei (2)

Egg (1) Synergids (2)

Development of a female gametophyte (embryo sac)

(b)

Within the ovule’s megasporangium is a large diploid cell called the megasporocyte (megaspore mother cell).

1

Three mitotic divisions of the megaspore form the embryo sac, a multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective tissue).

3

Female gametophyte (embryo sac)

Diploid (2n)

Haploid (2n) Figure 38.4b 100 µm

The megasporocyte divides by meiosis and gives rise to four haploid cells, but in most species only one of these survives as the megaspore.

2

Embryo sacs ◦ Develop from megaspores within ovules

In angiosperms ◦ Pollination is the transfer of pollen from an anther to

a stigma ◦ If pollination is successful, a pollen grain produces a

structure called a pollen tube, which grows down into the ovary and discharges sperm near the embryo sac Wind, water and animals

Concept 38.2: After fertilization, ovules develop into seeds and ovaries into fruits ◦ How?

After landing on a stigma ◦ A pollen grain germinates and produces a pollen tube

that extends down between the cells of the style toward the ovary

The pollen tube ◦ Then discharges two sperm into the embryo sac

In double fertilization ◦ One sperm fertilizes the egg ◦ The other sperm combines with the other cells

present within the megaspore; giving rise to the food-storing endosperm Why would we want endosperm to form ONLY in

fertilized ovules?

Stigma

Polar nuclei

Egg

Pollen grain

Pollen tube

2 sperm

Style

Ovary

Ovule (containing female gametophyte, or embryo sac)

Micropyle

Ovule

Polar nuclei

Egg

Two sperm about to be discharged

Endosperm nucleus (3n) (2 polar nuclei plus sperm)

Zygote (2n) (egg plus sperm) Figure 38.6

Growth of the pollen tube and double fertilization

If a pollen grain germinates, a pollen tube

grows down the style toward the ovary.

1

The pollen tube discharges two sperm into

the female gametophyte (embryo sac) within an ovule.

2

One sperm fertilizes the egg, forming the zygote.

The other sperm combines with the two polar nuclei of the embryo

sac’s large central cell, forming a triploid cell that develops into

the nutritive tissue called endosperm.

3

After double fertilization ◦ Each ovule develops into a seed ◦ The ovary develops into a fruit enclosing the seed(s)

Endosperm development ◦ Usually precedes embryo development

In most monocots and some eudicots ◦ The endosperm stores nutrients that can be used by

the seedling after germination In other eudicots ◦ The food reserves of the endosperm are completely

exported to the cotyledons

The first mitotic division of the zygote is transverse ◦ Splitting the fertilized egg into a basal cell and a

terminal cell

Figure 38.7

Ovule

Terminal cell

Endosperm nucleus

Basal cell

Zygote Integuments

Zygote

Proembryo

Cotyledons Shoot apex Root apex Seed coat

Basal cell

Suspensor

Endosperm Suspensor

Basal cell ◦ Generates “suspensors”

or anchors to parent plant ◦ As they elongate, embryo

is pushed deeper into protective tissues

Terminal cell ◦ Form the embryo and

cotyledon

The embryo and its food supply ◦ Are enclosed by a hard, protective seed coat

In a common garden bean, a eudicot ◦ The embryo consists of the hypocotyl, radicle, and

thick cotyledons

Figure 38.8a

(a) Common garden bean, a eudicot with thick cotyledons. The fleshy cotyledons store food absorbed from the endosperm before the seed germinates.

Seed coat

Radicle

Epicotyl

Hypocotyl

Cotyledons

The embryo of a monocot ◦ Has a single cotyledon, a coleoptile, and a coleorhiza

Figure 38.8c

(c) Maize, a monocot. Like all monocots, maize has only one cotyledon. Maize and other grasses have a large cotyledon called a scutellum. The rudimentary shoot is sheathed in a structure called the coleoptile, and the coleorhiza covers the young root.

