Reproductive Life Cycles of Vascular Plants
Plant life cycles are characterized by alternate
sporophytic and gametophytic generations.
Reproductive Life Cycles of Vascular Plants
The sporophyte produces
specialized reproductive
structures that facilitate
gamete production
through meiosis.
This initiates the
gametophytic generation.
Reproductive Life Cycles of Vascular Plants
Male and female gametes
have a haploid genetic
composition.
Fusion of these gametes
(fertilization) results in
a reproductive zygote
(embryo) that restarts
the sporophytic
generation.
Reproductive Life Cycles of Vascular Plants
Plants
The most primitive nonvascular and vascular
plants sexually reproduce
by spores.
The more advanced
vascular plants produce
seeds.
Non-vascular
Mosses
400 million years ago
Vascular
Seedless plants
Lycopods
Horsetails
Spores
Ferns
Seed plants
Seeds
Cycads
Ginkgo
Conifers
Angiosperms
300 million years ago
200 million years ago
100 million years ago
55 million years ago
Seed plants
Seed plants are separated into gymnosperms and angiosperms.
The seed habit is characterized by several anatomical
features that differentiate them from spore-producing plants:
(1) Rather than producing a single spore type (homospory),
seed plants produce a separate female megaspore and male
microspore (heterospory).
(2) The female gametophyte is retained on the mother plant
(sporophyte) and is enclosed within a protective maternal
seed coat.
(3) The ovule has an opening designed to receive pollen that
does not depend on water for male gamete transfer.
Reproductive Life Cycles of Vascular Plants
Seed Plants - Extinct seed producing plants
The evolution of the seed habit
began during the Devonian period
about 350 to 385 million years ago
in progymnosperms.
Fossil leaf
Progymnosperms were sporeproducing, but showed heterospory.
Heterospory is a first step in the
evolution of a true seed.
Progymnosperms are considered
the common ancestor of all seed
plants.
Archaeopteris
Reproductive Life Cycles of Vascular Plants
The “seed ferns” in the late Devonian
were the first plants to produce seedlike structures enclosed in female
tissue called cupules.
The cupules contained a single seed
(megaspore) within protective
coverings.
Cupules
Cupules
Alethopteris
Seed Plants - Gymnosperms
Gymnosperms are the oldest living
seed producing plants.
The term gymnosperm means
“naked seeds” and refers to the
absence of ovary tissue covering
the seeds, which is a characteristic
of angiosperms (flowering plants).
Gymnosperms include the cycads,
ginkgo, gnetophytes (Ephedra,
Gnetum) and the conifers (like pine,
fir, and hemlock).
Fir (Abies) cone
Cycads
Cycads appear in the fossil
record 320 million years ago.
Encephalartos
Cycads
The living cycads are palm-like in character and
mostly tropical and subtropical.
Cycas
Encephalartos
Cycads
Cycads produce male and female cone-like sporangia
on separate plants.
Male
Zamia integrifolia
Female
Cycads
Females in other cycads are
more cone-like.
Encephalartos
Lepidozamia
Cycads
Cycas nicely shows that the female is a
modified leaf called a megasporophyll
with attached sporangia.
Cycas
Cycads
Cycad seeds are covered with a
fleshy aril that resembles and
functions like a true fruit.
Encephalartos
Ginkgo
Only extant member from a
family developed over 120
million years ago.
Ginkgo is a unique gymnosperm
because it has a broad leaf and
is deciduous.
Ginkgo
Female ovule in Ginkgo are produced in pairs at the tips of
reproductive short shoots. There is a small opening at the
tip of the ovule to permit sperm to enter.
Reproductive short shoot
Pair of ovules
Ovule opening
Ginkgo
Males are produced on separate trees and are clusters of
sperm containing microsporangia that release pollen.
Microsporangia
Ginkgo
Ginkgo seeds are produced in autumn and have
an outer fleshy and inner hard seed coat.
Endosperm
Embryo
Intact seed with fleshy
covering.
Seed with outer coat
removed.
Cut seed showing
embryo and
endosperm.
Gnetophytes
Gnetophytes include Gnetum, Ephedra, and Welwitschia.
Gnetophytes appear to be the closest living relatives to
the Angiosperms.
Ephedra female (left) and male (right) plants.
Gnetophytes
Welwitschia is an unusual desert plant that produces only two
opposite strap-like leaves on either side of a central disc (stem).
