Mosses and Ferns

Stage One: Becoming multicellular

Stage Two: Developing sporangia

Occurred in waterEnables specialized tissues to develop

Enables dispersal on land

Stage Three: Developing a large sporophyteConfers competitive advantageProvides perennial spore production

Stage Four: Removing dependence of fertilization on a film of water

Enables survival in dry environments

Evolutionary developments necessary for plants to colonize land

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.

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

Soil development was minimal.

Devonian

plant community

Devonian plant community found at Rhynie, in Scotland. A reed-like marsh, 370-380 million years ago.

Simple dichotomous branching

MAIN FEATURES

!5 to 30 cm tall

No roots

Stomata with guard cells

Most had a central vascular strand

Cuticle

Asteroxylon had leaves –without a vascular connection

Sporangia

Asteroxylon

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 caseSo 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

Plants living in water release spores and gametes that swim and may be helped to dispersed by water movement

Stage Two: Developing sporangia

1. Dispersal

Meiosis in the sporangium producing haploid spores.

Mosses

Retention of the zygote by the female gametophyte

Archegonium

Zygote

Delayed meiosis and growth of the sporophyte by mitosis

Gametophyte

Developingsporophyte

Sporophyte – diploid plant that grows from the zygote and produces spores by meiosis

Gametophyte – haploid plant that develops from a spore and produces gametes by mitosis

Gametangium – a “case” holding gametes

Sporangium – the “case” holding spores

Gametes – collective term for sperm and egg

Spores – haploid, single cells produced by meiosis

Eight Terms to Learn to understand Alternation of Generations of Land Plants

Antheridium – The male gametangium

Archegonium – flask–shaped container holding the egg cell. (Ancient gonad) The female gametangium.

The word “phyte” is Greek for plant

Moss life cycleFig. 25.4, p. 406

Zygote grows, develops into a sporophyte while still attached to gametophyte.

Fertiliztion

zygote

Sperm reach eggs by moving through rain drops or film of water on the plant surface.

sperm-producing structure at shoot tip of male gametophyte.

egg-producing structure at shoot tip of female gametophyte.

Diploid StageHaploid Stage

Mature sporophyte (spore-producing structure and stalk), still dependent on 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.

rhizoid

Moss sporophyte

Top of capsule

Developing protonema

Moss antheridium and archegonium

HAPLOID

DIPLOID

Sporesn

Meiosis

Mitosis

Gametes (?)

Fusion (syngamy)

Mitosis

Zygote2n

Female gamete remains attached to the haploid thallus

Development of gametophyte thallus

Development of sporophyte thallus – remains attached to gametophyte

Important life cycle features of mosses

Hydrated

Dry

RAPID WATER LOSSRAPID WATER LOSSConstitutive CellularConstitutive Cellular

ProtectionProtection

Rehydrated

Induction of Induction of Recovery and Recovery and RepairRepairMechanismsMechanisms

Hormone ?Hormone ?

Dessication tolerance in Tortula ruralis

Fig. 25.5, p. 407

The species forms clumps- minimizing 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.

Spagnum – the bog-forming species

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

How can we characterize mosses?

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.

Large size enables competition as well as effective spore dispersal. The perennial root stock enables continued frond and spore production from year to yearSporophyte

originally grows from a gametophyte and then develops roots, rhizome, and fronds

Ferns

Fronds growing from a rhizome

Fern life cycleThe sporophyte (still attached to the gametophyte) grows, develops.

zygote

fertilizationDiploid Stage

Haploid Stage

egg

sperm

egg-producing structure

sperm-producing structure

mature gametophyte (underside)

Spores develop.

meiosis

sorus (one of the spore-producing structures)

Spores are released

Spore germinates, grows into a

gametophyte.

Archegonia

Antheridia

Sporangia Polypodium spp

sori

sporangia

http://departments.bloomu.edu/biology/chamuris/concepts2/labimg.html

A sorus

Magnified sporangia

Polypodium spp

Sporangia

Developing spores

Gametophyte

Polypodium spp

Gametophyte

Developing sporophyte

Arrangement of sporangia on two ferns

In lines on a broadleaved type At the end of the leavesAdiantumAsplenium

Cibotium menziesii in habitat in Hawaii.  Photo courtesy of Peter Richardson.

Cyathea australis with the uncurling croziers visible.  Photo courtesy of Scott Ridges

Tree ferns

Conducting tissue

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

How can we characterize ferns?

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).

Coal formation

Anaerobic conditions and periodic inundations of the sea

Early Carboniferous

Britain

Ice cap

EquatorAppalachians

Britain

Ice cap

EquatorAppalachians