Plant Diversity II
Evolution by Seed Plants
• cyanobacteria on land – 1.2 billion years ago
• 500 MYA – colonization by plants
• closest relatives of land plants = charophyceans
• molecular comparison of both nuclear and chloroplast genes
confirms morphological and biochemical conclusions that the
charophyceans are ancestors of plants
• plant share characteristics with other more primitive
organisms
– 1. multicellular, eukaryotic
– 2. photosynthetic autotrophs – brown, red, green algae
– 3. cell walls made of cellulose – green algae, dinoflagellates,
brown algae
– 4. chloroplasts with chlorophyll a and b – green algae, euglenids
and a few dinoflagellates
• A reminder: several unique traits seen in plants and
charophyceans
– 1. rose-shaped complexes for cellulose synthesis –called rosettes
• synthesize cellulose microfibrils for the cell walls
– 2. peroxisome enzymes –enzymes to help minimize the loss of organic
products as a result of photorespiration
– 3. flagellated sperm – some species of land plants have flagellated sperm
– 4. formation of a phragmoplast – involved in the synthesis of new cell walls
during mitosis - via the formation of new cross walls called cell plates
Charophyceans
Defining the Plant Kingdom
• land plants and green algae form a
single clade = Viridiplantae
• 1.2 billion years ago – split into 2
clades: Chlorophyta (algae) &
Streptophyta (land plants &
charophyceans)
• 475 million years ago – the land
plants began to evolve from the
charophyceans into non-vascular
plants
• 420 million years ago – evolution
of vascular plants
• traditional botanists call land
plants embryophytes
– embryo is nourished inside the
archegonium/female gametophyte
Defining the Plant Kingdom
• vascular plants now form a
single clade – 93% of all plant
species
– categorized into three smaller
clades
• 1. lycophytes – club
mosses and relatives
• 2. pterophytes – ferns and
relatives
• 3. seed vascular plants
– A. gymnosperms “naked seed” plants
– B. angiosperms –
flowering plants
• ANOTHER REMINDER: four key traits that define land
plants – absent in charophyceans
– 1. apical meristems
– 2. alternation of generations & multicellular,
dependent embryos
– 3. walled spores in sporangia
• two kinds of sporangia possible
– 4. multicellular gametangia
• male and female
Heterospory
• the rule among seed bearing vascular plants
• nearly all non-vascular plants are homosporous – produce
one kind of spore which gives rise to a
bisexual/monoecious gametophyte
• with the evolution of seed plants – development of
heterospory
– megasporangium located on modified leaves called
megasporophylls produce megaspores  female
gametophytes
– microsporangium located on modified leaves called
microsporophylls –produce microspores  male
gametophytes
– both are found on specialized reproductive structures
• e.g cones, flowers
Heterospory
• these sporangia can either be located on the
same plant = monoeicous
– meaning “one house”
– i.e. bisexual
• or they can be located on “male” and “female”
plants = dioecious
– reproduction requires a male and female plant
Seed plants
– three key reproductive adaptations
evolved in seed plants:
– 1. increasing dominance of the sporophyte
generation – reduced gametophyte
– 2. advent of the seed – ovules and eggs
– 3. evolution of pollen as an airborne agent
1. Reduced Gametophytes
• gametophytes of mosses and ferns are the dominant stage
• gametophytes of seed plants are mostly microscopic
• miniaturization allows for the development of their
gametophytes within the sporangium of the parental
sporophyte
– protects the delicate egg-forming gametophyte from environmental
stress
– also allows the growing gametophyte to derive nourishment directly
from the sporophyte
Bryophytes
Seedless Vascular
Sporophyte
(2n)
Sporophyte
(2n)
Gametophyte
(n)
Sporophyte dependent on
gametophyte (mosses and
other bryophytes)
Microscopic female
gametophytes (n) in
ovulate cones
(dependent)
Gametophyte
(n)
Large sporophyte and small,
independent game-tophyte
(ferns and other seedless
vascular plants)
Sporophyte (2n),
the flowering plant
(independent)
Microscopic male
gametophytes (n) in
