SEXUAL REPRODUCTION IN ANGIOSPERMS (FLOWERING PLANTS)

In contrast to gymnosperms, the reproductive structures of the angiosperms are found within structures called flowers, and the seeds, while developing, are enclosed within a fruit.

STRUCTURE OF A FLOWER

Pedicel

Pedicel supports the plant and connects it to the stem.

Receptacle is the part that other flower parts are attached.

Sepals are leaflike structures that form a ring around the base of the flower.They may be green or colored.

Petals are found above the sepals and they are white or colored. They surround the reproductive organs of the flower.

STRUCTURE OF A FLOWER

The stamen is male reproductive organ. It is made up of two parts;

filament and anther. Pollen grains are produced within anthers.

The pistil is usually called the female reproductive organ. It is made up of three parts;

Stigma: is enlarged area at the tip of the pistil that receives pollen.Style: supports the stigma.Ovary: found at the base of the pistil and contains ovules. The ovary develops into the fruit.

Stamens and pistils are essential organs of the flower. Corolla (complete circle of petals) and calyx (complete circle of sepals) are the accessory organs.

Plants that have flowers, which contain only pistils are considered as female, and the plants that have flowers, which contain only stamens are considered as male.

DEVELOPMENT OF GAMETESHaploid spores are produced by meiosis within the anthers of the stamen.

The spores undergo mitosis once, developing into pollen grains. These pollen grains are the young, male gametophytes.

Two haploid cells are found within the thick, protective wall of each pollen grain.

One of the cells in the pollen grain is the

tube cell (nucleus), while the other is generative cell (nucleus).

Once the pollen grains mature, anther bursts, exposing pollen to air.

DEVELOPMENT OF GAMETES IN ANTHERS

Anthers

microsporocyte (2n)

(anter ana hücresi)

meiosis

Each microspore undergoes mitosis

Vegetative nucleus

Generative nucleus

mitosis

Generative nucleus

Pollen tube nucleus (Vegetative nucleus)

Sperm nucleus

Pollen tube nucleusPOLLEN

Microspores (n)

DEVELOPMENT OF GAMETES IN OVARY

Every ovule within the ovary of the pistil has a small opening called micropyle.

In each ovule, meiosis of a single cell results in four haploid spores.

Three of these spores die, and the remaining spore undergoes mitosis three times.

DEVELOPMENT OF GAMETES IN OVARY

The resulting female gametophyte, called an embryo sac, has only seven cells but eight haploid nuclei.

Two of the nuclei, found within a large central cell are called polar nuclei. The egg cell, which is near the micropyle, is surrounded by two cells (synergids). Three other cells are the other end of the embryo sac. (antipodal cells)

DEVELOPMENT OF GAMETES IN OVARY

meiosis

n megaspore

3 mitosis

Embryo sac

megasporocyte (2n)

(Tohum taslağı ana hücresi)

Polar nuclei

synergids

Antipodal cells

egg

POLLINATION

In angiosperms, pollination is the transfer of pollen from an anther to stigma.

If pollen grain is transferred from an anther to stigma on the same plant, it is called self-pollination.

The transfer of pollen from the anthers of one plant to the stigma of another is called cross-pollination.

Artificial pollination occurs when pollen is intentionally transferred by humans from one plant to another.

FERTILIZATION

When a pollen grain reaches the stigma of a flower, it germinates.

A pollen tube grows down through the stigma and style and into the ovary.

It then enters the ovule through the micropyle.

FERTILIZATION

The tube cell nucleus and the generative cell nucleus pass from the pollen grain down the pollen tube.

As the generative nucleus moves down, it divides to form two haploid sperm nuclei.

FERTILIZATION

The two sperm nuclei enter the embryo sac. One fertilizes the egg cell to form a diploid zygote that develops into the sporophyte embryo. The other fuses with the two polar nuclei to form a triploid (3n) endosperm cell.

FERTILIZATIONBecause one sperm fertilizes the egg and the other fertilizes the two polar nuclei, the process is called double fertilization.

Following fertilization, the endosperm nucleus divides by mitosis to form the endosperm. Endosperm is the tissue that stores food for the developing plant embryo.

