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Ch9: Structure of Woody Plants, Cork cambium: Phellogen, Cork cells:…
Ch9: Structure of Woody Plants
Vascular Cambium
Initiation of the vascular cambium
vascular cambium: one of the meristems that produce the secondary plant body
cells location between the metaxylem and metaphloem of a vascular bundle (in woody plants) continue to divide: fascicular cambium
interfascicular cambium: connects on each side with the fascicular cambia
vascular cambia must be extended each year
ray initials
similar to fusiform cells
shorter and more or less cuboidal
arrangement of cambial cells
ray initials are typically grouped together in short vertical rows
uniseriate: only one cell wide
biseriate: two cells wide
multiseriate: many cells wide
fusiform initials may occur in horizontal rows(storied cambium) or non horizontal (nonstoried cambium)
a single shrub or tree only has one vascular cambium (cylindrical)
fusiform initials
long, tapered cells that produce two elongated cells during cell division
one is to be a fusiform initial, the other becomes secondary xylem or phloem
wood never forms to the exterior of the vascular cambium, bark never forms on the interior side
cambial cells produce narrow daughter cells
vascular cambium cells must occasionally divide longitudinally by anticlinal walls
Secondary Xylem
types of wood cells
secondary xylem (wood)
axial system: derived from fusiform initials
radial system: derived from ray initials
hardwoods: all basal angiosperms and eudicots
softwoods: woods from conifers (pines and redwoods)
two main types of ray parenchyma cells: upright & procumbent
in some plants, procumbent ray cells have no direct connection with axial cells, but upright cells do
ray perenchyma cells: upright cells and procumbent cells
may also contain ray tracheids: cells that are similar to perenchyma cells except they contain a secondary cell wall, circular bordered pits, and protoplasts
radial system of xylem is usually simple
angiosperms: only perenchyma cells arranged in rays (masses)
ray perenchyma cells store carbs/other nutrients during dormant periods, move materials over short distances
axial systems always contain tracheary elements
tracheids or vessels, or both
growth rings
vascular cambium becomes active when quiescence ceases
quiescence occurs during times of stress
late wood ring + early wood ring = annual ring (growth ring)
early wood: spring wood, the first wood formed, and it must have lots of wide vessels
(or in conifers: wide tracheids)
thickened cuticle, less transpiration, large # of vessels conducting rapidly leads to late wood (summer wood) production
late wood is stronger if it contains lots of fibers (or in conifers, narrow thick walled tracheids)
less vessels (late wood)
since the wood of growth rings have vessels throughout: diffuse porous
though species with vessels restricted mainly to early wood are considered "ring porous"
heartwood and sapwood
heartwood
is darker, drier, and more fragrant (inner)
sapwood
is the outer ring, lighter, moister
tracheids and vessels don't function forever, so the cambium produces new water-filled elements the next year (in the sapwood, new growth)
tylosis: a plug produced by wood parenchyma cells in the vessel to prevent fungus
reaction wood
produced in response to stresses on horizontal branches caused by gravity
in angiosperms: upper side of the branch (known as tension wood)
tension wood contains gelatinous lignin that exert tension on the branch preventing drooping
Outer Bark
cork/cork cambium
vascular cambium/secondary phloem pushed outward
results in tissues growing in circumference and being torn apart
cork cambium (
phellogen
)
cork cambium cells are cuboidal
outer cells:cork cells: phellem cells
phelloderm: inner layer, parenchyma
phellogen, phellem, phelloderm: periderm
all tissues outside the innermost cork cambium: outer bark. All secondary phloem in between the vascular cambium and the innermost cork cambium: inner bark
lenticels and oxygen diffusion
cork blocks out pathogens, retains water, but also blocks oxygen absorption
oxygen can enter when rounded cork cells mature (creates intercellular spaces)
rounded as opposed to cuboidal
these regions are called lenticels
located at bases, cracks of the bark
initiation of cork cambia
huge variability
cork cambia can form on stems (this cork cambia differs from cork cambia formed when the stem is older)
fibrous cells produce fibrous stringy bark
sclereid-filled phloem produces hard bark
Secondary Phloem
formed from the vascular cambium (like secondary xylem)
has an axial and radial system
axial system is responsible for conduction up and down the stem or root
contains companion cells and sieve tube members in angiosperms (sieve cells in conifers)
the size and shape of phloem rays matches the size and shape of xylem rays
ABOVE vascular cambium!!!
Secondary Growth in Roots
root vascular cambium: ray and fusiform initials, wood in root is similar to wood in shoot
sapwood and heartwood is as ring porous or diffuse porous as the stem is
conductivity requirements of root vs stem differ
Anomalous Forms of Growth
Anomalous Secondary Growth
alternative cambia produce secondary bodies that differ from the common type
sweet potatoes!! Multiple vascular cambia
included phloem: located between two bands of xylem
outermost second phloem
Unusual primary growth
palm trees (do not branch)
vascular cambium never develops
no true wood
"establishment growth"
Concepts
meristems: pith, cortex, epidermis, leaves, vascular bundles (
primary tissues
)
only plant body that ever develops in herbs
wood (secondary xylem), bark (secondary phloem and cork) are
secondary tissues
that develop in woody plants
wood and bark contain conducting tissues (greater capacity to move water and minerals up, and carbs down)
long lifespan means battling insects, fungi, environmental harshness, for longer
Cork cambium: Phellogen
Cork cells: Phellem
Periderm: phellem, phellogen, phelloderm
