Plant Reproduction Chapter 30 Table of Contents Section 1 Plant Life Cycles Section 2 Sexual...

Plant ReproductionChapter 30

Table of Contents

Section 1 Plant Life Cycles

Section 2 Sexual Reproduction in Flowering Plants

Section 3 Dispersal and Propagation

Section 1 Plant Life CyclesChapter 30

Objectives

• Describe the life cycle of a moss.

• Describe the life cycle of a fern.

• Describe the life cycle of a gymnosperm.

• Compare homospory and heterospory.


The Life Cycle of Mosses

• The life cycle of mosses alternates between clumps of gametophytes (the dominant generation) and a sporophyte that consists of a spore capsule on a bare stalk.

• Moss gametophytes produce gametes in two types of reproductive structures: antheridia (singular, antheridium), and archegonia (singular, archegonium).

Chapter 30

Click below to watch the Visual Concept.

Visual Concept

Alternation of Generations



The Life Cycle of Mosses, continued

• An antheridium is a male reproductive structure that produces hundreds of flagellated sperm by mitosis.

• An archegonium is a female reproductive structure that produces a single egg by mitosis.

• Sperm break out of the antheridia and swim to archegonia. One sperm fertilizes one egg to produce a diploid zygote.

• The zygote undergoes repeated mitotic division and forms a sporophyte, which remains on the gametophyte.



• Soon, the cells at the tip of the sporophyte will form a sporangium, called a capsule.

– A capsule of a moss is the part of the sporophyte that will create haploid spores.

• Mosses will only produce one type of spore. When only one type of spore is produced it is called homospory.



• When the spores are mature, the capsule will split open, and the spores are carried away by the wind.

• Spores that land in favorable environments may germinate and grow into new gametophytes.

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Life Cycle of a Moss


Chapter 30


Visual Concept

Life Cycle of Mosses



The Life Cycle of Ferns

• The life cycle of ferns is similar to mosses, but it is also different.

• Most ferns are homosporous and produce a sporophyte that grows from the gametophyte.

• However, the sporophyte, not the gametophyte, is the dominant generation in ferns.


The Life Cycle of Ferns, continued

• The fern’s gametophytes are tiny and are anchored to the soil by rhizoids. They can produce both antheridia and archegonia.

• Water must be present in order for the egg to be fertilized.

• Once the egg is fertilized, it will soon form a zygote and then a sporophyte.



• Once the sporophyte can survive on its own, the gametophyte will die.

• Once mature, a fern sporophyte will have leaves that are called fronds.

• Fronds grow from an underground stem, or rhizome, and contain cells on their underside that develop into sporangia.



• In many ferns, the sporangia on the underside of a frond are clustered together.

– A cluster of sporangia is called a sorus. The sorus will produce haploid spores.

• Once the spores have matured, they will be carried away by air currents. When the spores land, they may grow into new gametophytes.

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Life Cycle of a Fern


Chapter 30


Visual Concept

Life Cycle of Ferns



The Life Cycle of Gymnosperms

• Gymnosperms will produce two types of spores—male microspores and female megaspores.

– Microspores will grow into male gametophytes, and megaspores will grow into female gametophytes.

• When different types of asexual spores are produced it is called heterospory.


The Life Cycle of Gymnosperms, continued

• The microspores of heterosporous plants produce male gametophytes that stay attached to the sporophyte and develop into pollen.

• Pollen allows sexual reproduction in seed plants to take place independent of seasonal rains or other periods of moisture.



• However, gymnosperm sexual reproduction can take more than two years.

• During the first summer, a mature pine tree produces separate female and male cones, which produce male (microsporangia) and female (megasporangia) sporangia.

• The following spring, the cells in all sporangia undergo meiosis and divide to produce haploid spores.



• Megasporangia are haploid spores that produce megaspores, which develop into megagametophytes, or female gametophytes.

– A thick layer of cells called an integument surrounds and protects each megasporangium.

– Each integument has a small opening where pollen can enter, called the micropyle.

– Together, a megasporangium and its integument form a structure called an ovule.



• Microsporangia produce microspores, which develop into microgametophytes, or male gametophytes.

– A pollen grain is a microgametophyte of a seed plant.



• The male cones of a pine release huge numbers of pollen grains into the wind.

• When the pine pollen lands on a female cone, it will drift between the cone scales until they reach the ovules.

• The transfer of pollen, the male gametophyte, to ovules, the female gametophyte, is called pollination.



