EEB340 - Lecture 6.docx

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18 Mar 2012
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Lecture 6: Evolution of Vascular Tissue: Xylem
Evolution of Vascular Tissue
Evolutionary Trends
“Non-Vascular” Plants
Fossil Vascular Plants
Stelar Configuration
- 106 higher rate of water movement to tracheary elements than in parenchyma cells
- vessel members
o scalariform perforation plate
o simple perforation plate
- two general types of pits:
o simple pit
o bordered pit
- Gymosperm division Gnetophyta have evolved vessels
o Ex) Ephedra (in this division)
o Only Gymosperm to evolve vessels is in this division
The other divisions are now extinct
- R4: rate at which water moves through cell (hydrolic conductance) is related to
radius by r4
- In simple pits, the pull of water can move an air bubble into the adjacent cell may
inactivate many cells
o Harder to pull through with bordered pits (even some unique innovations
that prevent air bubble movement)
Non-Vascular Plants
- Anthocerophyta Hornworts (thalloid) very thin, no water conducting cells in
living plants or fossil record
- Hepatophyta Liverworts (thalloid) have water conducting cells
- Bryophyta mosses (leafy) have water conducting cells
The Water Conducting Cells
- secondary wall with polyphenolic compounds lignin-like, offer resistance to
decay
- primary wall slightly digested
- dead at maturity
- plasmodesmatal derived pores on their walls
- could be described as early precursor to what we know was tracheary elements
o would be more like tracheids because don’t have perforation plates
- tracheary elements have lignified secondary cell wall
o lignin provides strength important because negative pressure of water
would pull and cause cell wall to collapse
o dead at maturity by apoptosis
o lignin hydrophobic important for water movement
- vessel members shorter and wider
o evolved/derived from tracheids
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