THE LIFE CYCLE OF ANGIOSPERMSPacket #75

Chapter #38

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INTRODUCTION

Angiosperms are the predominant plant species on the planet.

Unlike gymnosperms, these plants produce seeds that have a covering.

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LIFE CYCLE OF ANGIOSPERM

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In the angiosperm, the sporophyte generation is dominant

Gametophytes are extremely reduced in size and nutritionally dependent on the sporophyte generation

Flowering plants are hetereosporous

Figure 38.2 (More Detail)Page 772

FIGURE 38.4THE DEVELOPMENT OF ANGIOSPERM GAMEOPHYTES

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FIGURE 38.4DEVELOPMENT OF POLLEN GRAINS

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3A 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. (SeeFigure 38.2b.)

Development of a male gametophyte (pollen grain)(a)

2Each 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 4microspores

Generativecell (willform 2sperm)

MaleGametophyte(pollen grain)

Nucleus of tube cell

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

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75 m

20 m

RagweedpollengrainFigure 38.4a

MEIOSIS

MITOSIS

KEYto labels

Haploid (2n)Diploid (2n)

Keyto labels

MITOSIS

MEIOSIS

Ovule

Ovule

Integuments

Embryosac

Mega-sporangium

Mega-sporocyte

Integuments

Micropyle

Survivingmegaspore

AntipodelCells (3)

PolarNuclei (2)

Egg (1)

Synergids (2)

Development of a female gametophyte (embryo sac)(b)

Within the ovule’smegasporangium is a large diploid cell called the megasporocyte (megasporemother cell).

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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).

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Female gametophyte(embryo sac)

Diploid (2n)

Haploid (2n) Figure 38.4b

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

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FIGURE 38.4DEVELOPMENT OF EMBRYO SACS

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LIFE CYCLE OF ANGIOSPERM

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Each microspore develop into a pollen grain.

Immature male gametophytes are contained within pollen grains. Each pollen grain has two

haploid (n) cells. One of each four

megaspores produced during meiosis develops into an embryo sac (female gametophyte). Embryo sac contains seven

cells with eight nuclei The egg cell and the

central cell, which contains two polar nuclei, participate in fertilization.

LIFE CYCLE OF ANGIOSPERM

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Pollen released from the anther

Pollen is carried to the sticky stigma

Flowers can self-pollinate or cross-pollinate. Transfer of pollen

from flowers of one plant to flowers of another plant is called cross-pollination

LIFE CYCLE OF ANGIOSPERM

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Pollen grain germinates after it sticks to the stigma.

Pollen grain now contains the mature male gametophyte and extends a tube down through the style.

After it reaches the ovary, the pollen tube penetrates the microphyle and discharges two sperm cells into the female gametophyte (embryo sac) Results in double

fertilization

POLLEN TUBE

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LIFE CYCLE OF ANGIOSPERM

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Double Fertilization Results in the formation

of a diploid zygote and triploid endosperm

One sperm nucleus fuses with the egg to form a diploid zygote.

The other sperm nucleus fuses with the two nuclei if the female gametophyte Cell is now triploid (3n).

Characteristic of flowering plants.

FIGURE 38.6DOUBLE FERTILIZATION

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Double fertilization ensures the endosperm will develop only in ovules where the egg has been fertilized Prevents the

angiosperms from wasting resources

LIFE CYCLE OF ANGIOSPERM

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After double fertilization, the ovule matures into a seed.

The zygote develops into the sporophyte embryo with a rudimentary root and either one or two seed leaves (cotyledons—monocots vs. dicots)

LIFE CYCLE OF ANGIOSPERM

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Seed consists of the embryo, endosperm, sporangium and a seed coat.

An ovary develops into a fruit as its ovules develop into seeds.

Dispersal occurs via wind or animals

Seed germinates if the environmental conditions are favorable.

Seed coat ruptures, the embryo emerges as a seedling and uses the stored food in the endosperm and cotyledons to begin growth.

MECHANISMS TO PREVENT SELF-POLLINATION (SELFING!)Chapter 38

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INTRODUCTION I

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Some flowers self-fertilize but most angiosperms have mechanism that prevent “selfing.”

Dioecious plants Unisexual

Ackee {Barbados}—Ginnup {Jamacia}

Bisexual flowers Stamens and carpels

mature at different times

Stamens and carpels are arranged in a way that makes self-pollination unlikely

INTRODUCTION IIBIOCHEMICAL BLOCKING

AKA Self-incompatibility Most common method that plants use to prevent

“selfing” Some plants recognize their own pollen and that

of closely related individuals Hmmm!

Biochemical block prevents pollen from completing its development and fertilizing the egg

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REVIEWImagine that…plants which are more simplistic organisms appear to have more complex reproductive cycles.

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REVIEW

Students List potential examination questions and/or here,

and on following slides, based on the packet.

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