Scutellum (cotyledon)

Coleoptile

Coleorhiza

Pericarp fused with seed coat

Endosperm

Epicotyl

Hypocotyl Radicle

As a seed matures ◦ It dehydrates and enters a phase referred to as

dormancy

Seed dormancy ◦ Increases the chances that germination will occur at a

time and place most advantageous to the seedling The breaking of seed dormancy ◦ Often requires environmental cues, such as

temperature or lighting cues Desert plants need a substantial rainfall Forest fire prone areas need heat Locations with harsh winters need exposure to cold

Germination of seeds depends on the physical process called imbibition ◦ The uptake of water due to low water potential of the

dry seed Allows for the expansion of seed and the splitting of

seed coat

Radicle-first to emerge (embryonic root) Shoot tip-breaks soil surface (hypocotyl) Hypocotyl straightens; raises cotyledon &

epicotyl Epicotyl spreads first leaves

Figure 38.10a

Foliage leaves

Cotyledon

Hypocotyl

Radicle

Epicotyl

Seed coat

Cotyledon Hypocotyl Cotyledon

Hypocotyl

Common garden bean. In common garden beans, straightening of a hook in the hypocotyl pulls the cotyledons from the soil.

(a)

Monocots ◦ Use a different method for breaking ground

when they germinate Coleoptile-pushes upward through the soil and

into the air Shoot tip grows up through coleoptile

Figure 38.10b

Foliage leaves

Coleoptile Coleoptile

Radicle Maize. In maize and other grasses, the shoot grows straight up through the tube of the coleoptile.

(b)

A fruit ◦ Develops from the ovary ◦ Protects the enclosed seeds ◦ Aids in the dispersal of seeds by wind or animals

Fruits are classified into several types ◦ Depending on their developmental origin

Figure 38.9a–c

Simple fruit. A simple fruit develops from a single carpel (or several fused carpels) of one flower (examples: pea, lemon, peanut).

(a) Aggregate fruit. An aggregate fruit develops from many separate carpels of one flower (examples: raspberry, blackberry, strawberry).

(b) Multiple fruit. A multiple fruit develops from many carpels of many flowers (examples: pineapple, fig).

(c) Pineapple fruit Raspberry fruit Pea fruit

Stamen

Carpel (fruitlet) Stigma

Ovary

Raspberry flower

Each segment develops from the carpel of one flower

Pineapple inflorescence

Stamen

Carpels Flower Ovary

Stigma Stamen

Ovule

Pea flower

Seed

Concept 38.3: Many flowering plants clone themselves by asexual reproduction

Many angiosperm species ◦ Reproduce both asexually and sexually

Sexual reproduction ◦ Generates the genetic variation that makes

evolutionary adaptation possible Asexual reproduction in plants ◦ Is called vegetative reproduction

Fragmentation ◦ Is the separation of a parent plant into parts that

develop into whole plants ◦ Is one of the most common modes of asexual

reproduction

In some species ◦ The root system of a single parent gives rise to many

adventitious shoots that become separate shoot systems

Figure 38.11

Asexual Sexual

Advantages ◦ No need for pollinators ◦ All traits are passed ◦ Clones are not as fragile

Disadvantages ◦ More likely to go extinct

Advantages ◦ Variation ◦ Seeds = wide dispersal ◦ Seeds = dormancy until

conditions are favorable

Many angiosperms ◦ Have mechanisms that make it difficult or impossible

for a flower to fertilize itself

Figure 38.5

Stigma

Anther with

pollen

Stigma

Pin flower Thrum flower

Dioecious ◦ Male and female plants

exist (separately) Parts of the plant

mature at different times

Structure prevents by design/location of parts

The most common anti-selfing mechanism in flowering plants ◦ Is known as self-incompatibility, the ability of a plant

to reject its own pollen ◦ What is “selfing?” Ensures seed production Good in groups of plants that are widely dispersed and

unlikely that pollen will be delivered reliably