CH 41: Plant Reproduction

Petals

Like sepals, they are also arranged around the receptacle in a whorl. They are often brightly colored and advertise the flower to pollinators. In some flowers, the base of the petals contains a gland called a nectary.

nectary

In some flowers, at the base of the petal. Nectary produces nectar, which is harvested by many of the animals that visit flowers.

How Are Male Gametophytes Produced?

In the anther, a diploid microsporocyte divides by meiosis to form microspores, which then divide by mitosis to form male gametophytes.

In most cases, animal pollination is an example of

mutualism, a mutually beneficial relationship between two species. Pollinators benefit by receiving food. Flowering plants gain because outcrossing takes place.

Spatial avoidance

occurs in dioecious species. In some species with perfect flowers, the anthers and stigma are so far apart that self-pollination is extremely unlikely.

Temporal avoidance

occurs when male and female gametophytes within a perfect flower mature at different times.

Molecular matching

occurs when pollination is blocked—if proteins on the pollen grain surface match proteins on the stigma—indicating that the pollen and stigma are from the same individual.

A typical angiosperm ovary contains one or more

ovules

Inside the ovary, female gametophytes are produced in

ovules

Flowers that contain both stamens and carpels are

perfect. Those that contain either stamens or carpels are imperfect.

Flashes of red light and far-red light can switch the

photoperiod response on and off

Pollination

the transfer of pollen grains from an anther to a stigma.

Long-day plants

bloom in midsummer when days are longest and nights are shortest. In long-day plants, high levels of CO stimulate production of the flowering hormone.

Stamens

reproductive structures that produce male gametophytes (pollen grains), which in turn produce sperm.

Meiosis produces haploid spores inside

sporangia

Pollination and fertilization are separated in

time and space.

Most dry fruits (nuts) are dispersed by

wind, animals, or mechanical action, or they simply fall to the ground. Fleshy fruits are commonly dispersed by animals.

Dormancy is usually a feature of seeds from species that

inhabit seasonal environments; scientists generally interpret it as an adaptation that allows seeds to remain viable until environmental conditions improve.

Simple fruits

like the apricot develop from a single flower that contains a single carpel or several carpels that are fused together.

Multiple fruits

like the pineapple develop from many flowers and thus many carpels.

Aggregate fruits

like the raspberry also develop from a single flower, but one that contains many separate carpels.

Seeds contain stored nutrients that increase

offspring success in crowded habitats. Seedlings can subsist on stored nutrients until they are well enough established to absorb water from the soil and feed themselves via photosynthesis

Recall that angiosperms in the group monocots have just

one cotyledon, whereas eudicots have two.

As a fruit matures, the walls of the ovary thicken to form the

pericarp that surrounds the seeds.

Each of these microspores divides mitotically to form a

pollen grain, a haploid, immature male gametophyte with two nuclei.

The triploid cell resulting from the second fertilization is called the

primary endosperm nucleus.

The first structure to emerge from the seed during germination is the

radicle, which begins developing into the root system.

As a seed matures, the embryo and endosperm develop inside the ovule and become surrounded by a covering called a

seed coat.

Even though they have been dispersed, seeds may not germinate for some time—a condition known as

seed dormancy

When photosynthesis begins in the cotyledons or the earliest seedling leaves;

seedlings no longer dependent on food reserves and are said to be established.

In eudicots, the shoot system with its cotyledons usually emerges

shortly after the radicle appears.

Exposing one leaf on a plant to the conditions necessary to induce flowering may result in the whole plant flowering, suggesting that the

signal to flower comes from leaves and travels to the apical meristem.

In most plant groups, fertilization is simply the combination of

sperm and egg and resulting formation of a diploid zygote.

Fertilization triggers the development of a young

sporophyte. In angiosperms, the first stage in the sporophyte's life is the maturation of the seed.

Pollination Syndromes

suites of flower traits that have co-evolved with classes of pollinators. They are close correlations between the structure of a flower and the size, shape, and behavior of its pollinators.

The advantage of selfing is

that it virtually assures successful pollination. However, offspring are usually less genetically diverse than are outcrossed offspring, and may suffer from inbreeding depression.

Every structure and process of plants exists to maximize

the chance that the individual will produce offspring.

