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Structure of Woody Plants (Vascular Cambium (Fusiform Initials (Two…
Structure of Woody Plants
Vascular Cambium
Initiation of the Vascular Cambium
The vascular cambium is one of the meristems that produce the secondary plant body.
The cells located between the metaxylem and metaphloem of a vascular bundle never undergo cell cycle arrest.
They continue to divide instead of maturing and they constitute the fascicular cambium.
Some mature parenchyma cells between vascular bundles come out of cell cycle arrest and resume mitosis, forming an interfascicular cambium that connects on each side with the fascicular cambia.
The terms "fascicular" and "interfascicular" are only used while the cambium is young.
Fusiform Initials
Long, tapered cells
Usual length is 140 to 160 micrometers in dicots and 700 to 8,700 in conifers
Periclinal wall is a wall parallel to the circumference of the cambium
Fusiform initials undergo longitudinal cell division
Two elongate cells are produced, one remains fusiform initial, but the other differentiates into a cell of secondary xylem or secondary phloem.
If the outer daughter cell remains a cambium cell, the inner cell develops into secondary xylem.
If the inner daughter cell remains a cambium cell, the outer cell develops into secondary phloem.
Continuous and wood never forms on the exterior and bark never forms interior.
Anticlinal walls are perpendicular to the cambium's surface
Vascular cells must occasionally divide longitudinally
Ray Initials
Similar to fusiform initials except they are short and more or less cuboidal
Also undergo periclinal cell division
One of the daughter cells remains a cambial ray initial and the other differentiating into either xylem parenchyma or phloem parenchyma depending on location.
Arrangement of Cambial Cells
Biseriate
Two cell wide
Multiseriate
Many cells wide
Uniseriate
One cell wide
Ray initial cells are grouped together in short vertical rows
Secondary Xylem
Types of Wood Cells
All cells formed to the interior of the vascular cambium develop into secondary xylem, or wood.
Secondary xylem contains all the types of cells that occur in primary xylem but no new ones.
Wood may contain:
Vessel Elements
Fibers
Sclereids
Tracheids
Parenchyma
Only real differences between primary and secondary xylem are the origin and arrangement of cells
Axial system is derived from the fusiform intitials
Radial system develops from the ray initials
Growth Rings
Annual rings classified by the changing conditions of the trees during specific seasons
Early wood is developed in spring with wide vessels, and late wood is produced in summer with smaller vessels.
Diffuse porous
Sugar Maple
Yellow Birch
Aspen
American Holly
Ring porous
Sassafras
Red Oak
Honey Locust
Heartwood and Sapwood
The lighter, moister outer region is sapwood
The dark wood in the center of a log is heartwood
The different regions exist because vessels and tracheid's do not function forever in water conduction
Reaction Wood
Wood produced to combat stress that would result in drooping of branches
Conifers form reaction wood located on the underside of the branch and it's known as compression wood.
Secondary Phloem
It also has an axial and radial system
Outer Bark
Cork and the Cork Cambium
Also called the phellogen
In secondary phloem the formation of the interfascicular vascular cambium results in a new cambium, known as the cork cambium
Differs a lot from vascular cambium in both structure and morphogenic activity.
All of its cells are cuboidal, like ray initials
After each division the inner cell is almost always cork cambium, and the outer cell differentiates into a cork cell (phellem cell).
In a few species the cork cambium may produce a cell or two to the inside that mature into a layer of parenchyma called phelloderm.
Known as periderm
All tissues outside the innermost cork cambium are the outer bark. All secondary phloem between the vascular cambium and the inner most cork cambium is the inner bark.
Lenticels and Oxygen Diffusion
Diffusion pathways for oxygen where intercellular spaces penetrate the cork layer.
These regions of aerenchymatous cork are lenticels
Initiation of Cork Cambia
In some species the first cork cambium arises before a twig or root is even 1 year old.
On stems this is often detectable as the surface color changes from green to tan.
In other species the first cork cambium forms only when that region is several years old.
Delayed formation of bark is common in plants that depend on cortex chlorenchyma for much of their photosynthesis
Cacti
Secondary Growth in Roots
Roots of conifers and woody angiosperms undergo secondary growth, as do the stems.
Anomalous Forms of Growth
Anomalous Secondary Growth
When alternative cambia produce secondary bodies that differ from the common type
Included Phloem
Secondary phloem located between two bands of xylem
Columns of some of the parenchyma cells undergo rapid division and produce narrow cells that differentiate into secondary vascular bundles in monocots.
Unusual Primary Growth
Palm trees are unusual in that their trunks do not taper at the tips like eudicots or conifer trees.
The palm trunk is all primary tissue consisting of vascular bundles distributed throughout ground tissue, each containing only primary xylem and primary phloem, which all derived from the shoot apical meristem.
The increase in width and addition of adventitious roots in palms is known as establishment growth, a form of primary growth.