inside these parts
of flowers
(dependent)
Seed
Vascular
Microscopic male
gametophytes (n)
in pollen cones
(dependent)
Sporophyte (2n),
(independent)
Microscopic female
gametophytes (n) in
inside these parts
of flowers
(dependent)
Reduced gametophyte dependent on sporophyte (seed plants:
gymnosperms and angiosperms)
2. Ovaries & Seeds
• seed plants are unique in that the megasporangium is retained with the
parent sporophyte
• the megasporangium contains the developing megaspore
• it is surrounded by layers of sporophyte tissue called integuments
– in gymnosperms – the megaspore is surrounded by only one integument
– angiosperms usually have two integuments
2. Ovaries & Seeds
• the megaspore (n) + megasporangium (2n) + integuments = ovule
• inside each ovule is a future female gametophyte that develops from the
megaspore
– gametophyte can produce one or more egg cells within the ovule
– these eggs are fertilized by the sperm that will develop from the pollen grain
– an embryo results  found within a seed
Integument
Female
gametophyte (n)
Egg nucleus (n)
Megasporangium
(2n)
Megaspore (n)
Unfertilized ovule
Micropyle
Fertilized ovule
Discharged
sperm nucleus (n)
Pollen grain (n)
containing male
gametophyte
Seed coat
(derived from
integument)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
Gymnosperm seed
2. Ovaries & Seeds
• seed = ovule after fertilization – contains the
embryo
– seed = embryo + food supply + seed coat
(from the integuments)
– allows for the developing embryo to resist
harsh conditions
Seed coat
(derived from
integument)
• evolutionary advantage of seeds:
– until seeds – the spore was the only protective
stage in the life cycle
– unlike spores – seeds carry their own food
supply
– unlike spores - a seed can remain dormant for
years following its release
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
Gymnosperm seed
•
•
•
•
the microsporangium produces microspores
microspores develop into pollen grains
a pollen grain contains the male gametophyte enclosed
within a pollen wall
outer wall is made by the sporophyte, inner wall is
made by the gametophyte within
–
•
•
•
outer wall = protected by a coating of sporopollenin
transfer of pollen to the ovule = pollination
pollen grains are carried away from the parent plant by
wind, insects
or they can travel to the female reproductive structures
within the same sporophyte
3. Evolution
of Pollen
• in order to fertilize the egg - the pollen grain
must germinate (grow)
– it produces a pollen tube
– pollen tube allows for the discharge of two
sperm (gametes) into the ovule
– sperm unites with the egg developing within
female gametophyte (within the ovule)
• in non-vascular plants (bryophytes) and
seedless vascular plants (ferns) – the sperm is
flagellated and swims to the female
gametophyte in order to fertilize the egg which
is also free living
• in vascular seed plants – the female
gametophyte produces an egg which never
leaves the sporophyte ovule
3. Evolution
of Pollen
Gymnosperms
• “naked seed” – seeds are not enclosed in
ovaries
• seeds are exposed on modified leaves
(sporophylls) that form cones or strobili
– sporophylls bear sporangia for spore
production
• 380 MYA – development of heterosporous trees
with woody stems – but did not bear seeds =
progymnosperms
• first seed plant in the fossil record – 360 MYA
• earliest fossils of gymnosperms – 305 MYA
• early Mesozoic era (250 MYA) - domination by
gymnosperms
• drier environment favored gymnosperms over
the bryophytes and ferns
• most common existing gymnosperms are the
conifers – spruce, pin, fir and redwood
Ponderosa pine
Gymnosperms
•
four gymnosperm phyla: Cycadophyta, Ginkgophyta,
Gnetophyta and Coniferophyta
– Phylum Cycadophyta – cycads
Cycas revoluta
• second largest group of gymnosperms
• 130 species survive
– Phylum Ginkgophyta - ginkos
• only one species left – Ginkgo biloba
– Phylum Gnetophyta – three genera alive today
• Gnetum – 35 species of tropical trees, shrubs and vines
(Africa and Asia)
• Welwitschia – one species, Welswitchia (Africa)
• Ephedra – 40 species, desert shrubs
Ginko biloba
Welwitschia
mirabilis.