Embryogenesis is

the process by which a single-celled zygote becomes a multicellular embryo.

When pollen grains land on a flower

they deliver sperm cells that fertilize the egg produced by the female gametophyte.

When the pollen tube reaches the micropyle, it grows

through it and enters the interior of the female gametophyte.

Although all asexual reproduction is based on mitosis, a wide array of mechanisms are involved:

Rhizomes corms plantlets Apomixis

Seed germination occurs in three phases:

Seeds take up water, consume oxygen, and synthesize proteins. Water uptake stops, the seed begins to manufacture the new mRNAs and proteins needed to support growth, and mitochondria begin to multiply. Water uptake resumes as growth begins. This second round of water uptake enables cells to enlarge and the embryo to burst from the seed coat.

There are three basic fruit types:

Simple fruits like the apricot develop from a single flower that contains a single carpel or several carpels that are fused together. Aggregate fruits like the raspberry also develop from a single flower, but one that contains many separate carpels. Multiple fruits like the pineapple develop from many flowers and thus many carpels.

Many plants have elaborate mechanisms to prevent selfing:

Temporal avoidance Spatial avoidance Molecular matching

The entire group of sepals in the flower is called the

calyx

Structures associated with pollination syndromes are thought to be adaptations—flowers and pollinators have adaptations that increase pollination frequency and feeding efficiency, respectively. This is an example of

coevolution

The entire group of petals in a flower is called the

corolla. The petals within the corolla can vary greatly in size, shape, and function

Fruits

develop from the flower's seed-producing organ and contain seeds.

By the time a seed matures, the three major tissue types have

differentiated in the embryo. The seed tissues then dry and the embryo stops growing.

Plants also respond to internal cues, such as

favorable nutritional status or hormones.

The carpel produces the

female gametophytes, which produce eggs.

Asexual reproduction does not involve

fertilization and results in the production of clones―genetically identical copies of the parent plant.

Biologists named the flowering hormone

florigen, even though the actual hormone had not yet been discovered.

Day-neutral plants

flower without regard to photoperiod.

Fertilization initiates

fruit development as well as seed and embryo development.

At the same time, the ovary around the ovule develops into a

fruit, which encloses and protects the seed (or seeds, if a single ovary contains multiple ovules).

an individual in the haploid phase of the life cycle is called a

gametophyte

Spores divide by mitosis to form multicellular,

haploid gametophytes

Dioecious plants

have either stamen- or carpel-producing flowers, but not both on the same individual.

angiosperm

Flowering plants

How Is Dormancy Broken?

For some types of seeds, the seed coat must be disrupted, or scarified, for germination to occur. For others, exposure to water, light, oxygen, or fire will initiate germination. In general, dormancy can be broken in response to a wide variety of environmental cues.

Why Did Pollination Evolve?

Groups that do not form pollen have sperm that must swim to the egg or are otherwise transferred through water. Pollination evolved, late in land plant evolution, into a much more efficient process when animals began to act as pollinators instead of relying on wind. The evolution of pollen (and seeds) allowed species to colonize drier habitats.

As the embryo continues to develop in embryogenesis, the long axis of the plant begins to emerge, and several structures develop:

The cotyledons, or seed leaves. The hypocotyl, or seed stem. The radicle, or embryonic root. Some embryos also have an upper portion of the embryonic stem called the epicotyl

In Embryogenesis, the terminal cell and its progeny sort into three groups:

The exterior protoderm forms the epidermis. The ground meristem just inside the exterior layer forms the ground tissue. The procambium in the core of the embryo becomes the vascular tissue

Embryogenesis in angiosperms is a four-step process:

The zygote divides to form two daughter cells. The lower basal cell divides to form a root tip cell and the suspensor while the upper terminal cell forms a cell mass that ultimately gives rise to all the cells in the embryo. The terminal cell and its progeny sort into three groups: The embryo continues to develop, the long axis of the plant begins to emerge, and several structures develop:

The Role of Drying in Seed Maturation

Water loss in seeds is an adaptation that prevents them from germinating on the parent plant. The dry condition of seeds also ensures that once they have dispersed from the parent plant, they will not germinate until water is available in the environment.

