Megasporogenesis: taking place inside the ovule inside the ovary.
megasporocyte
meiosis I
4 megaspores
meiosis II
3 disintegrate
functional
megaspore
mitotic divisions without cytokinesis megagametophyte
3 antipodals
central cell
cytokinesis
egg
2 synergids
Compare Fig. 41.7 Pg. 828
and Fig. 41.11 Pg. 832
central cell
egg
embryo sac
SYNGAMY
gametes
zygote
germination
mitosis
differentiation
mitosis
gametangia
differentiation
Gametophyte1N
antipodals
2N
Sporophyte
differentiation
differentiation
mitosis
germination
sporangium
mitosis
spores
sporocyte
ovule
MEIOSISmegasporocyte
synergids
http://www.bakerlite.co.uk/pics/Wales/South%20Wales/Vale/Trees/Tree-8.jpg
Life Cycle of Flowering Plants
The large plant is a sporophyte.
Its flower produces the sporangia; on the female side, the ovule of the carpel
The ovule contains a megasporocyte that divides by meiosis to make four megaspores
Three megaspores disintegrate.
The remaining megaspore cell divides by mitosis to make 8 nuclei.
Cytokinesis divides the 8 nuclei into seven cells of the megagametophyte (embryo sac)
The egg cell and the central cell are gametes that fuse with two sperm cells in syngamy
The ovule, with zygotic embryo and triploid endosperm, becomes a seed.
Compare Fig. 41.11 Pg. 832
In Flowering Plants, syngamy is a double event:
syngamy#2:
sperm
nuclei
2 x 1N polar nuclei + 1N sperm
= 3N primary endosperm cell
no miracle-grow, no compost!
syngamy#1:
1N egg + 1N sperm = 2N zygote
synergids
disintegrating
http://botit.botany.wisc.edu/courses/img/Botany_
130/Diversity/Angiosperms/Fertilization.jpg
In this slide, plasmogamy has
already occurred…but what
part of syngamy has not
occurred yet? karyogamy!
How about Dicots?
seed
coat
cells
Cells to become storage tissue
Polar Nuclei
Sperm Nucleus
Syngamy => Endosperm 3N
Egg Cell
Sperm Nucleus
Egg Nucleus
Syngamy => Zygote 2N
globular
embryo
In Dicots (Capsella),
the development lacks
competition, but the
sequence is similar.
These are globular
embryos with
suspensors.
suspensor cells
Compare Fig. 41.12
Pg. 833
Compare Fig. 23.5 Pg. 471
Protoderm
Ground
meristem
Undeveloped?
Provascular
Heart-shaped embryo
with suspensor
Compare Fig. 41.12 Pg. 833
Maturing embryo “torpedo”
Compare Fig. 41.12 Pg. 833
Mature
embryo
Capsella Seed:
Seed Coat
Endosperm
Embryo
Shoot Apex
Cotyledons eudicot
Hypocotyl
Radicle
Root Apex
Micropyle
Lettuce Seed Germination
radicle penetrates
seed coat
starch
Seed Coat
shoot apex
cotyledons
-amylase
sugar
dicot
RNA
Embryo
Compare Fig. 40.1
Pg. 801
red and
white light
stimulate
germination
DNA
phytochrome
radicle apex
water
photoactivation
imbibition
photoreversibility
660 nm
Pfr
Pr
730 nm
dark