Evolutionary developments necessary for plants to colonize land
Stage One: Becoming multicellular
Occurred in water
Enables specialized tissues to develop
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
Devonian plant community
MAIN FEATURES
Simple dichotomous branching
Sporangia
!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

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 spor angia 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
Meiosis in the sporangium producing haploid spores.
1. Dispersal
Developing sporophyte
Zygote
Gametophyte
Archegonium

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

Fig. 25.4, p. 406
Zygote grows, develops into a sporophyte while still zygote attached to gametophyte .
Fertiliztion
Sperm reach eggs by moving through rain drops or film of water on the plant surface .
Mature sporophyte (sporeproducing structure and stalk), still dependent on gametophyte.
rhizoid
Diploid Stage
Haploid Stage
Meiosis
Spores form by way of meiosis and are released.
Spores germinate. Some grow and develop into male gametophytes.
sperm-producing structure at shoot tip of male gametophyte.
egg-producing structure at shoot tip of female gametophyte .
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
Meiosis
Development of
Mitosis thallus
Gametes (?) remains attached to the haploid thallus
HAPLOID
Fusion (syngamy)
DIPLOID
Development of sporophyte thallus
– remains attached to gametophyte Mitosis
Zygote
2n

Dessication tolerance in Tortula ruralis
Hydrated
RAPID WATER LOSS
Constitutive Cellular
Hormone ?
Protection Induction of
Recovery and
Repair
Mechanisms
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

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

The sporophyte (still attached to the gametophyte) grows, develops.
zygote fertilization egg sperm
Diploid Stage meiosis
Haploid Stage eggproducing structure
Archegonia
Spores develop.
mature gametophyte
(underside ) spermproducing structure
Antheridia sorus (one of the spore-producing structures)
Spores are released
Spore germinates, grows into a gametophyte .

http://departments.bloomu.edu/biology/chamuris/concepts2/labimg.html
Polypodium spp sori sporangia
A sorus

g a n f i i e d s
M
Developing spores
Polypodium spp
Sporangia

Polypodium spp
Gametophyte
Developing sporophyte

Arrangement of sporangia on two ferns
In lines on a broadleaved type
Asplenium
At the end of the leaves
Adiantum

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

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

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
Appalachians
Ice cap
Equator
Britain

Appalachians
Ice cap
Equator
Britain