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Transcript
Wondrous Events in Evolution
Notes for Plant Taxonomy
Biology 4420 at Utah State University
Prepared by M.E. Barkworth
Factors important to success
For dominance, it is not enough to be good – one must be
better than one’s competitors
For survival, it is enough to be better than or as good as
one’s competitors in some situations.
Aspects of life to consider:
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Nutrition
Reproduction
Competition
Cost/benefits of structures and strategies
Land plants


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450 MYBP First evidence of land plants
Cutin, stomates,?mycorrhizal
partnerships
Bryophytes and tracheophytes more or
less simultaneous (give or take a million
years or so)
Tracheophytes

Diploid life style

Backup system in
place

Tracheids


More cellulose, lignin
Benefits outweigh
costs
Megaphyllous leaves

Microphyllous leaves




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One vein
No axillary branches
No leaf gaps
Restricts shape, size
Megaphyllous leaves




Better transportation
Diverse shapes
Leaf gaps so less
expensive
Benefits outweigh costs
microphyllous
leaves
megaphyllous
leaves
Tracheophytes can be tall …

What’s the advantage?


Sex at ground level so no still restricted
Sex requires water so still restricted



A film of water is adequate
Chemical guidance system works well
Gametophytes independent, need moisture
and nutrition source (soil) so still restricted
The solution is ….

Ovules and pollen grains



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Ovule provide food and protection for
female gametophyte
Pollen grain protects the wandering male
gametophyte
Still have to get the two together
Once fertilized, the ovule starts to
become a seed
Seeds 1

Integumented



Indehiscent


Protective cell layers
Integument(s) diploid
Does not split
Megasporangium

Where female spores
form and make
megagametophyte which
makes female gamete,
aka egg
Seeds 2

Megasporangium produces





Megaspores which form megagametophyte which
forms egg (female gamete)
Other cells of gametophyte provide nutrition –
endosperm
Megasporangium wall forms nucellus
Megasporangium is surrounded by diploid
tissue (integument) with vascular connection
to parent
Integument displays great diversity (think
seed coat)
Seeds 3

Advantages




Place genetically vulnerable phase,
gametophyte, in protected surrounding
Provide great environment for that difficult
event – mixing of DNA from parents
Great place for children to start life
Problem: How does male gamete get
there?
Solution - Pollen Grains

Armored airship for male
gametophyte




Armor is sporopollenin
Meiosis in microsporangia forms
first cell of microgametophyte
Male gametes develop through
mitotic divisions in
microgametophyte
Dispersal



Initially wind
Eventually insect
Some water
Seed and Pollen Grain



Sex above ground level
Adequate moisture from female parent
Eventually reduction number of cell
divisions and time in gametophyte
phase for both males and females
New problems


Bringing the male and female
gametes together
Watch for new developments as
plants solve this problem


“Educating” wind
Educating animals
Back to being tall



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Wood is energetically expensive
Need to minimize cost, maximize
benefit
Fibers, the strong part of wood,
primarily associated with vascular
bundles
Organization of vascular tissue
important
Vascular Tissue Organization
http://www.botany.hawaii.edu/faculty/webb/BOT311/Transition/invasion_landStele.htm

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Rod
Cylinder
Scattered
Circle, aha!
Circular cambium
“Good Wood” has circle
of vascular bundles that
subsequently are united
by cambium which form
xylem on inside and
phloem on outside
Important but Extinct Groups
•
Progymnosperms
(370 MYBP)



35 million years
12 m tall
Had “good wood”,
fernlike foliage and
reproduction
Important but Extinct Groups

Pteridosperms




Seeds
Fern like leaves
Some had “good wood”
Cycads, Pinophytes,
Gnetophytes, Angiosperms
may have evolved
independently from
different groups of
pteridosperms.