Vascular transport, mechanical support, storage: Plant stems (including vascular pathways, growth...
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Transcript of Vascular transport, mechanical support, storage: Plant stems (including vascular pathways, growth...
Vascular transport, mechanical support, storage: Plant stems (including vascular
pathways, growth forms, woody/herbaceous)
Questions
• For plants, APweb characters page• Any problems with the wiki?• For posting questions, please put your name
next to your post• Discussion• Questions?
Plant stems
• How do they accomplish these tasks?– Phloem (living cells) – Xylem
• Dead cells: Vessels, tracheids, fibers• Living cells: Parenchyma (axial and ray)
Conductivity increases to the fourth power of the radius
Perforation plates
Pits
Tracheid: Diam: 0.01-0.05 mm, L: ~1-5 mm
Vessel elementsDiam: 20-500 um, Vessel L: few mm to few m
Woody and pseudo woody clades Water conducting cells
Pits Support Living cells Wood
Ferns Tracheids, vessels
Homogenous Sclerenchyma bands
? Pseudo woody
Extant gymnosperms
Tracheids, except Gnetales
Torus/margo, with a number of exceptions
Tracheids 4-10% ray, 0% axial parenchyma
Woody, low density
Monocots (bamboos, palms)
Tracheids, vessels
Homogenous Fibers, tracheids
Ground parenchyma
Pseudo woody
Basal angiosperms, Magnolids, Eudicots
Mainly vessels
Mainly homogenous
Fibers, sometimes tracheids
11-30% ray, up to 11% axial parenchyma
Woody, variable density
Evolution of vessels
• Angiosperms: have higher rates of CO2 uptake and transpiration. They have literally transformed their surroundings (more next week!)
• To do this, they must be able to transport efficiently.
Question
• We know that derived angiosperm vessels are more efficient.
• Were basal angiosperms with vessels more efficient than gymnosperms and vesselless angiosperms?
*
*
*
* = vesselless
Herbs or pseudo woody
Vessels, homogenous pits, fibers
Tracheids, heterogeneous pits,
Herbs or pseudo woody
Basal
Derived
* Basal type with long thin vessels
Question
• We know that derived angiosperm vessels are more efficient.
• Were basal angiosperms with vessels more efficient than gymnosperms and vesselless angiosperms? – Answer: At the level of a given unit of stem, NO!
• So then what?
So, then what?
• We know basal angiosperms vessels are more efficient than tracheids at the conduit level. Meaning?
Perhaps…
• Less space needs to be devoted to vessels, so more diverse cell tissues can evolve or different allocation to tissues can arise… (Heteroxyly)
Why did vessels evolve?
• Findings: In the basal angiosperm with vessels– Vessels did not confer greater transport efficiency
at the stem level, nor greater photosynthetic ability
– Vessels did allow for “uncoupling” of the hydraulic and mechanical support functions
• Wood was denser and stems were stronger and energy per volume was greater in the species with vessels.
• Also, species with vessels grew taller.
Why did vessels evolve?
• So…– Why vessels were first important (heteroxyly)
appears to differ from why vessels may have led to the rise of the angiosperms (stem level efficiency)