Life Cycle of Vascular Land Plants

Life Cycle of Vascular
Land Plants
Heteromorphic Alternation
of
Generations
page 422-423, your text
Vascular plants
• Division/Phylum: Tracheophyta
tracheophytes, dominant plant group on
land today
• Their life cycles have:
- Dominant sporophyte generation
• Sporophyte conspicuous (what you see)
– Inconspicuous gametophyte generation
• Gametophyte inconspicuous and usually small (not
readily visible unless you know where to look)
Sporophytes: the diploid,
2n,
generation of a plant. Sporophyte =
“spore producing plant”.
Produces spores via meiosis
Examples: Almost everything you
think of as being a plant
Gametophyte: the haploid
generation (1n) of a plant.
Gametophyte = “gamete producing
plant”
This phase produces gametes (sperm
and eggs) via mitosis.
Examples: Prothallus of Ferns
Pollen Grains
Equisetum
Angiosperm Pollen
Selaginella
Gametophyte
Examples
Sporophyte
2n
diploid phase
Sorus (pl. sori) = a
cluster of sporangia
(sprg) found on the
lower surface of the
fern leaf
= horizontal stem
Lower surface = abaxial
surface
sorus
Annulus
Sprg
will
open
here
Spores
Thick walls
Thin
outer wall
Annulus
Stalk
a ring or patch of cells that
have thickened inner and side
walls-aid in sprgl dehiscence
= opening of sprg, which
allows spores to be shed
Sporangia each with annulus visible
Meiosis occurs in Sporangium
• Spore mother cells (2n) give rise to 4
genetically distinct haploid (n) spores.
• True for ALL land plants, even the
bryophytes (mosses, liverworts etc.)
2n
4n
Mitosis
2n
daughters
identical
2n
Crossing
over
4n
Prophase
I
Page 159 textbook also
ea. 2n
Meiosis
4 1n spores
all genetically different
Meiosis
2n cell has DNA replication = 4n, then 2 divisions
2n
Ο DNA replicates
=
Ο
4n
O 1n
O 1n
O 1n spores
O 1n
“reduction division”- diploid cell with two nuclear
divisions (only 1 DNA replication) reduced to 1n
(haploid)- 4 haploid products, all are genetically
different due to the crossing over of the
chromosomes (genetic material) at the time of
prophase I
Spore mother
cells
undergo meiosis
Spores are haploid products of meiosis and are
borne in tetrads = groups of 4
Trilete mark - place of attachment to other
spores in tetrad = Y-shaped mark,
a pressure mark left on spore
Tetrahedral tetrad
Spore germination
Spore
nucleus
(1n)
Young prothallus = chain of cells
produced by mitosis (nuclear division)
Fern Gametophyte (Prothallus)
Mature Gametophyte (Haploid, 1n)
ArchegoniaArchegonium (singular)
Sex organ that
produces the egg – flask
shaped
Antheridia- antheridium
(singular) –sex organ
that produces the sperm –
round
Rhizoids= elongate cells
used for the absorption of
water
Flattened, heart-shaped, thin, usually
photosynthetic
Antheridium (plural Antheridia)
Sperm
Jacket Cells
Immature
Mature
Sperm are products of mitosis
SEM of Ceratopteris sperm -- multiflagellate
Given that sperm is flagellated and
exposed to the outside world, it
needs water to swim. Thus, water
is needed for fertilization. This is the
case for all seedless plants.
Archegonia
Immature
Archegonium
Neck
cells
Neck Canal
Cells
Egg Nucleus
Egg Cell
Archegonium at maturity
Neck canal cells degenerate
and allow for passage
of sperm.
Sperm (n)
Egg Nucleus (n)
Egg Cell
Fertilization = syngamy = fusion of gametes
Produces a diploid zygote
Zygote divides by mitosis to produce a multicellular
EMBRYO (2n)
– we return to the sporophyte generation
Sporophyte
2n
Diploid
Fern gametophytes with young
sporophytes
Young
sporophyte
emerging from
gametophyte
Sporophyte 2n
Gametophyte 1n
Heteromorphic Alternation
of
Generations
Heteromorphic = different morphology i.e., the sporophyte and gametophyte look different
As opposed to: isomorphic (=sporophyte and gametophyte look identical) occurs in some algae
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
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
Homosporous Plants
sporangial wall
Ferns (most)
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
Psilotum
Equisetum strobilus
Sporangiophore with attached sprg
“Sporangium bearing branch=sporangiophore”
Selaginella life cycle – life cycle of a heterosporous plant
Pages 410-411 textbook
Selaginella
A heterosporous
plant
A
lycopod:
“club moss”
“spike moss”
“resurrection
plant”
Rock spike moss
Selaginella rupestris
S. lepidophylla
Resurrection plant
Selaginella
Spores are produced in cones or “strobili” (plural)
Strobilus is a term used when cones bear only spores
Selaginella
strobilus l.s.
2 sizes of
spores:
megaspores
&
microspores
ligule
Megasporangium
Microsporangium
ligule
Page 410 textbook
Endosporic
Gametophyte
development
Selaginella embryo and sporeling
Selaginella
Selaginella life cycle – life cycle of a heterosporous plant
Heterosporous Plants
Produce two sizes of spores
microspores microsporangium microsporophyll
megaspores megasporangium megasporophyll
Plants are free sporing = spores are shed from the sporangia
Gametophytes are endosporic-produced inside of the spore
walltwo gametophytes
1. Megagametophyte
2. Microgametophyte
- 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
Examples of heterosporous plants
Lycopods:
Selaginella
Isoetes
Heterosporous Aquatic Ferns
Order: Marsileales
Order: Salviniales
Fossil groups related to Equisetum
some calamites