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REPRODUCTION IN FLOWERING PLANTS.
Chapter 35
In flowering plants the diploid sporophyte generation is larger and nutritionally independent.
The haploid gametophyte generation, which is located in the flower, is microscopic in size and
nutritionally dependent of the sporophyte.
The gametophyte gives rise to gametes by mitosis.
Fertilization produces the diploid sporophyte, which produces spores by meiosis.
One haploid spore gives rise to the haploid gametophyte.
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. Petals may or may not
be present.
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.
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 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.
POLLINATION
Pollination is the transfer of pollen from the anther to the stigma.
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Plants can self-pollinate or cross-pollinate.
Flowering plants and their animal pollinators have evolved together.
Pollinators use nectar and pollen as sources of energy and protein.
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.
Insect-pollinated flowers often have yellow or blue petals and have scent.
Bird-pollinated flowers are often yellow, red or orange, and do not have scent.
Bat-pollinated flowers are creamy white and have strong scent.
Fly-pollinated flowers often smell like decaying flesh.
FERTILIZATION
Fertilization is the fusion of gametes.
Double fertilization is a unique phenomenon that occurs in angiosperms only.
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Egg and one sperm form the zygote.
The two polar nuclei and the second sperm form the endosperm.
Embryonic development follows a pattern:
Proembryo  globular embryo  heart-shaped embryo  torpedo stage  mature embryo.
A mature embryo consists of a radicle, hypocotyl, one or two cotyledons and the plumule.
The mature seed contains the embryo and the nutritive tissue (endosperm or cotyledons).
Seeds are enclosed in fruits. Fruits are ripened ovaries.
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.
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.
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.