reproductive differences between gymnosperms and angiosperms: gymnosperms: 1) ovules attached to...
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reproductive differences between gymnosperms and angiosperms:
gymnosperms:
1) ovules attached to a megasporophyll
- megasporophylls often arranged in a megastrobilus (ovulate cone)
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2) a) megagametophytes consist of many cells;
b) eggs produced in archegonia
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3) microsporangia attached to surface of a microsporophyll
- microsporophylls often arranged in a microstrobilus (staminate cone)
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4) microgametophytes are very small
- produce sperm (not in antheridia)- are pollen grains
5) pollination by wind dispersal of pollen
6) pollen tube grows to an ovule and sperm is released
- sperm have flagella and swim to egg, but not through external water
7) seeds develop on surface of megasporophylls
- seeds are not enclosed (“naked”)
gymnosperm = naked seed
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CHARACTERISTIC GYMNOSPERMS
Placement of ovules Attached to megasporophylls, in a megastrobilus
Size of megagametophyte Many cells
Archegonia Present
Placement of microsporangia (pollen sacs)
Attached to microsporophylls, in a microstrobilus
Size of microgametophytes Very small; several cells
Antheridia Absent
Pollination Mostly by wind dispersal of pollen
Sperm Have flagella; swim through ovule to reach egg
Seed location On surface of megasporophylls;not enclosed (naked)
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angiosperms:
1) ovules (and seeds) are enclosed within ovaries;
- part of flower structure
angiosperm = “vessel seed”
2) megagametophytes consist of only a few cells
- produce eggs, but not in archegonia
3) microsporangia carried on stamens;
part of flower structure
4) sperm do not have flagella
5) double fertilization occurs
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CHARACTERISTIC GYMNOSPERMS ANGIOSPERMS
Placement of ovules Attached to megasporophylls, in a megastrobilus
Located in an ovary, within a flower
Size of megagametophyte
Many cells A few cells
Archegonia Present Absent
Placement of microsporangia (pollen sacs)
Attached to microsporophylls, in a microstrobilus
Located on a stamen, within a flower
Size of microgametophytes
Very small; several cells Extremely small, a few cells
Antheridia Absent Absent
Pollination Mostly by wind dispersal of pollen
Various mechanisms - wind, insect, bat, rodent, etc.
Sperm Have flagella; swim through ovule to reach egg
No flagella; deposited at egg
Seed location On surface of megasporophylls; not enclosed in a fruit ("naked")
Located enclosed in a fruit that develops from the ovary
Double fertilization Absent Present
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Flower Structure:
flower stalk (peduncle) and floral appendages attached to it
receptacle
floral appendages = sterile appendages and fertile appendages
sterile appendages = sepals and petals
sepals
petals
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fertile appendages = stamens and carpels
stamens - are “specialized microsporophylls”
- include: filament
anther
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carpels “modified megasporophylls”?
- produce a tubular pistil
- each has a stigma ;
an ovary;
a style
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5) double fertilization
- megagametophyte of angiosperms usually has seven cells
- one egg, two synergids, three antipodals - each with one 1n nucleus
- one central cell containing TWO 1n nuclei = polar nuclei
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pollination - pollen is transferred from pollen sac to a stigma
pollen germination
pollen tube grows through style, to an ovule
- releases two sperm into the ovule
1) one sperm (1n) fuses with the egg (1n) - produces zygote (2n)
2) second sperm fuses with two polar nuclei-produces a triploid (3n) primary
endosperm nucleus
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following double fertilization:
- zygote develops into embryo (within the ovule)
- the primary endosperm nucleus produces endosperm tissue (all cells with 3n nuclei)
- will provide nutrients to the embryo
- integuments harden to form seed coat
** produces a seed, consisting of:
- seed coat
- endosperm (not present in gymnosperms)
- sometimes a perisperm
- embryo
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In flowering plants, ovules are enclosed in ovaries
- ovaries undergo changes as seeds form- produces a fruit
- will usually enlarge
may become fleshymay become dry, or hard
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fruit - provides additional protection for the embryo
- contributes to seed dispersal
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“typical” flower structure varies with species
common variations include variation in appendages:
# of each type
size
color
shape
other variations:
placentation type:
placenta
placentation
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parietal placentation:
- ovules attached to side walls of ovary
- ovary with one or more locules
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axile placentation:
- ovules attached to partitioning walls in ovary
- ovary has two or more locules
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free central placentation:
- ovules attached to central column of tissue
- ovary has one locule
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basal placentation:
- ovule(s) attached to base of ovary
- ovary has one locule
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fusion of various floral appendages
coalescence
e.g. formation of tubular corolla
adnation
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variations in ovary placement:
hypogynous flowers
- ovary is superior - other appendages attached lower
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epigynous flowers
- receptacle tissue surrounds ovary
- other appendages attached above the ovary
- ovary is inferior
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perigynous flowers
- basal portion of sepals, petals, and stamens form structure that surrounds the ovary
- ovary is superior
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variations related to presence or absence of floral appendages:
complete flowers
incomplete flowers
perfect flowers
imperfect flowers
staminate flower
carpellate flower
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species with imperfect flowers may be monoecious or dioecious:
monoecious species
dioecious species
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Embryo structure
- seed and embryo structure also varies with species
- embryo includes a root-shoot axis, and one or two cotyledons
- root-shoot axis consists of
a) the radicle
b) the plumule
- includes epicotyl and embryonic leaves attached
c) the hypocotyl
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- one or two cotyledons attached at hypocotyl/epicotyl junction
- two classes of flowering plants
- dicots, have two cotyledons
- monocots, have one cotyledon
- cotyledons are food-storage organs
- may remain small
- may become thick and fleshy
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monocot embryos include additional protective tissue layers:
- the coleoptile
- the coleorhiza