Structures formed by the parts of the pistil after pollination and fertilization;

OVARY FRUIT

EGG CELL EMBRYO

POLAR CELLS ENDOSPERM

SEEDEMBRYO SAC

OUTER TISSUES OF EMBRYO SAC

SEED COAT

FRUITS AND SEEDS

After fertilization, each ovule develops into a seed, and the ovary develops into fruit.

In angiosperms, the seed is always found in fruit.

The parts of the flower are not involved in the formation of fruit and die.

The ovary grows larger, and its wall thickens.

If a flower is not pollinated, a fruit usually does not form, and the flowers falls away.

TYPES OF FRUITS

Fruits are classified according to their origin;

A fruit that develops from a single ovary is called simple fruit. Ex: cherries and tomatoes

When several ovaries are found within one flower, an aggregate fruit forms.

In some plants, such as the pineapple, the simple fruits of many separate flowers fuse together to form multiple fruit.

TYPES OF FRUITS

TYPES OF FRUITS

Fruits that develop from single ovary (SIMPLE FRUIT)

Fruits that develop from more than one ovaries

(AGGRAGATE FRUIT)

Only ovary (or ovaries) is involved during fruit formation

(REAL FRUIT)

REAL – SIMPLE

FRUIT

Ex. Plum, peach, cherry

REAL- AGGRAGATE

FRUIT

Ex: raspberry (ahududu), blackberry (böğürtlen)

Ovary, sepals, petals and receptacle are involved during fruit formation

(FALSE FRUIT)

FALSE- SIMPLE

FRUIT

Ex: apple, pear, fig, elma, mulberry (dut)

FALSE- AGGRAGATE

FRUIT

Ex: strawberry

FLESHY FRUITS

DRY FRUITS

STRUCTURE OF THE SEED

Seed is formed from the embryo sac after fertilization. It is formed of three main parts;

1.Embryo: develops by mitosis from the fertilized egg

2. Endosperm: is food storage tissue (stores starch, protein, lipids or cellulose for the developing seedling)

3. Seed coat: protects the seed from unfavorable conditions

In addition to one or two cotyledons, the plant embryo has three parts:

*epicotyl

*hypocotyl

*radicle

The part of the embryo above the point of attachment of the cotyledons, it gives rise to terminal bud, leaves, upper stem

It is part of the embryo below the point of attachment of the cotyledons but above the radicle

It is the lowermost part of the embryo, the embryonic root

STRUCTURE OF THE SEED

STRUCTURE OF THE SEED

MONOCOTS AND DICOTS

SEED GERMINATION

SEED GERMINATION IN DICOT PLANT

SEED GERMINATION IN MONOCOT PLANT

DORMANCY• Seed dormancy is defined as a

state in which seeds are prevented from germinating even under environmental conditions normally favorable for germination. 

• These conditions are a complex combination of water, light, temperature, gasses, seed coats, and hormones.

• Living, non-dormant seeds germinate when soil temperatures and moisture conditions are suited for cellular processes and division

• Legumes are typical examples of physically dormant seeds; they have low moisture content and are prevented from intake of water by the seed coat.

FACTORS AFFECTING SEED GERMINATION

The essential factors for seed germination are;

Water:

A dormant seed contains 10-15% of water and is generally dehydrated. So the dormant seed has to absorb water to become active and exhibit germination. Water makes the seed coat soft, causes it to rupture after swelling and start germination. Water is also needed to bring in the dissolved oxygen for use by the growing embryo.

Oxygen:

In the dormant condition the seeds respiratory rate is very low and so oxygen is required in very small quantities. But for germination, oxygen is needed in large quantities. The seeds obtain this oxygen from the air contained in the soil. Seeds placed deeply in soil, fail to germinate because of lack of oxygen.

FACTORS AFFECTING SEED GERMINATION

Temperature:

Germination can take place over a wide range of temperature (5 - 400C), but the optimum for most of the crops is between 250 C to 300C.

Light:

Light has varied effects on germinating seeds of different plants. Some seeds need light for germination, while in some seeds germination is hindered by light.