• During pollination, the pollen grain is drawn into the ovule through the micropyle, and induces the ovule to produce archegonia and eggs.

• After pollination, the pollen grain begins to grow a pollen tube.

– The pollen tube is a slender extension of the pollen grain that enables sperm to reach an egg.



• Pine sperm do not have flagella and they do not swim to an egg. The pollen tube carries the maturing sperm to the egg.

• When the pollen tube reaches an archegonium, one sperm unites with an egg to form a zygote.

• Over the next few months, the zygote develops into an embryo as the ovule matures into a seed.

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Life Cycle of a Conifer


Chapter 30


Visual Concept

Life Cycle of Conifers


Section 2 Sexual Reproduction in Flowering PlantsChapter 30

Objectives

• Identify the four main flower parts, and state the function of each.

• Describe gametophyte formation in flowering plants.

• Relate flower structure to methods of pollination.

• Describe fertilization in flowering plants.


Parts of a Flower

• Early land plants lacked leaves and roots and consisted of only stems.

• Leaves evolved from branches of stems, and flowers are considered to be highly specialized branches and the parts of a flower to be specialized leaves.

• The specialized leaves of a flower form on the swollen tip of a floral “branch”, which is called a receptacle.


Parts of a Flower, continued

• Flower parts are usually found in four concentric whorls, or rings.

• The outer whorl is called the sepals, which protects the other parts of a developing flower before it opens.

• Petals make up the next whorl, and can vary drastically between plants. Some plants have brightly colored petals, and other plants have petals that are small or absent.



• The male reproductive structures are stamens, each of which consists of an anther and a filament, and are found on the third whorl of the flower.

– An anther contains microsporangia, which

produce microspores that develop into pollen grains.

– A stalklike filament supports an anther.



• The innermost whorl contains the female reproductive structures, which are called carpels.

• One or more carpels fused together make up the structure called a pistil, which is the main female reproductive structure in flowering plants.



– The base of a pistil contains the ovary, which will produce eggs in ovules.

– A style, which is usually stalklike, rises from the ovary.

– The tip of the style is called the stigma, which usually is sticky or has hairs in order to trap pollen grains.

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Floral Structure


Chapter 30


Visual Concept

Parts of a Flower


Chapter 30


Visual Concept

Parts of an Angiosperm



Gametophyte Formation

• In angiosperms, gametophytes develop within the reproductive structures of flowers.

• Embryo sacs, which are the female gametophytes in angiosperms, form within the ovary of the pistil.

• Pollen grains, the male gametophytes, form

within the anthers of the stamens.


Gametophyte Formation, continued

Embryo Sac Formation

• In flowers, ovules form in the ovary of a pistil.

• An angiosperm ovule consists of a megasporangium surrounded by two integuments, which do not completely enclose the megasporangium.

• At one end of the ovule is the micropyle, through which a pollen tube can enter.



Embryo Sac Formation, continued

• An ovule contains a large diploid cell called a megaspore mother cell.

• A megaspore mother cell undergoes meiosis and produces four haploid megaspores.

• The maturing megaspore undergoes three mitotic divisions, which produce a cell that has eight haploid nuclei.




• The haploid, megaspore nuclei migrate to certain locations within the cell.

• The nuclei are initially arranged in two groups of four at the top and bottom of the cell.

• Two nuclei will come to the center from the ends or poles and are called polar nuclei. These are the nuclei that will eventually fuse with the sperm cells.




• The other six nuclei within the megaspore mother cell help the polar nuclei be fertilized by the sperm and then eventually die after fertilization occurs.

• The megaspore mother cell is now called the embryo sac and contains eight nuclei—seven smaller cells and a large central cell that encloses all the other cells.




• The embryo sac is also known as the mature female gametophyte, or megagametophyte.

• The surrounding integuments and the embryo sac now form a mature ovule, which when fertilized will develop into a seed.

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Formation of A Female Gametophyte


Chapter 30


Visual Concept

Ovule Formation in an Angiosperm




Pollen Grain Formation

• An anther contains four microsporangia, or pollen sacs.

• Initially, the pollen sacs contain many diploid

cells, and are called microspore mother cells.

• Each of these microspore mother cells will undergo meiosis and produce four haploid microspores.



Pollen Grain Formation, continued

• Each microspore undergoes mitosis and produces two haploid cells that do not separate.

• Once the cells are haploid, a thick wall then develops around the microspore.