– Phylum Coniferophyta – largest group
•
•
•
•
“cone-bearing”
600 species of conifers
many are large trees
most are evergreens – retain their leaves throughout the
year
Ephedra.
Phylum Coniferophyta
•
•
•
•
also called Division Coniferophyta
575 species
largest genus – Pinus
leaves of conifers are always simple needles or
scales
• pine leaves – needles or needle-like
– arranged in clusters or bundles of two to five
leaves each bundle
– cluster = fascicle
Pine fascicle
Phylum Coniferophyta
• pine needle structure:
– needle is comprised of a outer epidermis
coated with a thick cuticle
– below that is one to two layers of cells =
hypodermis
– stomata are recessed in sunken cavities
– veins (vascular tissue) run down the center of
the needle and are surrounded by an
endodermis
– also contain resin canals – occur in other parts
of the pine
– these canals are lined with special cells that
secrete a resin – aromatic and antiseptic
• prevents water loss and fungal attacks on the needles
• deters insects
•
•
•
pine tree is the sporophyte
sporangia are located on scale-like leaves (sporophylls)
packed into cones – strobili (single = strobilus)
two types of cones produce two types of spores
– small pollen cones produce microspores – pollen
– larger ovulate cones produce megaspores – egg
– ovulate cones also known as seed cones – most
are woody
– cones of the juniper can resemble a fruit (berry)
Life Cycle: The
Pine
Life Cycle:
The Pine
Key
Haploid (n)
Diploid (2n)
Ovule
Ovulate
cone
Megasporocyte (2n)
Integument
Longitudinal
section of
ovulate cone
Micropyle
Pollen
cone
Megasporangium
Mature
sporophyte
(2n)
Microsporocytes
(2n)
Germinating
pollen grain
Pollen
grains (n)
(containing male
gametophytes)
MEIOSIS
MEIOSIS
Longitudinal
section of
pollen cone
Surviving
megaspore (n)
Sporophyll
Microsporangium
Seedling
Germinating
pollen grain
Archegonium
Egg (n)
Female
gametophyte
Seeds on surface
of ovulate scale
Germinating
pollen grain (n)
Food reserves
(gametophyte
tissue) (n)
Seed coat
(derived from
parent
sporophyte) (2n)
Discharged
sperm nucleus (n)
Pollen
tube
Embryo
(new sporophyte)
(2n)
FERTILIZATION
Egg nucleus (n)
Integument
• pollen cones: bear modified leaves or
sporophylls (microsporophylls) each
containing two microsporangia
– the microsporangium is comprised of
diploid cells called microsporocytes
(2n)
The Male Life cycle
– microsporocytes divide by meiosis to
form pollen grains which are haploid
– pollen grains contain the male
gametophyte – for the production of
sperm
The Male Life cycle
•
•
•
•
grains travel to the ovulate cone where
they begin to germinate and form a pollen
tube through which the sperm will travel
pollen tube “digests” its way into female
ovule through an opening called a
micropyle
pollen development results in the
production of 2 sperm cells within the
pollen tube
two sperm travel into the ovule to fertilizes
the eggs
The Male Life cycle
• ovulate cones: made up of
scales or megasporophylls
• base of the megasporophylls
are the ovules
The Female Life Cycle
Key
Haploid (n)
Diploid (2n)
Ovule
Ovulate
cone
Pollen
cone
Megasporocyte (2n)
Integument
Longitudinal
Micropyle
section of
ovulate cone
Megasporangium
Mature
sporophyte
(2n)
Germinating
pollen grain
Pollen
grains (n)
(containing male
gametophytes)
MEIOSIS
Surviving
megaspore (n)
Seedling
Germinating
pollen grain
Archegonium
Egg (n)
Seeds on surface
of ovulate scale
Female
gametophyte
