24–1 reproduction with cones and flowers

Copyright Pearson Prentice Hall

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Biology


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24–1 Reproduction With Cones and Flowers


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24–1 Reproduction WithCones and Flowers

Alternation of Generations


All plants have a life cycle in which a diploid sporophyte generation alternates with a haploid gametophyte generation.

Male and female gametes of a gametophyte plant join and form a zygote that begins the next sporophyte generation.

In some plants, the two stages of the life cycle are distinct, independent plants.


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In seed plants, gametophytes are found within tissues of the sporophyte plant.

In gymnosperms, they are found inside cones.

In angiosperms, they are found inside flowers.

Cones and flowers represent two different methods of reproduction.


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Life Cycle of Gymnosperms


What are the reproductive structures of gymnosperms?


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Reproduction in gymnosperms takes place in cones, which are produced by a mature sporophyte plant.

Gymnosperms produce two types of cones: pollen cones and seed cones.


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Pollen Cones and Seed Cones

Pollen cones produce the male gametophytes, also called pollen grains.

Pollen grain (N)(male gametophytes)


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Seed cones produce female gametophytes and are generally larger than pollen cones.

Female gametophytes develop in two ovules located near the base of each scale.


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Within the ovules, meiosis produces haploid cells that grow and divide to produce female gametophytes.

Each gametophyte contains egg cells.


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Pollination

The gymnosperm life cycle typically takes two years to complete.

The cycle begins as male cones release pollen grains.

Pollen grains are carried by the wind and reach female cones.


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Meiosis

Fertilization


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Male Cones

Mature sporophyte

Pollen cone

Pollen grain (N) (male gametophytes)


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Female Cones

Mature sporophyte

Seed cone

Cone scale

Ovules

Diploid cell (2N)

Ovule

Four haploid cells (N)

Female gametophyte (N)


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If pollen grains land on and enter an ovule, pollination occurs.

A pollen tube grows out of each pollen grain and releases sperm near an egg.

Egg cells

Discharged sperm nucleus

Pollen tube


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Fertilization produces a diploid zygote which develops into a new sporophyte plant.

Zygote (2N)(new sporophyte)


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This zygote grows into an embryo and is encased within what will develop into a seed.

Gametophyte tissue

Embryo (2N)

Seed coat(old sporophyte)

Seed


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The seed is then dispersed by wind.

When conditions are favorable, the seed germinates and its embryo grows into a seedling.


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Structure of Flowers

What are the reproductive structures of angiosperms?


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Flowers are reproductive organs that are composed of four kinds of specialized leaves: sepals, petals, stamens, and carpels.


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Sepals enclose the bud before it opens and protect the flower while it is developing.

Sepal


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Petals are often brightly colored and are found just inside the sepals.

Petals attract insects and other pollinators to the flower.

Petal


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The male parts of a flower consist of an anther and a filament, which together make up the stamen.

Filament

AntherStamen


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An anther is an oval sac where meiosis takes place, producing pollen grains.

Anther


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The filament is a long, thin stalk that supports an anther.

Filament


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The innermost floral parts are carpels, also called pistils, which produce the female gametophytes.

Carpel Style

Stigma

Ovary


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Each carpel has a broad base forming an ovary.

The ovary contains one or more ovules where female gametophytes are produced.

Ovary

Ovule


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The narrow stalk of the carpel is the style.

Style


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At the top of the style is the stigma—a sticky portion where pollen grains frequently land.

Stigma


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Ovary

Ovule

Carpel Style

Stigma

Ovary

Filament

AntherStamen

Parts of a Typical Flower

Sepal

Petal


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A typical flower produces both male and female gametophytes.

In some plants, male and female gametophytes are produced in separate flowers on the same individual.


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Life Cycle of Angiosperms


Reproduction in angiosperms takes place within the flower. Following pollination and fertilization, the seeds develop inside protective structures.


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Each flower contains anthers and an ovary.

Inside the anthers, each cell undergoes meiosis and produces haploid spore cells.

Each spore cell becomes a single pollen grain.


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The pollen grain usually stops growing until it is released from the anther and deposited on a stigma.

Pollen grains (N)(male gametophyte)

Stigma


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In the ovule, a single diploid cell undergoes meiosis to produce four haploid cells.

Haploid cell(N)

Ovule

Ovary (2N)


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Only one of the four cells undergoes mitosis to produce eight nuclei.

The eight nuclei and the surrounding membrane are called the embryo sac.

Pollen tubeEgg cell

Sperm

Polar nuclei

Embryo sac (N)(female gametophyte)


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The embryo sac is the female gametophyte.

One of the eight nuclei is the egg nucleus—the female gamete.

Pollen tube

Embryo sac (N)(female gametophyte)

Egg cell

Sperm

Polar nuclei


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When fertilization takes place, this cell becomes the zygote that grows into a new sporophyte plant.

Endosperm(3N)

Zygote (2N)


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Fertilization in Angiosperms


If a pollen grain lands on the stigma of a flower of the same species, it grows a pollen tube.


Pollen tubes

Ovule


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The pollen tube grows into the style, reaches the ovary, and enters the ovule.


Pollen tubes

Ovule


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One of the sperm nuclei fuses with the egg nucleus to produce a diploid zygote.

The zygote will grow into the new plant embryo.

Zygote (2N)


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The other sperm nucleus fuses with two polar nuclei in the embryo sac to form a triploid (3N) cell.

This cell will grow into a food-rich tissue known as endosperm, which nourishes the seedling as it grows.

Endosperm(3N)

Zygote (2N)


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Because two fertilization events take place between the male and female gametophytes, this process is known as double fertilization.


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Pollination

How does pollination differ between angiosperms and gymnosperms?


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Pollination

Pollination

Most gymnosperms and some angiosperms are wind pollinated, whereas most angiosperms are pollinated by animals.


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Pollination

Wind pollination

•is less efficient than animal pollination

•relies on weather

Animal pollination

•plants have bright colors and sweet nectar to attract animals

•benefits both the plants and the animals that pollinate them


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24–1


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24–1

In a gymnosperm, fertilization occurs inside the

a. ovule.

b. pollen tube.

c. seed cone.

d. pollen cone.


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24–1

The pollen-producing structure of angiosperms is the

a. stigma.

b. carpel.

c. anther.

d. sepal.


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24–1

In an angiosperm, a structure that results from fertilization is the

a. female gametophyte.

b. pollen grain.

c. zygote.

d. ovary.


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24–1

Which term applies to the chromosome number of a seed plant embryo?

a. haploid

b. diploid

c. triploid

d. polyploid


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24–1

Pollination in most gymnosperms and some angiosperms is carried out by

a. water transport.

b. insects.

c. wind.

d. birds and bats.

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24–1 reproduction with cones and flowers

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