– The resulting two-celled structure is a pollen grain, which is the male gametophyte, or microgametophyte.



Pollen Grain Formation, continued

• The larger of the two cells is the tube cell, from which the pollen tube will form.

• The generative cell, which is enclosed in the tube cell, will divide by mitosis to form two sperm.

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Formation of A Male Gametophyte


Chapter 30


Visual Concept

Pollen Grain Formation


Chapter 30


Visual Concept

Parts of a Pollen Grain



Pollination

• In flowering plants, pollination occurs when pollen grains are transferred from an anther to a stigma.

• Pollination that involves just one flower, flowers on the same plant, or flowers from two genetically identical plants is called self-pollination.

• In contrast, pollination that involves two genetically different plants is called cross-pollination.


Pollination, continued

• Flower structure promotes self-pollination in plants that have flowers with petals that completely enclose both the male and female flower parts.

• Pollen can be dispersed by water or air. The flowers of such wind-pollinated angiosperms are small and lack showy petals and sepals.

• Successful wind pollination depends on four conditions: the release of large amounts of pollen, the ample circulation of air or water, proximity of other plants that it can pollinate, and dry weather.


Pollination, continued

• Bright petals and distinctive odors attract animals that feed on pollen and nectar, a nourishing solution of sugars.

• Many different kinds of animals can be pollinators.

• When these animals gather nectar, pollen sticks to their bodies. As they collect more nectar, the animals deposit some of the pollen on other flowers. This is how the animals pollinate other flowers.

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Visual Concept

Flowers and Animal Pollinators



Fertilization

• For fertilization to occur in angiosperms, a pollen grain must land on a stigma and then absorb moisture. The pollen grain will then germinate.

• After fertilization, germination occurs. Germination is when the tube cell forms a pollen tube, and then grows through the stigma and style to the ovary.


Fertilization, continued

• The pollen tube will grow into an ovule through the micropyle on the ovary.

• After the pollen tube penetrates the ovule, two sperm can travel through the pollen tube and reach and fertilize the egg.



• One of the two sperm will fuse with the egg and form a diploid zygote, which will eventually develop into an embryo.

• The second sperm fuses with the two polar nuclei, producing a triploid (3n) nucleus. This nucleus then develops into tissue called endosperm.



• The endosperm provides nourishment for the embryo. Endosperm can be used up by the developing embryo or be could still be on the seed after the embryo is fully mature.

– This process of two cell fusions, which is called double fertilization, is unique to angiosperms.

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Visual Concept

Fertilization of a Flower


Chapter 30

Life Cycle of an Angiosperm


Section 3 Dispersal and PropagationChapter 30

Objectives

• Describe adaptations for fruit and seed dispersal.

• Name the three major categories of fruits.

• Compare the structure and germination of different types of seeds.

• Recognize the advantages and disadvantage of asexual reproduction.

• Describe human methods of plant propagation.


Dispersal of Fruits and Seeds

• One reason for the success of the seed plants is the development of structures that are adapted for dispersing offspring—fruits and seeds.

• Fruits and seeds are dispersed by animals, wind, water, forcible discharge, and gravity.

• Gymnosperms do not produce fruits, but their cones help protect seeds and aid in seed dispersal.


Types of Fruits

• Botanists define a fruit as a mature ovary.

• Many different types of fruits have evolved among the flowering plants.


Types of Fruits, continued

• Fertilization usually initiates the development of fruits.

• Fruits protect seeds, aid in their dispersal, and

often delay their sprouting.

• Fruits are classified mainly on the basis of two

characteristics: how many pistils or flowers form the fruit, and whether the fruit is dry or fleshy.

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Visual Concept

Development of a Fruit



Structure of Seeds

• A seed is a plant embryo.

• A seed is protected and surrounded by a protective coat called the seed coat.

• The structure of seeds differs among the major groups of seed plants and non seed plants.

• Angiosperm seed structure differs between monocots and dicots and differs when compared to gymnosperm seeds.


Structure of Seeds, continued

• A dicot seed has many parts.

• Between the two cotyledons of a dicot are the parts that make up the rest of the embryo.



• The parts of a dicot are the plumule, the epicotyl, the hypocotyl, the radicle, and the hilum.

– The shoot tip, along with any embryonic leaves, is called the plumule.

– The epicotyl extends from the plumule and is attached the cotyledons.