Gnetophytes
Welwitschia
Welwitschia reproductive structures
are produced on the margin of the
central disc. There are separate male
and female plants.
Male
Female
Welwitschia
Gnetophytes
Welwitschia
Welwitschia seeds have a
papery outer seed coat.
Conifers
Conifers (cone-bearing) represent the largest group of genera
in the gynmosperms dating back to 290 million years ago.
Abies
Picea
Keteleeria
Conifers
Conifers include:
Araucariaceae – Agathis and Araucaria.
Pinaceae – Abies, Cedrus, Larix, Picea, Pinus, Pseudolarix,
Pseudotsuga, Tsuga, and Sciadopitys.
Cupressaceae – Chamaecyparis, Cupressus, Juniperus,
Microbiota,Thuja, Platycladus, and Thujopsis.
Taxodiaceae – Taxodium, Metasequoia, Cryptomeria,
Cunninghamia, Taiwania, Sequoia, and
Sequoiadendron.
Taxaceae – Cephalotaxus, Taxus, and Torreya.
Taxaceae
Members of the Taxaceae produce males and
females on separate plants.
The female is cone-like and produces a drop of fluid
at the tip of the ovules to capture air borne pollen.
Taxus
Cephalotaxus
Taxaceae
Males shed wind-borne pollen.
Cephalotaxus
Taxus
Wind-borne pollen
Taxaceae
Unlike many of the other conifers,
members of the Taxaceae produce
seeds with a fleshy outer coat.
Taxus
Cephalotaxus
Conifer life cycle
Conifer Reproductive Cycle - Pine
Conifer life cycle
Pollen formation
The male gametophyte is
produced in a staminate cone.
Staminate cone
Conifer life cycle
Pollen formation
Four haploid pollen
grains are produced
through meiosis.
The male gametophyte
is a winged pollen grain
spread by the wind.
Conifer life cycle
Ovule formation
In many gymnosperms, the female gametophyte is produced in
the axils of the ovulate cone between protective scales.
The ovulate cone
consists of many spirally
arranged ovuliferous
scales subtended by a
cone bract.
Ovulate cone
Each ovuliferous scale
has a pair of ovules on
its surface.
The ovuliferous scale will
form the seed wing that
covers the mature seed.
Spruce
(Picea)
Ovuliferous
scale
Conifer life cycle
Ovule formation
Megaspore mother cell
The haploid female gametophyte
is developed within the nucellus
(megasporangium) from the
megaspore mother cell by meiosis.
Nucellus
Cone bract
Ovuliferous
scale
Nucellus
Megaspore
mother
cell
Ovuliferous
scale
Cone bract
Conifer life cycle
Fertilization
Within the female gametophyte, two archegonia are formed
each with one haploid egg cell. Only one egg cell will be
fertilized and develop into an embryo within the ovule.
Micropyle
Archegonia
Integuments
Ovule
Conifer life cycle
Fertilization
The pollen grain contains two nuclei – one is
the tube cell and one is the generative cell.
Following pollen germination, the generative
nucleus divides into additional sperm nuclei.
Pollen grain
Germinating pollen
Sperm
nuclei
Pollen
tube
Generative
cell
Tube
cell
Tube nucleus
Conifer life cycle
Fertilization
The pollen germinates and the
pollen tube enters the ovule and
deposits the sperm nuclei.
Micropyle
Pollen
Ovule
The sperm nucleus and egg nucleus
fuse to complete fertilization and
form the 2n zygote.
However, fertilization in most
conifers does not occur until
months after the pollen tube
enters the ovule.
In pines, it can take over a year
between pollination and egg cell
fertilization.
Egg
nucleus
Conifer life cycle
Fertilization
Fusion of the egg and
sperm cells completes
fertilization and
results in a new zygote.
Pollen
tube
Sperm
nucleus
Egg
nucleus
Archegonium
Conifer life cycle
Fertilization
Following gamete fusion, cells
organize to form an embryo tier of
cells and a suspensor tier.
The suspensor cells elongate and
several multiple embryos
(polyembryos) are formed inside a
single ovule.
Initially, there can be as many as 12
developing embryos.
However, it is usual that one embryo
becomes dominate and continues to
develop as the seed matures.
Suspensors
Embryos
Conifer life cycle
Fertilization
In most conifers, the mature seed is attached to a wing
derived from the ovuliferous scale and the embryo that will
be the next sporophytic generation.
