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Chapter 36
PLANT REPRODUCTION
Only one group of plants produces flowers, the angiosperms.
Bryophytes (mosses, liverworts, etc.), pteridophytes (ferns, ground pines), gymnosperms
(conifers, ginkgo, etc.) do not produce flowers.
PLANT REPRODUCTION - AN INTRODUCTION
Flowers are a collection of organs designed to produce gametes, attract gametes, and develop
seeds.
Sexual reproduction
Most plants reproduce sexually.
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Meiosis: reduction in the number of chromosomes.
Gametogenesis: Formation of sperm and eggs.
Fertilization: fusion of sperm and egg.
Flowers having both male and female organs are called perfect.
Flowers having either male or female organs are called imperfect.
Many plants are capable of outcrossing, that is, they exchange gametes with other plants of
the same species.
Many plants are also capable of self-fertilization.
Plant life cycle
Plants have a characteristic life cycle called alternation of generations.
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Sporophyte generation is diploid, 2n, and produces haploid spores through meiosis.
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Gametophyte generation is haploid, n, and produces haploid gametes through mitosis.
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Fertilization restores the diploid stage; meiosis restores the haploid stage.
Asexual reproduction.
Offspring are formed without the fusion of gametes.
Offspring are genetically similar to the parent plant.
Stems, leaves and roots may be adapted to asexual reproduction.
Apomixis is the formation of seeds without fertilization, akin to parthenogenesis is animals.
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The new generation can come from an unfertilized ovule or from a vegetative cell.
Modified stems may give rise to independent plants in time:
Rhizomes are underground horizontal stems that may or may not be fleshy.
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They produce new plants by fragmentation of the old rhizome.
Tubers are fleshy underground stems enlarge for food storage.
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Tubers produce independent plants once the parent plant dies.
Bulbs are modified underground buds attached to short stems with storage leaves.
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It frequently forms axillary buds that separate and grow into independent plants.
Corms are short, erect underground stems that store food in their tissues and are covered with
papery leaves.
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Small axillary buds give rise to new corms.
Stolons or runners are horizontal, aboveground stems.
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They produce buds that give rise to small plants that root and become independent.
Modified leaves can produce plantlets that break off and give rise to new plants.
REPRODUCTIVE STRUCTURES
Different groups of plants have their own characteristic male and female reproductive structures.
Angiosperms are the most abundant group with about 230,000 species.
Flower formation begins in the apical meristem.
Flower parts are modified leaves. They differentiate under the influence of environmental
signals.
Day length, hormones and other cues.
Photoperiod is the length of daylight in a 24-hour day.
Short-day plants (long-night plants) flower when the night length is equal to or greater than
some critical period.
 Plant detects the shortening of the day or lengthening of the night.
 Minimum critical night length varies with the species.
 Fall flowers like poinsettias and chrysanthemums.
Long-day plants (short-night plants) flower when the night length is equal to or less than
some critical period.
 Plant detects the lengthening of the day and shortening of the night.
 Maximum critical night length varies with the species.
 Spring flowers.
Plants in the tropics are not sensitive to day length because the day length variation is minimal.
Desert plants are sensitive to watering, the rainy season, rather than day length.
When light, water and nutrients are present in favorable amounts, the plant may begin to flower
before the arrival of the correct day length.
Gibberellins or other hormones could trigger the production of reproductive organs.
Genes respond to the signal to flower.
Researchers have confirmed that different genes are responsible for the response to different
flowering signals.
Blazquez and Wigel found that genes controlling flower development have promoters with
binding sites for more than one type of transcription activator.
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Day length and hormones (GA)
Structure of the flower.
The flower is a modified branch apex, and is involved in sexual reproduction.
Reproductive and accessory organs are normally arranged in whorls or circles of structures:
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Sepals, petals stamens, and carpels.
The whorls of organs sit on an enlarged branch end called the receptacle.
The sepals form the calyx and protect the flower bud.
The petals form the corolla and attract animals to assist in pollination.
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Petals may or may not be present.
Nectaries may be located at the base of petals and contribute to pollination.
The stamens are the male reproductive organs.
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They consist of a filament and an anther.
Pollen grains form in the anthers.
Each pollen grain contains two cells; one produces two sperm nuclei, and the other
produces a pollen tube to transfer the sperm nuclei to the ovule.
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A pollen grain represents a male gametophyte.
The carpels are the female reproductive structures.
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A flower may have one or several carpels.
Carpels may be separate or fused.
Carpel usually has a style and stigma.
Ovary is another name for the lower portion of the carpel.