• The parts of a dicot are the plumule, the epicotyl, the hypocotyl, the radicle, and the hilum.

– The hypocotyl connects the cotyledons to the radicle.

– The radicle is the embryonic root.

– The hilum is a scar that marks where the seed was attached to the ovary wall.



• A monocot seed has many of the same parts as a dicot.

• One difference between the two is that a monocot seed does not store nutrients like a dicot does.



• A gymnosperm seed has some of the same parts as a seed plant.

• A difference between gymnosperms and seed plants is that a gymnosperm has needlelike cotyledons instead of leaves.

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Seed Structure


Chapter 30

Structure and Function of Seeds



Seed Germination

• Although its embryo is alive, a seed will not germinate, or sprout, until it is exposed to certain environmental conditions.

• Many seeds will not germinate even when exposed to conditions ideal for germination.

– These seeds exhibit dormancy, or a state of reduced metabolism in which growth and development do not occur.


Seed Germination, continued

Conditions Needed for Germination

• Most mature seeds are very dry and must absorb water to germinate and grow.

• Many seeds also need light for germination, and some even need certain or extreme temperatures to germinate.

• In order to keep growing, the seed must get oxygen.



Process of Germination

• The first visible sign of seed germination is the emergence of the radicle.

• Soon after the radicle breaks the seed coat, the shoot begins to grow.

• The type of seed determines how the shoot will break through the ground.



Process of Germination, continued

• In some seeds the hypocotyl curves and becomes hook-shaped. In these seeds, once the hook breaks through the soil, the hypocotyl straightens.

– This straightening pulls the cotyledons and the

embryonic leaves into the air, and they begin photosynthesis.



Process of Germination, continued

• In contrast, other seeds remain underground and transfer nutrients from the endosperm to the growing embryo.

• In these type of seeds, the plumule is protected by a sheath that pushes through the soil.

• Once the sheath has broken through the soil surface, the plumule grows up through the sheath and the first leaf unfolds.

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Seed Germination


Chapter 30


Visual Concept

Parts of a Seed



Asexual Reproduction

• Asexual reproduction is the production of an individual without the union of gametes, and is quite common in the plant kingdom.

• Asexual reproduction can be an advantage to individuals that are well-adapted to their environment.

• A disadvantage of asexual reproduction is the lack of genetic variation among the offspring.


Asexual Reproduction, continued

• In nature, plants reproduce asexually in many ways, including the development of spores and vegetative reproduction.

– Vegetative reproduction is asexual reproduction involving vegetative (nonreproductive) parts of a plant, such as leaves, stems, or roots.

– Many structures specialized for vegetative reproduction have evolved in plants.

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Stems Modified for Vegetative Reproduction


Chapter 30

Methods of Vegetative Plant Propagation



Propagation of Plants by Humans

• Many species of plants are vegetatively propagated by humans from various plant structures.

• People have also developed several methods of propagating plants from other vegetative parts, such as roots and even tissue samples.

• These methods include layering, grafting, and using cuttings and tissue cultures.


Propagation of Plants by Humans, continued

Cuttings

• In some plants, roots will form on a cut piece of a stem, or shoots will form on a piece of a root.

• Pieces of stems and roots that are cut from a plant and used to grow new plants are called cuttings.

• Cuttings are widely used to propagate houseplants, ornamental trees and shrubs, and some fruit crops.



Layering

• In some species roots will form on stems where they make contact with the soil.

• People often stake branch tips to the soil or cover the bases of stems with soil to propagate such plants.

• The process of causing roots to form on a stem is called layering.



Grafting

• Grafting is the joining of two or more plant parts to form a single plant, by attaching a bud or small stem of one plant to the roots or stems of a second plant.

• Grafting enables the desirable characteristics of two cultivars to be combined.

• Grafting is used to propagate fruit and nut trees and many ornamental trees and shrubs.



Tissue Culture

• The production of new plants from pieces of tissue placed on a sterile nutrient medium is called tissue culture.

• Unlike most animal cells, plant cells contain functional copies of all the genes needed to produce a new plant. Thus, it is possible for a whole plant to regrow from a single cell.



Tissue Culture, continued

• Because plants can be grown by using tissue cultures, millions of identical plants can be grown from a small amount of tissue.

• Tissue culture is used in the commercial production of orchids, houseplants, cut flowers, fruit plants, and ornamental trees, shrubs, and nonwoody plants.

Chapter 30


Visual Concept

Parts of Plants Eaten as Food


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