EEB340 - Lecture 5

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18 Mar 2012

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Lecture 5: Evolution of Vascular Tissue: Xylem
Evolution of Vascular Tissue
Origins in Time
Growth Forms prostrate vs. upright
Water conducting cells tracheary element in xylem
- protonema: filamentous or thalloid in structure
o single-cell layer in thickness
o develops from a spore
o no parenchymatous growth at first
- lignin is very similar to cutin and sporopollenin
o resistant to decay (microbes)
o absorbs UV light
o resistant to water loss water cannot move through a wall that contains
lignin because so hydrophobic
minimal interaction between lignin in water conducting cells and the
water (good feature)
o provide structural rigidity
o resistant to impact
o resitant to bending
- central core of cells are also water conducting cells
o we don’t know if support or water conduction was the first function
suggested that water conduction was secondary to support
- water must move through pits to move from tracheid to tracheid
- water moves through perforation plates in vessel elements minimizes resistance
- thalloid liverworts and hornworts would create carpets of photosynthetic cells
eventually run out of room and are competing
o only place to grow is up
- branchings became dichotomous equal forks being developed
o how liverworts colonize environment
- Silurian: can get support for this height from epidermal cells (hoop like support)
and turgor from parenchyma cells
o Beyond 0.48 meters need other structures
- Plants used upright structure to decrease water loss and maximize sunlight capture
(without leaves)
- In first land plants, secondary wall would not be as developed, but importantly
would have contained ligning
- Rynia (0.5m tall) above 0.48m so developed some support tissue
- Psilophyton (1m tall) central core of support tissue (cells contain ligning)
o Hypodermal sterome cells with very thick walls
- Where secondary wall is sparse, cell walls can still elongate secondary wall acts
as cement
o Water conducting can develop as cell elongating
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