An ovary may be formed by various fused carpels.
Pistil is another name for the female reproductive structure.
A pistil may be formed by a single carpel or by several fused carpels.
The ovary contains one or several ovules.
The ovule produces and contains the embryo sac.
The embryo sac produces two polar nuclei and one egg.
The egg and the polar nuclei are involved in the process of double fertilization.
THE ANGIOSPERM LIFE CYCLE
Producing the female gametophyte.
OVULE → MEGASPORE → MEGAGAMETOPHYTE (EMBRYO SAC) → EGG
1. The ovule consists of an inner tissue called the nucellus and one or two protective layers
called integuments.
2. The integuments form one small opening at one end of the ovule, the micropyle.
3. The one diploid cell in the nucellus produces four haploid cells or megaspores through
meiosis.
4. Three nuclei degenerate and one haploid nucleus remains.
5. The functional megaspore enlarges at the expense of the other cells of the nucellus.
6. This nucleus inside the cell divides mitotically twice, forming first four nuclei and then eight
nuclei.
7. The eight nuclei separate and form six small cells and one large cell, the megagametophyte
or female gametophyte.
8. Three cells migrate towards the micropylar end, one cell becomes the egg and the two
others are called synergids eventually degenerate.
9. Three other cells migrate to the end opposite to the micropyle.
10. Two nuclei remain the in the center of the large cell. These two nuclei are called the polar
nuclei.
11. At this point this structure is called the female gametophyte or megagametophyte or
embryo sac.
Producing the male gametophyte
ANTHER → SPOROGENOUS TISSUE → MICROSPORE → MICROGAMETOPHYTE (POLLEN GRAIN) →
GENERATIVE CELL → MALE NUCLEI
1. The anther and the filament make the male reproductive organ of the plant.
2. Inside the anther, the diploid cells of the sporogenous tissue divides meiotically to produce
haploid microspores.
3. Each diploid cell produces four haploid microspores.
4. Each of the four cells forming called now microspores, divide mitotically to form pollen grain
made of generative cell enclosed in a larger vegetative cell.
5. A very resistant outer layer of sporopollenin called the exine, and an inner layer of pectin
called the intine surrounds this two-cell structure or pollen grain.
POLLINATION AND FERTILIZATION
Pollination and fertilization are two different things.
Fertilization occurs in all plant groups, from mosses to flowering plants.
Pollination occurs in gymnosperms and in angiosperms.
Pollination
Pollination is the transfer of pollen from the anther to the stigma.
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Plants can self-pollinate or cross-pollinate.
During pollination the male and female gametophytes are brought together.
Animals or wind usually carry pollen (male gametophytes) to the stigma of the flower.
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Water is the medium of pollen transport in some specialized cases.
Flowering plants and their animal pollinators have evolved together. This is mutualistic
relationship between the plant and the animal.
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Pollinators use nectar and pollen as sources of energy and protein.
Plants are pollinated and outcrossing can take place.
Some plant species deceive the pollinators and do not provide food.
Mosses and their relatives, and pteridophytes do not form pollen are the first groups to evolve.
Gymnosperms form pollen and are wind pollinated.
Angiosperms are the last group of plants to evolve and are mostly animal and wind pollinated.
Wind and animal pollination eliminates the need of water for sexual reproduction and allows
plants to colonize upland environments away from water.
Pollination by animals can be a very precise process and a more efficient process.
Evolutionary changes in animals affect plant populations and vice versa. Angiosperms and
animal pollinators are an example of coevolution.
It is estimated that insects pollinate about 70% of flowering plants.
About 30% of our food come from crops pollinated by bees.
Plants pollinated by wind often have reduced or absent petals, produce large amounts of pollen
and do not have scent.
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Insect-pollinated flowers often have yellow or blue petals and have scent.
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Bird-pollinated flowers are often yellow, red or orange, and do not have scent.
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Bat-pollinated flowers are creamy white and have strong scent.
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Fly-pollinated flowers often smell like decaying flesh.
Self-incompatibility
Some plants can self-fertilize if pollen from the same plant fall on the stigma.
Other plants cannot self-fertilize: these plants are self-incompatible.
There are several mechanisms involved in the compatibility or not of the pollen and the stigma.
The S locus in the cabbage family:
1. The "S locus" consists in reality of three loci.
2. There are multiple alleles of these genes, up to 50.
3. The proteins coded by these loci are located one in the membrane of the stigma cells,
another in the cell wall of the stigma cells, and the third is secreted by mature pollen
grains.
