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Structure of Woody Plants (Ray Initials (Also undergo periclinal cell…
Structure of Woody Plants
Derived sets of tissues from the meristems
Primary tissues
Cortex
Vascular bundles
Pith
Leaves
Epidermis
Secondary tissues
Vascular cambium
Cork cambium
Secondary xylem
Are wood
Secondary phloem
Bark
Wood and bark contain conducting tissues
Gives plants a better capacity to move water and minerals upward and carbohydrates downward
Increased numbers of leaves and roots that can be supported
Secondary cork
Bark
After a portion of a stem or root matures its conducting capacity is set
All provascular cells have differentiated into either primary xylem or primary phloem
Capacity is correlated with the needs of stems and roots
Vascular Cambium
Meristem that produces the secondary plant body
Cells between the metaxylem and metaphloem of vascular bundle stops dividing and differentiate into conducting tissues
In woody species the cells located in this position never undergo cell cycle arrest
Continue to divide instead of maturing
Constitute the fascicular cambium
Mature parenchyma cells between vascular bundles come out of cell cycle arrest and resume mitosis
Forms interfascicular cambium that connects on each side with the fascicular cambia
Makes a complete cylinder
Fascicular and interfascicular are used only while the cambium is young
Fusiform Initials
Long, tapered cells
When they undergo longitudinal cell division with a wall parallel to the circumference of the cambium (a periclinal wall) it produces two elongated cells
One cell remains the same and the other differentiates into a cell of secondary xylem or secondary phloem
Regardless of which cell changes one always remains as cambium
Vascular cambium cells must occasionally divide longitudinally by anticlinal walls
Thereby increasing the number of cambial cells
Without anticlinal divisions cambial cells would be stretched wider and could not function
May occur in a storied cambium or a nonstoried cambium
Ray Initials
Similar to fusiform initials except that they are short and more or less cuboidal
Also undergo periclinal cell divisions
The elongated fusiform initials produce the elongate cells of wood and phloem
Ray initials produce short cells mostly just storage parenchyma and in gymnosperms, albuminous cells
Typically grouped together in short vertical rows either uniseriate, biseriate, or multiseriate
Two basic types of ray parenchyma cells
Upright cells
Have direct connection with axial cells
Procumbent cells
No direct connection with axial cells
May contain tracheids
Horizontal, rectangular cells that look somewhat like parenchyma cells but have secondary walls, circular bordered pits, and protoplasts that degenerate quickly after the secondary wall is completed
Types of Wood Cells
Secondary xylem, known as wood
All cells formed to the interior of the vascular cambium that develop
Contains all of the types of cells that occur in primary xylem but no new ones
Only difference between primary and secondary xylem are the origin and arrangement of cells
Axial system
Derived from the fusiform initials
Always contains tracheary elements, which carry out longitudinal conduction of water through the wood
Contains fibers that give the wood flexibility and strength
Complexity of the axial system of wood varies
Radial system
Develop from the ray initials
Of xylem is simple, and in contains only parenchyma, arranged as uniseriate, biseriate, or multiseriate masses called rays
Hardwoods
Term used for wood of all basal angiosperms and eudicots
Softwoods
Woods from conifers such as pins and redwoods have few or no fibers and thus have a softer consistency
Growth Rings
1st wood formed is the early wood
Must have proportion of wide vessels or in conifers, wide tracheids
Cuticle has thickened, transpiration is less, and large #'s of newly formed vessels are conducting rapidly
Wood produced at this time is known as late wood
Annual ring
Early and late wood together make up 1 year's growth
Diffuse porous
Because the wood of a growth ring has vessels located throughout
Ring porous
Species with vessels restricted mainly to early wood
Heartwood and Sapwood
Heartwood
The dark wood
One annual ring is converted to heartwood each year
Becomes wider with age
Sapwood
Lighter, moister outer region
New layer is formed each year by the vascular cambium
Has a more or less constant thickness
Tylosis
Wood parenchyma cells adjacent to vessels psuh bubbles of protoplasm through the pits into the vessel, forming a plug
Cork and Cork Cambium
Cork cambium
Also called the phellogen
All cells are cuboidal
Inner cell almost always remains cork cambium, whereas the outer cell differentiates into a cork cell
Phellem cell
Phelloderm
May produce a cell or two to the inside that mature into a layer of parenchyma
Periderm
The layers of cork cells, and phelloderm
Outer bark
All tissue outside the innermost cork cambium
Inner bark
All secondary phloem between the vascular cambium and the innermost cork cambium
Anomalous secondary growth
Because alternative cambia produce secondary bodies that differ from the common type
Unusual Primary Growth
Palm trees trunks do not taper at the tips like those of eudicot or conifer trees
Do not branch as well
Palm trunk is all primary tissue
Consisting of vascular bundles distributed throughout ground tissue
Each bundle contains only primary xylem and primary phloem
All derived from the shoot apical meristem
The increase in width and addition of adventitious roots in palms
Establishment growth
