Bryophytes: land plants that have a dominant gametophyte generation
and lack lignified (true) vascular tissue. Their unbranched sporophytes
produce a single sporangium. They do not have true leaves, roots or
stems.
Gametophyte: the 1n gamete producing generation
Gametangium: a general term for a structure where gametes are
produced
Sporophyte: the 2n generation in which meiosis occurs and
meiospores are produced
Sporangium: a general term for an enclosure where spores are made
Bryophytes
Mosses
Vascular plants
Liverworts
lignin
at least 475 million
years ago
Hornworts
cuticle
sporophyte
multicellular gametangia
Charophytes
Chara
Coleochaete
Other charophytes
Other green algae
Evidence that plants are descended from
green alga:
Green algae have chlorophyll a and b and
carotenoids and store starch. All land
plants and some green algae have
cellulose cell walls and phragmoplasts.
All plants are oogamous and they have
embryos that depend on the female
gametophyte during development.
Three lineages of bryophytes:
Liverworts (Marchantiophyta)
Hornworts (Anthercerotophyta)
Mosses (Bryophyta)
First appeared 475 million years ago. (mammals appeared 200 M
years ago)
The ancestors of bryophytes are the green algae.
Innovations of the bryophytes:
-multicellular gametangia called antheridia and archegonia
-multicellular sporophyte that is dependent on the gametophyte
-cuticle
-stomata
-do not have “true” lignified vascular tissue
sporangium
All bryophytes have the same
overall structure. The
dominant generation is the
gametophyte. The antheridia
and archegonia occur on the
gametophyte. Fertilization
occurs in the archegonium.
The sporophyte grows out of
the gametophyte.
Meiosis occurs in the
sporophyte and meiospores
are released. Meiospores
germinate to become
gametophytes.
calyptra
seta
leafy
gametophyte
rhizoids
sporophyte
cover cells
neck canal
neck
sterile
jacket
fertile
cells
egg cell
venter
archegonium
antheridium
The dominant
generation in
bryophytes is the
gametophyte
generation.
Gemmae are an
asexual
reproductive
structure in
bryophytes.
Gemmae are
multicellular bodies
that give rise to
new
gametophytes.
gametophyte
thallus
gemmae cups
pore and air
chamber
Bryophytes do not have true
leaves but there is some
cellular differentiation.
epidermis
chlorenchyma
parenchyma
Some liverwort gametophytes
make special structures to
house their gametangia.
The male structure is called
the antheridiophore and the
female structure is called the
archegoniophore.
thallus
archegoniophore
gemma cup
antheridiophore
thallus
Liverworts in Glendalough, Ireland
The life cycle of
the liverwort
Marchantia
Liverworts, they are primitive but diverse, there are 9000 species. Liverworts are
the most primitive (extant) plants.
Liverwort sporangia
Liverworts, undetermined species from Puerto Rico
Liverworts, undetermined species from Dominican Republic
Liverworts, undetermined species from Dominican Republic
Hornworts - the gametophyte forms symbiotic relationships with cyanobacteria
(Nostoc) that fix nitrogen. There are around 100 species of hornworts. Each
cell has one large chloroplast (similar to algae) rather than many.
columella
sporophyte
splitting
capsule
(sporangium)
gametophyte
Hornwort lifecycle
Note, hornworts have stomata on
their sporophytes. All other plants
have stomata except liverworts.
Gametophytes can be unisexual or
bisexual.
columella
Hornwort sporophyte
pseudoelaters for spore
ejection
spore tetrads
sporocytes
meiosis
sporangium
(capsule)
meristematic region
gametophyte tissue
foot
Life cycle of a moss
There are 9500 species of
moss.
paraphyses
antheridia
neck
archegonium
neck canal
venter
egg cell
stalk
Fig. 22-15c, p. 396
Moss gametophytes: three phases, protonema, bud and leafy phases. Bud
formation is induced by cytokinin.
spore
protonema
bud
rhizoids
Moss stem showing large epidermal
cells specialized for storing water.
large
epidermal cells
leptoids
Mosses have some primitive
vascular tissue. Hydroids are
dead empty cells for water conduction,
similar to xylem but not lignified. Leptoids
are living and similar to phloem in that
sugar can be transported in leptoids.
hydroid
outside
center
Asexual reproduction in mosses:
- protonema continue to produce buds
- leafy tissue can produce protonema
- rhizoids sometimes can also produce buds
- gemmae can be produced
operculum
Moss sporangia are complex. They have a
cuticle, epidermis with stomata, a cortex,
central conducting tissue. The cap is called
the operculum, with triangular peristome
teeth below it.
The teeth move in response to atmospheric
humidity to disperse spores.
peristome
teeth
spores
Spores can remain dormant for years.
columella
seta (stalk)
tip or
operculum
capsule
peristome teeth
capsule
peristome
teeth
Tracheophytes, early seedless vascular plants (Ch 21 in Mauseth)
* They have lignified vascular tissue.
* Branched sporophytes are independent of gametophyte
Water is still required for movement of sperm from the antheridia to
archegonia.
Monilophytes
Earliest trachiophytes are the
Rhyniophytes.
Only known from fossils, named
after Rhynie Scotland.
True ferns
Horsetails heterospory
(in some)
Dicotomous branching, no leaves,
no roots. They had aerial stems, rhizomes
and rhizoids.
Ophioglossalean
ferns
leptosporangia
heterospory
(in some)
Whisk
ferns
Marattialean ferns
megaphylls
Isoetes
Selaginella (Zosterophyllophyta)
(Lepidodendrids)
heterospory
heterospory
Seed plants
chloroplast DNA inversion
Lycopodium
group
reduced gametophyte
Lycophytes
(Other Rhyniophytes)
(Aglaophyton)
(Rhyniophytes)
lignin
(Rhynia)
strongly lignified vascular tissue
(xylem and phloem)
branched, independent sporophyte
Bryophytes
sporangia
Rhynia sporophyte (from fossils).
Few examples of gametophytes are
known.
upright
stems
rhizome
rhizoids
Rhynia
stem
cross
section
epidermis
cortex
phloem
xylem
Why did the sporophyte generation become dominant in land plants?
There are theories:
1. Diploid cells are protected from mutation by the presence of two copies of each
gene? (Increased exposure to solar radiation causes more mutations.)
2. Since sperm required water for dispersal, maybe gametopyte size was more
restricted than sporophyte size. And size was important to produce more spores,
and later to compete for sunlight.
The Lycophytes:
True leaves, roots and stems. The bryophytes do not have true leaves, roots or
stems.
The rhyniophytes are the first to have have true stems with central conducting
tissue.
Lycophytes are the first to have true leaves, roots, and stems.
Lycopodium is homosporous and has subterranean gametophytes and laterally
attached sporangia.
Selaginella is heterosporous.
Isoetes is heterosporous and has
secondary growth.
Lepidodendron (extinct)
35 m in height, 1m diameter stem
True stems, true roots,
true leaves.
pith
primary xylem
secondary xylem
cambium
phloem
cortex
cork
The club mosses,
Lycopodium
This is Huperzia,
another
club moss.
The sporangia are in the
axils of the microphylls.
sporangium
microphylls
Cross section of a microphyll of Huperzia,
note single unbranched vein and stomata.
air spaces
epidermis
mesophyll cells
stomate
vein
epidermis
air spaces
The club moss stem,
note central vascular area and
lignified cells (sclerenchyma).
xylem
phloem
sclerenchyma
parenchyma
cortex
epidermis
parenchyma
cortex
sclerenchyma
phloem
xylem
Lifecycle of a club moss.
Next time, finish Tracheophytes