Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
Plants moved from water to land; had to undergo some modifications:
1. Water is no longer main channel by which plants spread gametes
a. wind
b. animals (insects, etc.)
2. Embryos are protected in seeds
3. Vegetative reproduction is sexual or asexual depending on the
environment
Alternation of Generations in Angiosperms
-alternation of multicellular haploid gametophyte with diploid
sporophyte generation
-sporophyteÆrecognizable “plant”
-produces haploid spores
-spores undergo mitosis to produce multicellular male or
female gametophyte
-maleÆpollen grains
-femaleÆembryo sac
-gametophytes produce gametes (sperm and egg) by mitosis
-gametes fuse to form a zygote
-develops into a multicellular sporophyte
-sporophyteÆdominant stage in the angiosperm life cycle
-gametophyte stages are totally dependent on sporophyte
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
FlowersÆreproductive structure of angiosperm sporophytes
-4 sets of modified leaves:
1. sepalsÆenclose and protect floral bud before opening
2. petalsÆattract pollinators
3. stamensÆwhere male gametophytes develop (pollen grains)
4. carpels (pistils)Æ female gametophytes develop (embryo
sac)
Flower parts
1. carpel (pistil)Æfemale reproductive structure consists of:
a. ovaryÆwhere eggs are kept
b. styleÆlong tube connecting stigma with ovary
c. stigmaÆsticky surface that “catches” pollen grains
2. stamenÆmale reproductive structure
a. antherÆwhere pollen is made and stored
b. filamentÆstalk
3. sepals
4. petals
Variations of flowers:
1. Complete flowerÆa flower with sepals, petals, stamens, and carpels
2. Incomplete flowerÆa flower that is missing one or more of the parts
listed for a complete flower
ex: grassesÆdo not have petals
3. Perfect flowerÆhas both stamen and carpel (may still be incomplete
without petal or sepal)
4. Imperfect flowerÆmay have:
a. stamen, but no carpel (male)
b. carpel, but no stamen (female)
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
Development of Male and Female Gametophytes:
A. Pollen Development
-develops in anthers
-pollenÆextremely durable
-formation of pollen grain:
within anther, diploid microsporocytes undergo meiosis to
produce 4 haploid microspores
↓
haploid microspore undergoes mitosis-gives rise to:
a generative cell
a tube cell
↓
the wall of the microspore thickens and becomes sculptured
into a species specific pattern (evolutionary link between plant
species)
↓
2 cells and thickened wall are the pollen grain (male
gametophyte)
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
B. Embryo Sac development
-forms within ovuleÆstructure within ovary that contains female
sporangia
-development:
Megasporocyte in each ovule undergoes meiosis to form 4
haploid megaspores (usually 1 survives)
↓
Remaining megaspore grows and nucleus undergoes 3 mitotic
divisions- forming 1 large cell with 8 nuclei (haploid)
↓
Membranes partition this into a multicellular embryo sac
Within embryo sac:
1. Egg (when fertilized, becomes embryo) is located at one end
2. Two other cells surround eggÆsynergids (protect egg; guide tube
nucleus)
3. At opposite end are 3 antipodal cells (function unknown; later
degenerate)
4. Other 2 nuclei (polar nuclei-become endosperm, food for embryo)
are located at center of sac
5. At end of sac with egg is the micropyle
-opening of embryo sac
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
Embryo sac (8 nuclei)
Pollen grain
Pollination
-placement of pollen onto stigma of carpel
-some plants use wind, animals, or cross/self pollinate
Most angiosperms have mechanisms to prevent self-fertilization:
(contribute to genetic variation)
1. stamen and carpel mature at different times in some species
2. structural arrangement of flowers in many species pollinated by
animals reduces chance that pollinators will pollinate same flower
3. other species are bioincompatible
-if pollen grain lands on stigma of the same flower, a
biochemical block prevents pollen grain from developing and
fertilizing the egg
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
Self-fertilization Preventative Mechanisms
-contribute to genetic variation
-stamen and carpel mature at different times in some species
-structural arrangement of flowers in many species pollinated by
animals reduces chance that pollinators will pollinate same flower
-other species are bio-incompatible