The lower basal cell divides to form

a root tip cell and the suspensor, through which nutrients can travel from the parent plant to the developing embryo, while the upper terminal cell forms a cell mass that ultimately gives rise to all the cells in the embryo. (Embryogenesis)

Seeds contain

an embryo and a food supply surrounded by a coat.

The mature seed consists of

an embryo, a food supply in the form of endosperm, and a seed coat.

In monocots, the radicle and the coleoptile, which covers the young shoot, emerge

at the same time.

Land plants are characterized by a life cycle with two

distinct multicellular forms—one diploid and one haploid. An individual in the diploid phase of the life cycle is called a sporophyte, while an individual in the haploid phase of the life cycle is called a gametophyte. This type of life cycle is called alternation of generations.

Although some seeds have high levels of abscisic acid (ABA), ABA levels have not been strictly correlated with

dormancy. Researchers have concluded that there is no universal mechanism for initiating and maintaining seed dormancy.

Male gametophytes (pollen grains) are portable. Female gametophytes are

encased in an ovary and are retained in the flower.

It undergoes a series of mitotic divisions that form a triploid tissue called

endosperm

Flowering can be stimulated by

external cues, internal cues, or both.

Each ovule contains a cell called a ____ inside a structure called a ______.

megasporocyte megasporangium

Sporophytes produce spores by

meiosis

Alternation of Generations

meiosis does not lead directly to the formation of gametes as it does in humans and other animals. Instead it leads to the production of haploid cells called spores

In the carpel, a diploid megasporocyte divides by

meiosis to form a megaspore, which then divides by mitosis to form the female gametophyte.

Gametophytes produce gametes by

mitosis

Endosperm stores

nutrients that will be needed by the embryo.

The zygote divides to form

two daughter cells.

In many angiosperms, the embryo sac contains eight haploid nuclei and seven cells. The large central cell contains

two polar nuclei.

Rhizomes are

underground horizontal stems from which shoots and roots emerge. If the individuals emerging from the nodes become separated from the parent plant, they represent asexually produced offspring.

corms

underground modified stems used in asexual reproduction

Plants fall into three main categories with regard to photoperiodism and flowering:

1. Long-day plants bloom in midsummer when days are longest and nights are shortest. 2. Short-day plants bloom in spring, late summer, or fall, when days are shortest. 3. Day-neutral plants flower without regard to photoperiod.

There are four basic organs that are essentially modified leaves:

1. Sepals. 2. Petals. 3. Stamens. 4. One or more carpels. All four organs are not necessarily present in all flowers.

Each stamen consists of:

A stalk called the filament. Pollen-producing organs called anthers.

Fertilization in angiosperms involves four steps:

After landing on a stigma, the pollen grain absorbs water and germinates, or resumes growth. The male gametophyte produces a long projection called a pollen tube that grows down the length of the style to the ovary. The generative cell travels down the tube and divides, forming two sperm. When the pollen tube reaches the micropyle, it grows through it and enters the interior of the female gametophyte. Fertilization—the fusion of sperm and egg to form a diploid zygote—occurs.

Does Pollination by Animals Encourage Speciation

Evolutionary changes in the size or food-finding habits of a pollinator affect the angiosperm populations Changes in flower size and shape affect the insects pollinating the plant population. Because mutation continuously introduces variations in these traits, insect and angiosperm populations frequently change, diverge, and form new species.

Several structures and processes are common to all land plant life cycles:

Meiosis produces haploid spores inside sporangia. Spores divide by mitosis to form multicellular, haploid gametophytes. Gametophytes produce gametes by mitosis. Fertilization occurs when two gametes fuse to form a diploid zygote. The zygote then grows by mitosis to form the sporophyte

Advantage and disadvantage of asexual reproduction

The advantage of asexual reproduction is that it is very efficient. The major disadvantage is that genetically similar populations are more likely to succumb to diseases

The carpel consists of:

The stigma, the moist tip that receives pollen. A slender stalk called the style. The ovary at the base of the carpel.

Sexual Reproduction of plants:

Unifies the diversity of reproductive systems in plants. is based on meiosis and fertilization. Meiosis is nuclear division that halves the chromosome number. Fertilization is the fusion of haploid gametes that produces a diploid zygote. Sperm are male gametes that contribute genetic information (DNA), but few or no nutrients, to the offspring. Eggs are female gametes that contribute both DNA and a store of nutrients to the offspring.

