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Structure of Woody Plants (Secondary Xylem and Phloem (Growth rings /…
Structure of Woody Plants
Vascular Cambium
vascular cambium-meristem that produces the secondary plant body
herbaceous species- cells located btwn metaxylem and metaphloem vascular bundle
stop dividing and differentiate into conducting tissue
woody species- cells never undergo cell cycle arrest
instead they divide and constitute the fascicular cambium (fc)
some mature parenchyma cells resume mitosis forming imterfascicular cambium ; connects to fc
vascular cambium (VC) has to extend every year
during the next growth season apical meristem extends and continues growth
fusiform initials and ray initials for cell type
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Fusiform Initials and Ray Initials
long tampered cells
when it undergoes longitudinal cell division produces two elongate cells
one cell becomes fusiform other secondary xylem and phloem
outer cell remains cambium , inner becomes secondary phloem
inner remains cambium, outer becomes secondary xylem
cambial cells produce narrow daughter cells
inner daughter cells cause cambial to push outward
longitudinally anticlinal walls increase the number of cells
ray initials are short and more cuboidal
very similar to fusiform
biggest difference is ray produces short cells
mostly parenchyma and albuminous cells
Arrangement of Cells
Ray- typically grouped in short vertical rows
fusiform- can be horizontal pattern or irregular
Secondary Xylem and Phloem
all cells formed interior of VC develop into secondary xylem "wood"
arrangement of secondary xylem reflects fusiform and ray initials
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axial system- derived from fusiform
contains treachery elements, carry out longitudinal conduction of water
some immature cells undergo transverse divisions and differentiate into column of xylem parenchyma
important for temporary water reservoir
in agiosperms, axial contains fibers, gives wood strength and flexibility
"hardwoods" basal angiosperms and eudicots / lots of fibers
"softwoods" conifers/ few or no fibers
radial system- derived from ray initials
store carbohydrates and other nutrients
upright cells- have direct connection to axial cells
procumbent cells do not
Growth rings / Heartwood and Sapwood
first wood formed is early wood or spring wood
late wood "summer wood" have lower proportion vessels
first and last make up one years growth called annual ring
VC is small during times of climate stress; summer, winter
VC gets larger during the spring
the dark wood is the center of the log "heartwood"
lighter, moist outer region is called sapwood
the two regions exist because vessels and tracheas do not
branches and trunks that are not vertical have a lateral stress
they form reaction wood, have gelatinous fibers which offer more support
Outer Bark
Cork and Cork Cambium
goes through a similar process that ifc goes through
difference is in secondary phloem- now referred to cork cambium or phellogen
cork cambium has cuboidal cells like ray initials
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inner cell remains cork cambium and outer differentiates into cork cell
can produce a cell into the inside to mature into phelloderm
all tissues outside cork cambium make outer bark
secondary phloem btwn VC and innermost cork cambium is inner bark
all these layers are known as the periderm
periderm only offers temporary protection
Other important things to know
intercellular spaces penetrate the cork layer forming lenticels
they keep folding in on each other to create a bigger opening for oxygen to enter
Anomalous Forms of Growth
Anomalous Secondary Growth
different type of growth then conifers and woody angiosperms
sweet potatoes have an increased storage parenchyma
has numerous VC around individual vessels or group of vessels
secondary tissue are an irregular matrix of parenchyma , few sieve tubes, vessels and VC
Phloem included
after a short period cambium cells stop dividing and differentiate into xylem
cells in outermost secondary phloem reactivate and differentiate into new VC
secondary phloem between two bands of xylem is called included phloem
Unusual Primary Growth
Palm trees , trunks do not taper and they do not branch
truck is primary tissue consisting of vascular bundles
only contain primary xylem and phloem
never develops a VC
palm seedling don't have a full set of leaves and a wide trunk
as the years go on the trunk increases in width and the leaves increase and adventitious roots
this is called establishment growth