4. If the proteins secreted by the pollen are the same as one or both of the proteins in the
cell membrane and wall of the stigma, the pollen grain does not form a pollen tube.
Similarity of alleles means that they are probable from the same plant.
5. If the proteins secreted by the pollen tube are different from both of those in the cell wall
and the cell membrane of the stigma, the pollen tube forms. The pollen comes from a
different plant.
Fertilization
Fertilization is the fusion of gametes.
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It restores the diploid condition in the zygote.
The male gametophyte produces a long tube that grows through the stigma, the style and
enters the micropyle of the female gametophyte (syn. embryo sac).
The generative cell of the pollen divides to form two sperm nuclei that move into the pollen tube.
Double fertilization is a unique phenomenon that occurs in angiosperms only.
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Egg and one sperm form the zygote, 2n.
The two polar nuclei and the second sperm form the endosperm, 3n.
The endosperm stores food for the developing embryo.
The ovule will develop into a seed and the ovary will develop into a fruit.
THE SEED
The zygote is the first cell of the sporophyte.
As the seed matures inside the ovary, the ovule and the embryo become surrounded by a tough
seed coat.
At the same time, the ovary develops into a fruit that protects the seeds and aids in their
dispersal.
Embryogenesis
Embryonic development follows a pattern:
The zygote divides into two cells; the upper cell develops into the embryo proper, and basal cell
develops into a column of cells that brings in nutrients and gibberellins to the developing
embryo.
The suspensor is short lived and undergoes a process of programmed cell death or apoptosis.
Proembryo  globular embryo  heart-shaped embryo   mature embryo.
A mature embryo consists of a radicle, hypocotyl, one or two cotyledons and the plumule.
Radicle: embryonic root.
Hypocotyl: embryonic or seed stem.
Seed maturation
Cotyledons: embryonic or seed leaves.
Plumule: embryonic bud or meristem.
The mature seed contains the embryo and the nutritive tissue (endosperm or cotyledons).
During maturation seeds undergo a drying process.
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Water makes more than 80% of the plant cell but only 5 - 20% of the seed cells.
Germination will occur only when the seed absorbs water, when water is available in the
environment.
Sugars seem to be involved in maintaining the stability of the cell membranes in seeds by
forming a viscous sugary material with little or no water.
When water becomes available, the sugary viscous liquid dissolves and germination starts.
Fruit development and seed dispersal.
Seeds are enclosed in fruits.
Fruits are ripened ovaries and may or may not include other parts of the flower.
The ovary wall or pericarp thickens during maturation.
Fruit classification
1. Simple fruits develop from a single pistil.
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One or several carpels involved.
May be fleshy (e.g. berries) or dry (e.g. grains).
2. Aggregate fruits develop from a single flower with many separate ovaries (e.g.
blackberries, gumballs).
3. Multiple fruits develop from a many flowers growing closely together on a common axis
(e.g. pineapple).
4. Accessory fruits develop from tissues other than the ovary (e.g. strawberry, apples, and
pears).
Seeds and fruits are adapted to various means of dispersal, including wind, water, animals and
explosive dehiscence.
Dry simple fruits are classified as dehiscent (to break open) or indehiscent (not break open)
Most dry fruits are dispersed by the wind or fall to the ground. Some dry fruits have hooks that
attach to the fur of animals or clothes and are transported far away.
Animals are the most common dispersal agents of fleshy fruits. The seeds pass through their
digestive track unharmed and germinate after the animal defecates at a different location.
Seed dormancy
Seed dormancy is characteristic of plants that live in seasonal climates.
There is not one single mechanism involved in seed dormancy.
In some plants, dormant seeds have high concentration of abscisic acid (ABA).
When the dormant seeds are exposed to water, the abscisic acid washes out and germination
starts.
Some seeds have a very thick and resistant coat that does not allow water or oxygen to reach
the embryo. These seeds must be scarified by abrasion, fire or passage through the digestive
track of an animal.
Other seeds must undergo a period of cool or freezing weather before germination.
Seed germination
During germination, glucose breakdown may be entirely anaerobic.
Three phases have been distinguished in seed germination:
Phase 1:
Phase 2:
Phase 3:
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Increase in water uptake, oxygen consumption, and protein production.
No new mRNA is transcribed at this point.
The processes are controlled by mRNA stored in the seed.
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Water uptake stops.
Newly transcribe mRNA is translated into proteins.
Mitochondria begin to multiply.
Proteins are manufactured to support growth.
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Water imbibition is resumed.
Cells enlarge and the embryo burst through the seed coat.
Radicle emerges and begins to penetrate into the soil.
Hypocotyl elongates.