A spore is

a cell that grows directly into an adult individual.

Plants undergo alternation of generations, in which

a diploid sporophyte phase alternates with a haploid gametophyte phase.

In angiosperms, the walls of the ovary develop into

a fruit that encloses the seed or seeds.

pollen grain

a haploid, immature male gametophyte with two nuclei.

At the immature stage, the male gametophyte consists of two cells:

a small generative cell enclosed within a larger tube cell. a small generative cell enclosed within a larger tube cell.

Self-fertilization (selfing) occurs when

a sperm and an egg from the same individual combine to produce offspring.

Fertilization occurs when

a sperm and an egg unite to form a diploid zygote.

The wall of a pollen grain develops

a tough outer coat that protects the male gametophyte when the pollen is released from the parent plant into the environment

After landing on a stigma, the pollen grain

absorbs water and germinates, or resumes growth.

Flower formation begins when

an apical meristem stops making energy-harvesting stems and leaves, and becomes a floral meristem able to produce the modified leaves that comprise flowers.

Seeds consist of

an embryo and nutrient stores surrounded by a protective coat.

Female gametophytes are encased in

an ovary, are retained in the flower, and produce an egg.

Meiosis and pollen formation occur in the

anther.

Photoperiodism is

any response by an organism that is based on photoperiod, the relative lengths of day and night.

The egg cell is located

at one end of the gametophyte near an opening to the ovule called the micropyle.

Short-day plants

bloom in spring, late summer, or fall, when days are shortest. In short-day plants, high levels of CO inhibit production of the flowering hormone.

Inside the anther, structures called microsporangia contain

diploid cells called microsporocytes.

Monoecious plants

have separate stamen- and carpel-producing flowers on the same individual.

Phytochrome

is tied to timekeeping in plants. Plants have a clock that is reset each morning. Clock proteins rise during the day and trigger expression of a gene called CONSTANS (CO).

In plants, the ability to measure photoperiod is important because

it allows the plant to respond to seasonal changes in climate and the availability of resources and pollinators.

Sepals are

leaflike structures that make up the outermost part of the flower. They are usually green and photosynthetic. They are arranged in a circle or whorl attached to the receptacle and enclose the flower bud as it develops and grows.

External environmental cues may involve

length of day and night, the arrival of seasonal rains, or other mechanisms.

Apomixis occurs when

mature seeds form without fertilization occurring, resulting in seeds that are genetically identical to the parent.

The megasporocyte divides by

meiosis to produce four haploid megaspores, of which three degenerate.

Some species produce plantlets, which form from

meristematic tissue located along the margins of its leaves. When the plantlets mature, they drop off the parent plant and grow into independent individuals.

In angiosperms, male and female gametophytes are

microscopic and are produced inside flowers

In angiosperms, the male and female gametophytes are

microscopic, physically separate, and completely dependent on the sporophyte for nutrition.

Microsporocytes undergo meiosis, producing four haploid

microspores

Wind-pollinated species invest in making large numbers of

pollen grains; animal-pollinated species make fewer pollen grains but invest in structures that attract and reward animals.

The male gametophyte produces a long projection called a

pollen tube that grows down the length of the style to the ovary. The generative cell travels down the tube and divides, forming two sperm.

flower is a reproductive structure that

produces gametes attracts gametes from other individuals, nourishes embryos develops seeds and fruits.

The four basic organs that are essentially modified leaves are attached to a compressed portion of stem called the

receptacle

Outcrossing advantages & disadvantages

riskier in terms of the likelihood of pollination, but its advantage is that it results in more genetically diverse offspring that may be much more successful at warding off attacks from pathogens.

Outcrossing, which is much more common than self-fertilization, occurs when

sperm and eggs from different individuals combine. It results from cross-pollination—when pollen is carried from the anther of one individual to the stigma of a different individual

An individual in the diploid phase of the life cycle is called a

sporophyte

The male gametophyte is considered mature when

the haploid generative cell produces two sperm cells via mitosis.

In angiosperms double fertilization occurs

the nuclei of the egg and one sperm unite to form the zygote; the other sperm nucleus fuses with the two polar nuclei, forming a triploid (3n) cell.