Germinating
pollen grain (n)
Food reserves Seed coat
Discharged
(gametophyte (derived from
sperm nucleus (n)
tissue) (n)
parent
sporophyte) (2n)
Pollen
tube
Embryo
(new sporophyte)
(2n)
FERTILIZATION
Egg nucleus (n)
Integument
• each ovule contains one
megasporangium
– the megasporangium contains
a cell called a megasporocyte
or a megaspore mother cell
(2n)
– the megaspore mother cell
undergoes meiosis to form 4
haploid cells
– only one survives as the
megaspore (n)
– the remaining degenerate
The Female Life Cycle
Key
Haploid (n)
Diploid (2n)
Ovule
Megasporocyte (2n)
Integument
Longitudinal
section of
ovulate cone
Micropyle
Megasporangium
Germinating
Pollen
pollen grain
grains (n)
(containing male
MEIOSIS
gametophytes)
– the surviving megaspore develops
into the female gametophyte
(archegonia + eggs)
– the female gametophyte contains
two (or three) archegonia - each
will form an egg inside (two or
three eggs form by mitosis)
– the archegonium with egg +
surrounding tissue = female
gametophyte
The Female Life Cycle
Archegonium
Egg (n)
Female
gametophyte
Germinating
pollen grain (n)
Discharged
sperm nucleus (n)
Pollen
tube
archegonium
egg
female gametophyte
tissue fills the ovule
Egg nucleus (n)
– the germinating pollen grain
develops its pollen tube and two
sperm enter into the ovule
through the micropyle
– fusion of egg nucleus and sperm
nucleus  Zygote (2n)
– both eggs in the female
gametophyte may be fertilized
The Female Life Cycle
Surviving
megaspore (n)
Seedling
Germinating
pollen grain
Archegonium
Egg (n)
Seeds on surface
of ovulate scale
Female
gametophyte
Germinating
pollen grain (n)
Food reserves
(gametophyte
tissue) (n)
Discharged
sperm nucleus (n)
Pollen
Embryo
(new sporophyte)
(2n)
Seed coat
tube
(derived from
parent
sporophyte) (2n)
FERTILIZATION
Egg nucleus (n)
Integument
– the ovule with the zygote is now
called the seed
– the developing embryo is retained
within the female gametophyte
(within the ovule)
The Female Life Cycle
Surviving
megaspore (n)
Seedling
Germinating
pollen grain
Archegonium
Egg (n)
Seeds on surface
of ovulate scale
Female
gametophyte
Germinating
pollen grain (n)
Food reserves
(gametophyte
tissue) (n)
Discharged
sperm nucleus (n)
Pollen
Embryo
(new sporophyte)
(2n)
Seed coat
tube
(derived from
parent
sporophyte) (2n)
FERTILIZATION
Egg nucleus (n)
Integument
A simplified pine cycle
• conifer pollen arrives before the egg is
mature
• more than a year may pass between
pollination & fertilization!!!
Pollination
Pollination
• MITOSIS within the pollen grain produces
three cells – 2 small cells (which degenerate)
and one large cell
• this large cell divides to form: a generative
cell and a tube cell
• the tube cell elongates to form the pollen
tube
• the generative cell forms 2 sperm
• as the eggs mature – the
pollen tube is developing its
two sperm cells
• so the eggs and sperm mature
at the same time
germinating pollen grains
with pollen tube
air bladder
tube
cell
generative cell
The Pine Embryo
•
•
•
•
following fertilization – the zygote does not immediately form
– the first cells to form elongate as a suspensor
– serves to connect the developing embryo to the food source of the seed
embryonic development is similar to angiosperms
the zygote splits into a basal cell and a terminal cell
– terminal cell  embryo
– basal cell  suspensor
the embryo combined with the integuments (derived from the ovule) and its food source
– known as the seed