Vascular transport, mechanical support,
storage: Plant stems (including vascular
pathways, growth forms, woody/herbaceous)
Questions
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• 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
Vessel elements
Diam: 20-500 um, Vessel L: few mm to few m
Tracheid:
Diam: 0.01-0.05 mm, L: ~1-5 mm
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
Ferns
Water
conducting
cells
Pits
Support
Living cells
Wood
Tracheids,
vessels
Homogenous
Sclerenchym
a bands
?
Pseudo
woody
Extant
Tracheids,
gymnosperms except
Gnetales
Torus/margo, Tracheids
with a number
of exceptions
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
Vessel elements
Tracheid
Question
• We know that derived angiosperm vessels are
more efficient.
• Were basal angiosperms with vessels more
efficient than gymnosperms and vesselless
angiosperms?
Herbs or pseudo woody
Tracheids, heterogeneous pits,
*
Vessels, homogenous pits, fibers
Basal
*
Derived
Herbs or pseudo woody
*
* = vesselless
* 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://tolweb.org/Deuterostomia/2466
Elpistostege
http://universe-review.ca/R10-19-animals.htm