Seed coat
Embryo
A pair of mature
seeds in pine
Cone
bract
Wing
Endosperm (1n)
Female gametophyte
Spruce (Picea) seed
Seed
Wing
Seed
Conifer life cycle
Fertilization
In gymnosperms, there is only a
single fertilization event
between the sperm and egg
nuclei.
Endosperm
Female gametophyte (1n)
Therefore, the endosperm is not
triploid as in most angiosperms,
but is derived from the female
gametophyte (megasporangia)
that is haploid.
It is still often referred to as
the endosperm or as the female
gametophyte storage tissue.
Seed coat
Embryo
Mature pine seed
Angiosperm life cycle
Angiosperms are true flowering
plants.
The term angiosperm means
“enclosed seeds” and refers to
the female ovary tissue (carpels)
that forms the fruit surrounding
angiosperm seeds.
Angiosperms are the dominant
plant type on Earth with
approximately 250,000 species,
compared with only about 8,000
living species of gymnosperms.
Akebia quinata
Angiosperm life cycle
One reason for angiosperm
success and diversity is the
mutualistic co-evolution of
animals (especially insects)
as pollinators and seed
dispersers.
Angiosperm life cycle
Angiosperms present an incredible diversity
of flower forms and colors.
Angiosperm life cycle
Flowers are the sexual organs in angiosperms.
The male organ is the stamen and the female is the pistil.
Stamen
Anther Pollen
Filament
Petals
Stigma
Stigma
Style
Anthers
Pistil
Petal
Ovule
Ovary
Ovary
Sepal
Style
Receptacle
Pedicel
Receptacle
Pedicel
Angiosperm life cycle
Angiosperm life cycle
Pollen development (Microsporogenesis)
Male gametes are formed in the pollen grains (microspores)
that are produced within the stamen of the flower.
Stamen
Anther
Pollen grains
Anther
Pollen
grain
Filament
Angiosperm life cycle
Pollen development (Microsporogenesis)
There are four
pollen sacs in each
lily anther.
Microsporangia
(Pollen sacs)
Microspore
mother cells
Cross-section lily stamen
Angiosperm life cycle
Pollen development (Microsporogenesis)
Microspore mother cells (Microsporocytes) will divide via
meiosis to become the pollen grains. The tapetum is a
layer of nutritive cells surrounding the developing pollen.
Microsporangia
(Pollen sacs)
Microspore
mother cells
initiating
division
Tapetum
Angiosperm life cycle
Pollen development (Microsporogenesis)
Many pollen grains matures within each pollen sac.
Eventually the anther will open to release the pollen.
Pollen sacs
Pollen
Angiosperm life cycle
Pollen development (Microsporogenesis)
The outer layer of the pollen grain
is called the exine. The exine
provides protection for the pollen
grain.
The exine tends to be smooth in
wind-pollinated plants and rough or
spiked in insect-pollinated plants.
Hibiscus pollen with a rough exine indicating
that the pollen is carried by insects.
Angiosperm life cycle
Pollen development (Microsporogenesis)
A mature pollen grain typically contains two nuclei; one generative
nucleus and one tube nucleus. The outer surface of the pollen grain
has an outer exine and inner intine interrupted by several pores. The
pollen tube will exit (germinate) through one of the pores.
Tube nucleus
Pore
Pore
Tube
nucleus
Intine
Generative
nucleus
Exine
Pore
Generative
nucleus
Angiosperm life cycle
Pollen germination
Generative
nuclei
Tube
nucleus
Pore
Hydration of the
pollen grain enables
the pollen tube to
emerge through a pore
on the pollen surface.
Pollen
tube
Soon after the pollen
tube elongates, the
tube nucleus followed
by the generative
nucleus enters the
tube.
As the nuclei move
down the tube, the
generative nucleus
divides to form
two male sperm
nuclei that will
unite with female
egg cells during
fertilization.
Angiosperm life cycle
Pollen germination
The pollen tube moves
down the style towards
the ovule.
The journey may be
short, less than ½ an
inch in beet; or it might
be a long distance (over
5 to 15 inches) as in
lily or corn.
Pollen
Stigma
Style
Pollen
tube
Generative
nuclei
Tube
nucleus
Angiosperm life cycle
Pollen germination
The tube nucleus acts to
guide the pollen tube, while
the generative nuclei will
eventually fuse with female
egg cells.
