vascular transport, mechanical support, storage: plant stems (including vascular pathways, growth...
TRANSCRIPT
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?
Ferns
Gymnosperms
Angiosperms
A digression into woody vs. herbaceous
Primary vs. Secondary growth
Groover. 2005. What genes make a tree a tree. TREE. 5:210
Plant stems
• What are the functions of a plant stem?
Plant stems
• How do they accomplish these tasks?– Phloem (living cells) – Xylem
• Dead cells: Vessels, tracheids, fibers• Living cells: Parenchyma (axial and ray)
Transport: Water!
Evapotranspiration
Cohesion-tension
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
Embolisms
• What happens when air gets in?• Freeze- and drought-induced cavitation
Embolisms
• What happens when air gets in?• Drought-induced cavitation: air seeding
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
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.
Peforation plates
Pits
Tracheid
Vessel elements
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?
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)
From water to land: Evolution of tetrapod limbs
http://universe-review.ca/R10-19-animals.htm