Transcript 20 plants to land

Plants to land
- What is a plant?
- What are the challenges of land?
- How were these challenges met?
Refer to chapter 23 in text.
- What is a plant?
Short form: a truly multicellular photoautotroph.
photoautotroph: self energy and carbon capture
(it photosynthesizes)
truly multicellular: showing tissue specialization
Contrast with unicellular photoautotrophs, algae,
(which, being unicellular eukaryotes, are protists)
Kelp are (very) large communal algae,
without actual tissue specialization↓
wildflowers.jdcc.edu/Common%20Wood%20Violet.html
http://www.smbaykeeper.org/images/site_images/Giant_kelp_adult.jpg
Prior to 500 million years ago the land was mostly barren:
- bare rock and sand,
with maybe prokaryotes scattered about.
There had been colonial algae in the seas for about 2 billion years,
and many complex animals were present, all marine.
← Artist’s rendition of the
Cambrian explosion
(505 mya)
based on Burgess Shale fossils
discovered in western Canada
in 1909 by Charles Walcott.
http://www.cartage.org.lb/en/themes/sciences/Paleontology/Paleozoology/EarlyPaleozoic/cambdiorama.gif
What are the challenges of land?
1. Dehydration
While living in water there is still the problem of
water balance, but not like being surrounded by air.
2. Support
Aquatic algae are supported by the water.
3. Nutrient acquisition/gas exchange
These algae are surrounded by the nutrients they need,
suspended in the water itself.
4. Nutrient dispersal
Algae are generally small, or thin, and don’t have far
to move materials internally.
5. Dispersal of reproductive structures
Again, surrounded by water, reproductive cells are
hydrated and washed away, but on land…
These challenges were met in a step-wise progression,
reflected in modern examples.
Charophyceans are the algae most closely related to plants:
- similar cellulose-making apparatus (for cell walls)
- same peroxisome enzymes
(take care of photosynthesis byproduct)
- sperm same as those of sperm-producing plants
- similarity in cell plate formation in mitosis
- apical growth, lengthening filaments, like stems
- gaps through cell walls (plasmodesmata) for cell-cell communication
http://www.life.umd.edu/labs/delwiche/Strp/Chlorophyta/charophyceae/Coleo-irregularis.jpg
But these were still all living in the water…
Vascular
Roots
Leaves
Seeds
Flowers
No (very thin/flat)
No (rhizoids, to anchor only)
No
No (spores)
No
They do have a water-retaining
outer surface.
http://taggart.glg.msu.edu/bot335/bryo.htm
These were the dominant plants
for 100 million years.
Most of their life they are
haploid (more later).
Sperm have to swim in moisture
←haploid gametophyte with
diploid sporophyte sticking out
http://www.davidlnelson.md/Cazadero/CazImages/Moss_sporangium2.jpg
http://flickriver.com/photos/imbala/6101817600/
Bryophyte (e.g. mosses↓ liverworts and hornworts→)
the earliest land plants
Lycopodium venustulum ↓
Selaginella plana →
(note sporangia-bearing strobili… ‘clubs’)
http://courses.bio.indiana.edu/B300-Knox/Images/Selaginella_plana.jpg
Lycophytes (e.g. “club mosses”..
NOT actual mosses)
Vascular Yes (strong, hydrophobic lignin in cell walls)
Roots
Yes, off of rhizomes (underground stems)
Leaves Sort of (single vein microphylls)
Seeds
No (windblown spores and
flagellated sperm)
Flowers No
- Leaf forms evolved independently
(not direct ancestor of modern
branched-vein leaves)
- Forests of lycophytes contributed
to coal deposits from Carboniferous era;
most genera are extinct.
- Spores are flammable…
http://www.palaeos.com/Plants/Lycophytes/Images/Lycopodium.jpg
http://nouveaufauves.files.wordpress.com/2008/04/new-fern-fronds5.jpg
Pteridophyte/ Filicophyta (e.g. ferns)
Vascular
Roots
Leaves
Seeds
Flowers
Yes
Yes (rhizome and root)
Yes (megaphylls, or “fronds”)
No (spores)
No
Two other groups,
horsetails and whisk ferns,
seem more like lycophytes –
They have lost traits through
devolution.
http://www.davidlnelson.md/Cazadero/CazImages/Fern_structure.jpg
Vascular
Roots
Leaves
Seeds
Flowers
http://www.pinegenome.org/i/redwood.jpg
Gymnosperms (e.g. conifers
and cycads) “naked seeds”
Yes
Yes
Yes (often needles)
Yes (not in fruit)
No
Redwoods are conifers →
Sagos are cycads ↓
http://www.biologyreference.com/images/biol_02_img0213.jpg
- Taxonomically not one clade.
- Wind-borne pollen
Angiosperms (i.e. all
flowering plants)
Yes
Yes
Yes
Yes (in fruit)
Yes
Google images
Vascular
Roots
Leaves
Seeds
Flowers
-
-
http://www.sky-bolt.com/images/curcumaflower.jpg
Evolution of flowers and fruit has
provided for a wide range of
reproductive strategies (e.g. coevolution of animal pollinators)
Two broad divisions:
←monocots (e.g. ginger)
and eudicots (e.g. dogwood)↑
(More on this later.)
Alternation of generations
Along with the trend to gain the traits noted,
there has been a trend to spend less time in a haploid state.
(recall: diploid (2n) is 2 sets of chromosomes,
usual for most eukaryotes,
while haploid (n) is one set,
as is gametes in sexual reproduction.)
The next series of slides goes over the life cycles showing this:
There is a lot of stuff. Don’t panic.
We will have more details on angiosperm fertilization later
Bryophytes
note
-The gametophyte (n)
is the conspicuous
stage.
-Sporophyte is the
only diploid (2n) stage,
depends on the
gametophyte.
-Sperm (upper right)
have to swim in
moisture to egg.
Pteridophytes
note
- Dominant frond is diploid (2n).
- Haploid gametophyte still vegetative.
- Sperm still need to swim
Gymnosperms
note
- Megaspore,
microspore, and
food reserves
are the haploid
structures
- Microspore
(pollen) is
wind blown.
Angiosperms
note
- As with gymnosperms, little is haploid.
- A triploid (3n) food storage structure (endosperm)
has been added.
- Pollen may be carried by wind or pollinators.
Alternation of generation, in summary:
What do you think are the advantages of haploidy?
Of diploidy?
Describe how each of the 5 major plant groups
represents adaptations to life on land.
Charophyceans have a lot in common with land plants:
Why aren’t they aquatic plants, then?
photoautotroph.
conifer
Charophycean
cycad
cellulose
Angiosperm
Bryophyte
alternation of generations
rhizoid
diploid
lignin
haploid
Lycophyte
rhizome
gametophyte
Pteridophyte
sporophyte
Filicophyta
pollen
Gymnosperm
endosperm