The pollen tube enters the
micropyle (a natural opening
between the integuments)
releasing the generative
nuclei into the embryo sac.
Angiosperm life cycle
Pollen germination
The interaction between the pollen and stigmatic surface
is important for pollen germination and tube growth. This
interaction is a way to force cross-pollination.
Angiosperm life cycle
Pollen germination
Sporophytic self-incompatibility. Each pollen contains
genes of both S1 and S2 alleles, and the pollen tube will
only grow down a style with a different genotype.
Angiosperm life cycle
Pollen germination
Gametophytic self-incompatibility. Each pollen grain has
a single S allele. Pollen tube will not grow down a style
where that allele is represented.
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
In the angiosperm flower, the female gametophyte consists of nucellar
tissue that is surrounded by either a single or a double outer tissue layer
called the integuments. The integuments will become the seed coat.
Integuments
Integuments
Nucellus
Nucellus
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
In the nucellus, a megaspore mother cell forms that will undergo
meiosis and become the female egg cells within the egg sac.
Outer
Integument
Inner
Integuments
Megaspore
mother cell
Nucellus
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Soon after the completion
of meiosis, the egg sac is
formed and haploid (1n)
nuclei organized according
to their future function.
Funiculus
Ovary
Ovule
A gap is retained between
the enveloping
integuments called the
micropyle.
This is the opening where
the pollen tube will enter
the embryo sac.
Micropyle
Integuments
Egg sac
Nucellus
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
The integuments grow to cover the nucellus and
continues to enclose the embryo sac creating the ovule.
The ovule eventually will become the seed.
Megaspore mother cell
forms in the nucellus.
Megaspore mother cell
undergoes meiosis.
Haploid cells form in
the embryo sac.
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Ovule development over time. A common form of ovule development has
the ovule turn along the placenta (funiculus) and become inverted.
Funiculus
Nucellus
Integuments
Micropyle
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Ovules vary in their orientation and shape in the ovary. Three common
types include orthotropous, anatropous, and hemianatropous.
Stigma
Style
Ovary
Integuments
Ovule
Egg sac
orthotropous
anatropous
hemianatropous
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Nuclei in the embryo sac from by meiosis. In meiosis I, there is an
initial cell division to give two cells with diploid nuclei.
At the end of meiosis II, there are four linear haploid nuclei formed.
Only one nucleus survives to duplicates to form the archegonia in
gymnosperms or the contents of the embryo sac in angiosperms.
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
In angiosperms, the most common arrangement of
cells in the embryo sac is called the Polygonum type
and occurs in about two-thirds of flowering plants
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
The Polygonum type of embryo
sac has seven cells (eight nuclei)
that occupy specific locations
that dictate their function.
Egg sac
Angiosperm life cycle
Different forms of ovule formation
Angiosperm life cycle
Ovule formation
Embryo sac development (Megagametogenesis)
Angiosperm life cycle
Double fertilization
In angiosperms, sexual
reproduction involves
double fertilization.
One male and one female
egg nuclei fuse to form
the zygote.
The second male and two
female egg nuclei fuse to
form the triploid
endosperm.
Angiosperm life cycle
Double fertilization
The pollen tube enters
the ovule through the
micropyle and deposits
the two male nuclei into
the embryo sac.
Angiosperm life cycle
Double fertilization
Synergid function
Synergids produce a
chemical that attracts
the pollen tube to the
micropyle.
Arrests tube growth.
Ensures the proper
release of the sperm cells.
Angiosperm life cycle
Double fertilization
The exact function of
antipodal cells is not
completely understood,
but they disintegrate
soon after fertilization
of the egg cell.
Angiosperm life cycle
Double fertilization
The pollen tube enters the synergid and
deposits the two male nuclei.
Female
egg nuclei
Embryo
sac
Pollen tube entering
the embryo sac
Angiosperm life cycle
Double fertilization
The egg cell to form the
zygote (2n embryo).
Embryo sac
The central cell and its
two polar nuclei to form
the 3n endosperm.
Central cell
Egg cell
Double fertilization in lily.
Angiosperm life cycle
Double fertilization
Most seeds of angiosperms mature
to include three basic parts.
The diploid embryo from the
fusion of male and female
gametes.
The triploid endosperm from the
fusion of one male and two female
gametes.
The diploid seed coat developed
from the integuments derived
from diploid maternal tissue.
Seed coat
Embryo
Endosperm