CHAPTER 38 PLANT REPRODUCTION. Sexual Reproduction & Biotechnology. Floral Organs. Sepals and petals are nonreproductive organs. Sepals : enclose and protect the floral bud before it opens; usually green and more leaf-like in appearance.
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CHAPTER 38 PLANT REPRODUCTION
Sexual Reproduction & Biotechnology
Sepals and petals are nonreproductive organs.
Sepals: enclose and protect the floral bud before it opens; usually green and more leaf-like in appearance.
In many angiosperms, the petals are brightly colored to attract pollinators.
Stamens: male reproductive organs
Carpels: female reproductive organs
Ovary- base of the carpel
Embryo Sac (female gametophyte), i.e., seed
Stigma- platform for pollen grain
Style- slender neck, connects ovary and stigma
The stamens and carpels of flowers contain sporangia, within which the spores and then gametophytes develop.
The male gametophytes are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers.
The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries.
Pollination begins the process by which the male and female gametophytes are brought together so that their gametes can unite.
Pollination- when pollen released from anthers lands on a stigma.
Each pollen grain produces a pollen tube, which grows down into the ovary via the style and discharges sperm into the embryo sac, fertilizing the egg.
The zygote gives rise to an embryo.
The ovule develops into a seed and the entire ovary develops into a fruit containing one or more seeds.
Fruits disperse seeds away from the source plant where the seed germinates.
Complete Versus Incomplete Flowers
Complete: possess sepals, petals, stamens, and carpels
Incomplete: lack one or more of these components
Perfect Versus Imperfect Flowers
Perfect: possess both stamens and carpels
Imperfect: possess either stamens (staminate) or carpels (carpelate), but not both
Monoecious: both staminate and carpellate flowers are found together on the same plant (e.g., corn).
Dioecious: staminate flowers occur on separate plants from those that carry carpellate flowers (e.g., date palms).
Mature pollen grains are entire haploid male gametophyte plants.
Microsporangia in anthers produce microsporocytes that undergo meiosis and become haploid microspores.
Each microspore undergoes mitosis to produce two-celled male gametophyte plants (pollen grains).
One cell is the generative cell while the other is the tube cell.
Entire mature female gametophyte plants spend their entire lives supported by the parent sporophyte.
Megasporangia in ovules produce megasporocytes that each undergo meiosis and become four haploid megaspores.
Only one of these four cells become functional megaspores, the remaining three degenerating.
The haploid nucleus of the megaspore undergoes three mitotic divisions to produce a multinucleate cell with eight haploid nuclei.
Cytokinesis divides these nuclei into seven cells: three antipodal cells, two synergids, one egg cell, and one binucleate central cell.
Together these cells form the mature female gametophyte or embryo sac.
Fertilization involves a double fertilization event.
After attachment to the stigma, the haploid generative cell of a pollen grain undergoes mitosis to produce two sperm nuclei.
The two sperm nuclei migrate down the pollen tube as it elongates through the style to the ovary containing the ovules.
One sperm nucleus enters the egg cell; the other enters the binucleate central cell of the female gametophyte.
The central cell is trinucleate for a while.
Fusion of all three haploid nucleus yield one triploid nucleus.
Mitosis of the triploid central cell produces the multinucleate triploid endosperm tissue.
This endosperm tissue represents a source of stored organic energy to be used by the developing sporophyte embryo (derived from the zygote) and to be used during seed germination.
The development of angiosperm gametophytes involves meiosis and mitosis.
The male gametophyte begins its development within the sporangia (pollen sacs) of the anther.
Within the sporangia are microsporocytes, each of which will from four haploid microspores through meiosis.
Each microspore can eventually give rise to a haploid male gametophyte.
A microspore divides once by mitosis and produces a generative cell and a tube cell.
The generative cell forms sperm.
The tube cell, enclosing the generative cell, produces the pollen tube, which delivers sperm to the egg.
Typical Monocot (e.g., corn)
endosperm present in substantial quantities in mature seed.
cotyledon absorbs nutrients from endosperm during seed germination.
Typical Dicot (e.g, garden bean)
endosperm completely absorbed into cotyledons during seed maturation.
Other Dicots (e.g., castor bean)
endosperm only partially absorbed by cotyledons during seed maturation.
remainder of endosperm absorbed by cotyledons during germination.
As the seeds are developing from ovules, the ovary of the flower is developing into a fruit, which protects the enclosed seeds and aids in their dispersal by wind or animals.
Pollination triggers hormonal changes that cause the ovary to begin its transformation into a fruit.
If a flower has not been pollinated, fruit usually does not develop, and the entire flower withers and falls away.
The ovary wall becomes the pericarp, the thickened wall of the fruit
Other flower parts wither and are shed.
However, in some angiosperms, other floral parts contribute to what we call a fruit.
Development of a pea fruit (pod)
Protection of the enclosed seed (e.g., pea pods).
wings for wind dispersal (e.g., maple).
hocks and barbs for attachment to animal fur or avian feathers (e.g., cocklebur).
sweet, fleshy fruit encouraging ingestion and dispersal of seeds by animals (e.g., cherry).
Function: allows seeds to germinate at the most optimal time.
Length of dormancy
Signals triggering the end of dormancy.
occurrence of water
period of cold temperature
Asexual and Sexual Reproduction in the Life Histories of Plants
consequence of the existence of meristematic tissues and indeterminate growth in plants
typically involves fragmentation
=production of seeds without fertilization
diploid cell in ovule develops into embryo
Shoot or stem cuttings generate roots.
Cloning from single leaves.
Potato eyes used to generate whole potato plants.
Plant tissue culture.
Injecting foreign DNA into host cells
the sum of all of the changes that progressively elaborate an organism’s body; involves growth, morphogenesis, and cellular differentiation
an irreversible increase in size resulting from increases in cell number and size
the development of form
the specialization of cells into different types with different functions
Plant and Animal Development
Indeterminant growth in plants means that morphogenesis is a continuous, never-ending process.
Plant morphogenesis involves oriented cell division and growth but not the migration of cells to different parts of the plant body.
Ring of microtubles (preprophase band) in the cortex of the cell determines the division plane.
Preprophase band of microtubules disappears, leaving an imprint of actin microfilaments.
hold nucleus in place until spindle apparatus forms.
directs the movements of cell plate vesicles.
Orientation of Mitosis
Involves changes in gene expression, not in the genomic information of the cells.
development of specific structures in specific locations
depends upon positional information of the cells
also related to positional consequences of cell division and elongation
Genetic Basis for Pattern Formation in Flower Development