Angiosperm life cycle

Today’s lecture
1.
2.
3.
4.
5.
Finish pine life cycle as typical gymnosperm
Compare seed plants to non-seed plants
Angiosperm reproductive characters
Angiosperm life cycle
Angiosperm advancements
Pinus life cycle –seed plant
Pages 442-443 text
Conifer sperm are non-flagellate
Spermatogenous cell divides in pollen tube to
produce 2 sperm
pollen tube
sperm
nuclei
egg
nucleus
egg cell
cytoplasm
Pinus life cycle –seed plant
Pages 442-443 text
Nucellar apex
Cycads & Ginkgo have
multiflagellate sperm
Seed evolution and changing life cycles
Homospory, heterospory and the seed habit
Seed evolution steps
homospory
Megasporangium
surrounded by
sterile telomes =
branches
heterospory
Free telome
tips
Evolution of the seed/ovule
Homospory
Produce 1 size of spore
Land plant life cycles
Exosporic gametophyte development
Free living gametophytes with multicelluar sex organsantheridia and archegonia
Free sporing plants (=shed their spores from the sporangia)
Need water for spore germination, gametophyte development
and fertilization to take place
Heterospory
Produce 2 sizes of spores (megaspores and microspores)
Endosporic gametophyte development (inside of spore wall)
Free sporing plants
Only need water for fertilization as gametophytes develop
inside the spore wall
Seed habit
• Plants produce 2 sizes of spores
• Endosporic gametophytes
• Megasporangium never sheds its
megaspore – i.e., it is not free sporing
• Has eliminated the need for water in the
completion of the life cycle
• New to seed plants-the integument,
pollen tubes that deliver the sperm to the
archegonia or close to it.
Cycads-swimming sperm
Conifers-non-flagellate sperm
Angiosperm
life cycle
Definitions
• Angiosperms = flowering plants
• “Angio” = container “sperm” = seeds
Contained seeds – seeds of angiosperms
are contained in a structure termed a carpel
Reproductive Characters of
angiosperms
• Carpel encloses the seeds
• Double integument for seed
• Flowers – determinate stem tip with
leaves and modified leaves that bear
the pollen and seeds
• 7-celled, 8 nucleate megagametophyte
• 3-celled microgametophyte
• Double fertilization
• Endosperm
stigma
stamen
anther
style
filament
pistil
ovary
petal
ovules
sepal
receptacle
FLOWER
pedicel
Flower parts
Pistil = stigma, style &
ovary
Gynoecium = all the carpels
Androecium = all stamens
Calyx = all the sepals
Corolla = all the petals
Stamen = filament & anther
Anther = 4 pollen sacs
Tepals = collective term used
when sepals & petals look alike
Anther x.s.
4 pollen sacs
Vascular
Bundle of
Connective
Monocot flower x.s.
Note: in this flower the sepals and petals often
look alike (colored) therefore = tepals
Lily flower
Carpels
• The units that contain the seeds
• Part of the ovary
• If the ovary has more than one carpel you usually see
more than one locule (chamber containing seeds)
• You can sometimes tell how many carpels are in a flower
by looking at the tip of the style. Number of style tips or
lobes = number of carpels
• Carpels are leaves that have rolled up to enclose the
ovules. Later a number of these may have fused
together to form syncarpous ovaries (i.e., they evolved
from leaves)
• Note: stamens are also modified leaves with attached
pollen sacs
Angiosperm
Ovule
Central cell
Nucellus
7 celled embryo sac = megagametophyte
Meiosis takes place in the pollen sac of the anther
Microspore mother cells/ microsporocytes (2n) undergo meiosis
Four microspores result from each division- these are
Haploid (1n).
Tapetum – nutritive tissue (also lays down the sporopollenin walls)
Pollen tetrads – after meiosis
Pollen shed in a 2-celled stage
Microgametophyte
is only 3 cells in
total
Tube cell or
Tube cell (vegetative cell) controls
The growth of the pollen tube
Generative cell divides to form the
2 sperm
pollen
Megagametophyte development
Young developing ovule with
megasporocyte or megaspore mother cell –
undergoes meiosis
Embryo sac = megagametophyte
• Monosporic embryo sac development = after
meiosis four haploid megaspores result. 3
abort and 1 goes on to divide and form the
megagametophyte. Mono=one – Most
angiosperms
First division of the megaspore nucleus
Most angiosperms have monosporic embryo sac
development
Two nucleate megagametophyte stage
Second mitotic division
Four-nucleate phase
Angiosperm
Ovule
Central cell
Nucellus
7 celled embryo sac = megagametophyte
Mature embryo sac
•
1.
2.
3.
4.
Contains 8 nuclei, 7 cells
Egg
2 synergids (“nurse cells”)
3 antipodal cells
2 polar nuclei in the central cell
Note: There are no archegonia
Gametophytes in angiosperms are very
reduced
Microgametophyte is 3 cells– mega. is 7cells
A trend in reduction in the size of vascular
plant gametophytes over time.
Embryo sac = megagametophyte
micropyle
Egg & synergids
at this end
Central cell
Polar nuclei
3 antipodal
cells
Central
cell
Pollen on stigmatic surface (stigma)
Pollination in Angiosperms
.
The transfer of pollen from the anther
to the stigma
.
Pollination droplet
Integument
Megaspore
Ovule
enclosed
in
carpel
Pollination in gymnosperms
Nucellus
The transfer of pollen from the pollen
sac to the micropyle of the seed/ovule
Mature
microgametophyte
&
pollen tube growth
Tube nucleus
Angiosperm
Ovule
Double
fertilization
Central cell
2 sperm
Nucellus
Pollen tube
7 celled embryo sac = megagametophyte
Primary endosperm
nucleus
3n
(degenerate)
xx x
(degenerate)
x x
Zygote 2n
Double fertilization
• 1 egg + 1 sperm = 2n zygote
• 2 polar nuclei + 1 sperm = 3n endosperm
(primary endosperm nucleus)
• Primary endosperm nucleus ÷ to form
endosperm tissue cellular (or free nuclear)
• “Endosperm” only found in angiosperms
Becomes the food supply for the
developing embryo (3n tissue)
After double fertilization
M
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Endosperm divides 1st then the zygote
M
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Endosperm can be cellular or free nuclear
Capsella young dicot embryo
2n
2n
3n
Capsella mature embryo l.s.
Angiosperm life cycle
Pages 472-473