-if pollen grain lands on stigma of the same flower, a biochemical block
prevents pollen grain from developing and fertilizing the egg
Double Fertilization
-when a compatible pollen grain lands on a stigma of an angiosperm
double fertilization occurs
Æunion of 2 sperm cells with 3 cells of the embryo sac
Pollen grain (2 nuclei)
Adheres to stigma
↓
germinates (continues in path of development)
↓
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
tube nucleus
generative nucleus
↓
↓
extends a pollen tube between
style cells to ovary
↓
directed by a chemical
attractant, (Ca), tip of
pollen tube enters
through micropyle
and discharges 2 sperm
nuclei into egg sac
divides by mitosis
to form 2 sperm
↓
fuses
fuses with 2 polar
with egg
nuclei, 3n nucleus
(2n)
↓
↓
endosperm (food
zygote
storing tissue)
After double fertilization, each ovule will develop into a seed
OvaryÆfruit surrounding the seed
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
Development of Seed and Nutrients
A. Development of the endosperm:
-begins before embryo development
-goes through several cell divisions (multicellular)
-rich in nutrients and provides energy to the developing
embryo
-monocotsÆendosperm stocks nutrients, which are available
after germination
-dicotsÆendosperm food reserves are exported to the
cotyledons; therefore, mature seeds have no endosperm
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
B. Embryogenesis (embryo development)
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
C. Structure of the Mature Seed:
-in mature seeds, embryogenesis ceases until germination
-seed dehydrates until water content is only 5-15% of total weight
-embryo is surrounded by endosperm, cotyledon, or both
-seed coat is formed from ovule
Structure of Seeds
Conditions of seed germination
1. water
2. passage through animal’s digestive system
-seed coat may be too tough (some)
3. oxygen
-needed for respiration
4. light
5. temperature
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
-vary species to species, also individual to individual
6. increase in hormones
-especially, gibberellins
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
Vegetative Propagation (Asexual)
A. In plants-plantlets
-meristematic tissues are composed of dividing, undifferentiated
cells that can sustain or renew growth indefinitely
-parenchyma cellsÆcan also divide and differentiate into various
types of cells
-ex: KalanchoeÆasexually reproduces offspring
B. In agriculture
-most are based on the ability of plants to form adventitious roots
and shoots
-objectiveÆto improve crops, orchards, ornamental plants
1. Cuttings
a. shoot or stem cuttings
-at cut end of shoot-mass of dividing, undifferentiated
cells formÆcalled callus
-adventitious roots develop from the callus
-cuttings may come from stems, leaves (ex: African
violets), or from specialized storage stems (ex:
potatoes)
2. Combining of 2 plant species via grafting twig of one plant onto
root/stem of another plant species
-plant providing root systemÆstock; twigÆ scion
-quality and type of fruit is usually determined by scion,
although sometimes, stock can alter a characteristic of
shoot system developing from scion
-ex: dwarf stocks retard growth of normal scion shoots
Plants can clone themselves via:
1. Fragmentation
-separation of parent plant into parts that will become whole plants
-most common form
-some species of dicotsÆ
-parental root system develops adventitious (grow from stems)
roots that may become separate shoot systems
-ex: stolons
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Plant Reproduction and Development Notes
AP Biology
Mrs. Laux
2. ApomixisÆproduction of seeds without meiosis and fertilization
-diploid cell in ovary gives rise to embryo
-skips gametophyte stage/fertilization
-ovules mature into seedsÆdispersed
-ex: dandelions
Both sexual and asexual reproduction have been featured in adaptations of
plants to their environments
A. Benefits of Sexual Reproduction:
1. Generates variation
-asset when environment changes
2. Production of seed
a. disperse to new locations
b. will remain dormant until conditions become
favorable
B. Benefits of asexual reproduction:
1. plants can clone many copies of themselves in a short
period of time
2. progeny are mature fragments of parent plant, not fragile
seedlings
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