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Mosses and Ferns
Evolutionary developments necessary for plants
to colonize land
Stage One: Becoming multicellular
Occurred in water
Enables specialized tissues to develop
Stage Two: Developing sporangia
Enables dispersal on land
Stage Three: Developing a large sporophyte
Confers competitive advantage
Provides perennial spore production
Stage Four: Removing dependence of
fertilization on a film of water
Enables survival in dry environments
History of evolution of major plant types on land
Stage One of adaptation to living on land
The land that plants colonized was hostile to life.
Soil development was minimal.
Land plants required several adaptations to be successful
that require
multi-cellular tissues
:
mechanical strength for support,
exposed light catching surfaces,
anchoring system,
conducting system for water,
system for obtaining mineral nutrients,
a way to restrict water loss in desiccating air,
a means of reproducing and dispersing on land
Devonian plant community found at Rhynie, in Scotland.
A reed-like marsh, 370-380 million years ago.
Asteroxylon
MAIN FEATURES
Simple dichotomous branching
Sporangia
!5 to 30 cm tall
No roots
Stomata with guard cells
Most had a central vascular strand
Cuticle
Devonian
plant
community
Asteroxylon had leaves –
without a vascular connection
Plants living in water release spores and gametes that swim
and may be helped to dispersed by water movement
To live on land plants faced two challenges for their reproduction:
1. Dispersal
2. Fertilization
Dispersal was solved first – through production of sporangia.
angeion is Latin for case
So a sporangia is a spore case
The important feature of sporangia is that they lift spores
above the ground so they can be
dispersed by the wind
Stage Two: Developing sporangia
Mosses
Retention of the
zygote by the
female
gametophyte
Delayed meiosis
and growth of
the sporophyte
by mitosis
Developing
sporophyte
Zygote
Gametophyte
Archegonium
Meiosis in the
sporangium
producing haploid
spores.
1. Dispersal
Eight Terms to Learn to understand
Alternation of Generations of Land Plants
Spores – haploid, single cells produced by meiosis
The word “phyte” is Greek for plant
Gametes – collective term for sperm and egg
Gametophyte – haploid plant that develops from a spore and
produces gametes by mitosis
Gametangium – a “case” holding gametes
Archegonium – flask–shaped container holding the egg
cell. (Ancient gonad) The female gametangium.
Antheridium – The male gametangium
Sporophyte – diploid plant that grows from the zygote and
produces spores by meiosis
Sporangium – the “case” holding spores
Moss life cycle
Fig. 25.4, p. 406
Mature sporophyte (sporeproducing structure and stalk), still
dependent on gametophyte.
Zygote grows,
develops into a
sporophyte
while still
attached to
zygote
gametophyte.
Diploid Stage
Fertiliztion
Sperm reach eggs by
moving through rain drops
or film of water on the plant
surface.
Haploid Stage
rhizoid
sperm-producing
structure at
shoot tip of male
gametophyte.
egg-producing
structure at shoot
tip of female
gametophyte.
Meiosis
Spores form by way of
meiosis and are
released.
Spores germinate. Some grow
and develop into male
gametophytes.
Other germinating spores grow and
develop into female gametophytes.
Moss sporophyte
Top of capsule
Developing protonema
Moss antheridium and archegonium
Important life cycle features of mosses
Spores
n
Development of
Mitosis
gametophyte
thallus
Female
gamete(?)
Gametes
remains attached to
the haploid thallus
Meiosis
HAPLOID
Fusion (syngamy)
DIPLOID
Development of
sporophyte thallus
– remains attached
to gametophyte
Zygote
2n
Mitosis
Dessication tolerance in Tortula ruralis
RAPID WATER LOSS
Constitutive Cellular
Induction of
Hormone ?
Protection
Recovery and
Repair
Mechanisms
Hydrated
Dry
Rehydrated
Spagnum – the bog-forming species
The species forms
clumpsminimizing
surface area to
volume ratio.
Unique leaf cells (hyaline cells)
of Spahgnum species enable the
plant to absorb up to 20 times
its own dry weight of water.
Fig. 25.5, p. 407
How can we characterize mosses?
1.
Plants accumulate matter and make
growth
2.
Plant growth is an organized process
following rules of anatomy and
morphology
3.
Plants maintain their heat and water
balance
4. Plants have a life cycle with
reproduction and dispersal
5. Evolution is a constant process
Stage Three: Developing a large sporophyte
In ferns the
sporophyte is only
dependent on the
gametophyte for
obtaining nutrient,
water, and physical
support when it is
first formed.
Sporophyte
originally grows
from a gametophyte
and then develops
roots, rhizome, and
fronds
Ferns
Large size enables
competition as well
as effective spore
dispersal. The
perennial root stock
enables continued
frond and spore
production from year
to year
Fronds growing
from a rhizome
Fern life cycle
The sporophyte (still
attached to the
gametophyte)
grows, develops.
sorus (one of the
spore-producing
structures)
zygote
Diploid Stage
fertilization
egg
sperm
Haploid Stage
eggproducing Archegonia
structure
spermproducing
structure
Antheridia
meiosis
Spores develop.
mature
gametophyte
(underside)
Spores are
released
Spore
germinates,
grows into a
gametophyte.
http://departments.bloomu.edu/biology/chamuris/concepts2/labimg.html
Sporangia
Polypodium spp
sori
sporangia
A sorus
Polypodium spp
a
g
n
i
f
i
e
d
Sporangia
Developing spores
Polypodium spp
Gametophyte
Gametophyte
Developing sporophyte
Arrangement of sporangia on two ferns
In lines on a broadleaved type
Asplenium
At the end of the leaves
Adiantum
Tree ferns
Cibotium menziesii in habitat in
Hawaii. Photo courtesy of Peter Richardson.
Conducting tissue
Cyathea australis with the uncurling croziers
visible. Photo courtesy of Scott Ridges
How can we characterize ferns?
1.
Plants accumulate matter and make
growth
2.
Plant growth is an organized process
following rules of anatomy and
morphology
3.
Plants maintain their heat and water
balance
4.
Plants have a life cycle with
reproduction and dispersal
5.
Evolution is a constant process
Coal formation
Jungle-like forests of the Carboniferous were
dominated by giant ancestors of club mosses,
horsetails, ferns, conifers, and cycads.
Most of the plant fossils found in the coals and
associated sedimentary rocks show no annual
growth rings, suggesting rapid growth rates and
lack of seasonal variation in the climate (tropical).
Anaerobic conditions and periodic inundations of
the sea
Early Carboniferous
Equator
Appalachians
Britain
Ice cap
Equator
Appalachians
Britain
Ice cap