Seed plants

Today’s lecture
1. Review of homospory &
heterospory
2. Some examples of heterospory
3. Seed plant groups
4. Seed plant terminology,
definitions, and homology
3. Concept of gymnosperm
4. Seed plant life cycle
5. New lecture outline
Homosporous Plants
sporangial wall
Ferns (most)-10,000 spp.
Equisetum-scouring rush
Lycopodium-club moss
Psilotum-whisk fern
spores
sporophyll
Early land plants (fossil groups)
Sporophyll= “sporangium-bearing leaf”
Strobilus= cone with sporangia & spores
Homosporous plants have:
1. One size of spore produced in the sporangium
2. Plants are “free sporing” – i.e., they shed their
spores from the sporangium
3. Each spore germinates to form a free-living
gametophyte (exosporic gametophyte
development)
4. Water is needed in the life cycle for the sperm to
swim to the egg, but also for spore germination
and gametophyte growth which takes place on
the ground
Heterosporous Plants
1. Produce two sizes of spores
microspores microsporangium microsporophyll
megaspores megasporangium megasporophyll
2. Plants are free sporing = spores are shed from the sporangia
3. Gametophytes are endosporic-produced inside of the spore walltwo gametophytes
Megagametophyte
Microgametophyte
4. - Water still needed for fertilization,
- but not for gametophyte growth which takes place inside
of the spore wall and
- not for spore germination, although needed for release of
sperm
Selaginella
strobilus l.s.
2 sizes of
spores:
megaspores
&
microspores
ligule
prothallial cell
Endosporic
Gametophyte
development
Selaginella life cycle – life cycle of a heterosporous plant
Pages 410-411 textbook
Examples of heterosporous plants
Lycopods:
Selaginella
Isoetes
Fossil Lepidodendraceae
Heterosporous Aquatic
Ferns
Marsilea
Salvinia & Azolla
Fossil groups related to Equisetum
some calamites
Advantages of Heterospory
1. Water not needed for gametophyte growth:
endosporic gametophyte development
2. Water not needed for spore germination to
form these gametophytes.
3. Insures genetic recombination: megaspores
& microspores have different genes. Why?
(Hint: NOT because they are large and small spores)
Advantages of Heterospory
1. Water not needed for gametophyte growth:
endosporic gametophyte development
2. Water not needed for spore germination to
form these gametophytes.
3. Insures genetic recombination: megaspores
& microspores have different genes. Why?
Because they are products of meiosis.
They can come from the same parent plant, but
remember the results of meiosis: 4 haploid products.
All genetically different because of crossing over
during prophase I of meiosis.
Seed plants
Gymnosperms
Gymnosperms
• Used to be a formal group, now informal
• “gymno” = naked “sperm” = seed i.e.,
naked-seeded plants
• Includes cycads, conifers, gnetophytes,
ginkgos - extant groups (living)
• Many extinct fossil groups
• New major group = new terminology
• Groups differ in a number of characters
including the types of structures on
which the seeds are borne
Cycads
Cycas megasporophylls
Ginkgophytes-Ginkgo biloba – maidenhair tree
A “living fossil”
Gnetophytes – Gnetum
Welwitschia
Ephedra
Conifers – cone-bearing plants
Sequoia taxodiaceous Cupressaceae
Picea (spruce) Pinaceae
Leaf variability in conifers
Agathis
Araucaria
Araucaria
Araucariaceae
Pseudotsuga
Thuja (compound cones)
b
os
Douglas fir
b=bract (leaf), os=ovuliferous scale (modified shoot system)
hemlock
M = micropyle (hole)
Parts of a typical seed or ovule
IN = integument = seed
coat
PC = pollen chamber
MW
N = nucellus = ?
MW = megaspore wall
(megaspore membrane)
A = archegonium with egg
MG= megagametophyte
tissue
Cycad ovule
Parts of a typical seed or ovule
M = micropyle (hole)
IN = integument
PC = pollen chamber
N = nucellus =
megasporangium
MW
MW= megaspore wall
(megaspore membrane)
A = archegonium with egg
MG= megagametophyte
tissue
Cycad ovule
3 aborted megaspores
Products of meiosis – a linear tetrad
of 4 spores
3 aborted spores &
1 functional megaspore
Functional megaspore develops an endosporic
megagametophyte
Megaspore enlarges &
produces a
megagametophyte like
Selaginella, with free nuclear
then cellular phases
Parts of a typical seed or ovule
M = micropyle (hole)
IN = integument = seed coat
PC = pollen chamber
N = nucellus =
megasporangium
MW
MW= megaspore wall
(megaspore membrane)
A = archegonium with egg
MG= megagametophyte
tissue
Cycad ovule
Seed Definitions
• Seed is a fertilized ovule (embryo inside)
• Ovule is an unfertilized seed
In gymnosperms (=naked-seeded plants)
the ovule can develop to full size without
fertilization of an egg
Seed = an integumented megasporangium
Nucellus = the megasporangium
The integument is a NEW STRUCTURE not
seen in seedless vascular plants
Gymnosperm
pollen
cones
microsporophyll
pollen
pollen sac
Pollen cone with microsporophylls that
bear abaxial pollen sacs (=microsporangia)
containing pollen grains (~=microspores)
Spores
vs.
Pollen
Produced by non-seed plants vs.
seed plants
Both are products of meiosis in a microsporangium
Both have sporopollenin walls
Have one cell when shed
vs.
More than 1 cell
Germinate proximally
(same side of grain as
the trilete suture)
vs.
Germinate distally
(opposite trilete)
Pine pollen –
microgametophyte
development
corpus
p
a
A,B,C – Meiosis
D – microspore nucleus
E, F, G – First division of
microspore nucleus to form
the first prothallial cell =p
and another cell (sometimes
called an antheridial initial = a)
H, I – Second division to
produce a 2nd prothallial cell
J, K – 3rd division produces the
generative cell (g) and tube cell (t)
This is the stage where the pollen
is shed (4-celled stage)
L – Division of generative cell
to form a sterile cell (s) and a
spermatogenous cell (spc)
and the pollen tube begins to
grow - t=tube nucleus
saccus
spc
shed here
g
Each microgametophyte has
a total of 6 cells
2 prothallial cells
a sterile cell (stalk cell)
a tube cell (with tube nucleus)
2 sperm
It is shed at a 4 celled stage
I
I
Pine pollen
& microgametophytes
Pollen tube
Corpus=body
Saccus=air bladder
used to orient grain in
the pollination droplet
saccus
corpus
distal
surface
Pollen terminology
• Pollen grain = microspore w/ distal
germination
• Pollen sac = microsporangium
• Pollen cone = microsporangiate cone
• Microsporophyll
• Microgametophyte
• Pollination =
In gymnosperms,
the transfer of
pollen from the
pollen sac to the
seed micropyle
Pollination droplets in gymnosperms
Droplet formed by
breakdown of
nucellar tissue
forming the
pollen chamber
Cycas
Ginkgo
In nature these
seeds are oriented
with the micropyle
facing downward
and pollen floats
upward into
micropyle
Pinus
Abies
Zhang et al. 2012
Pinus life cycle –seed plant
Pages 442-443 text