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Structure of Woody Plants (Secondary Xylem # # (Growth rings (These two…
Structure of Woody Plants
Concepts of Woody Plant Structure
Primary tissue
Tissues that derive from the apical meristem
Epidermis
Vascular bundles
Pith
Cortex
Leaves
Secondary tissue
Tissues derived from other meristems
Meristems
Vascular cambium
Cork cambium
Tissue types
Wood (secondary xylem)
Bark (secondary phloem)
Usually only in trees and shrubs
Wood and bark contain conducting material
Giving the plant a greater capacity to transport materials
Leaf and root numbers increase
In turn causes more photosynthesis
Descended from one group of woody plants 370 million years ago
Vascular Cambium
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Initiation of the vascular cambium
Fascicular cambium
Cells that continue to divide between the metaxylem and metaphloem
Interfascicular cambium
Parenchyma cells that resume mitosis between vascular bundles
Vascular cambium is both of these cambia
Extended each year
Found mostly in roots and stems
Has only two cell types
Fusiform initials
Ray initials
Fusiform initials
Long tapered cells
140-462 micrometers in dicots
700-8,700 micrometers in conifers
Periclinal walls are formed during longitudinal cell division
Two cells are formed
One stays a fusiform initial
The other forms a secondary xylem of secondary phloem
Depends on whether it is the inner (X) or outer cell (P)
Anticlinal walls form perpendicular to the cambium surface during division
Ray initials
Similar to fusiform initials, but make short cells
Storage parenchyma
albuminous cells
Arrangement of Cambium cells
Ray initials are typically grouped in short vertical rows
Fusiform is either horizontal rows (storied cambium)
Or irregularly (nonstoried cambium)
The ratio of fusiform initials and ray initials is constant through a tree
Secondary Phloem
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Also formed from the vascular cambium
Size, shape, and number match xylem rays
Secondary Xylem
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Types of wood cells
All cells interior to the vascular cambium
Axial system
Derives from fusiform initials
Always contains tracheary elements
Hardwoods
Wood from angiosperms and eudicots w/ lots of fibers
Softwoods
Wood from conifers w/ few or no fibers
Radial system
Derives from ray initials
Upright cells
Cells that connect the two systems
Procumbent cells
Stores starch in these cells
Ray tracheids look like parenchyma cells
AKA wood
Growth rings
Early wood has a wide proportion of wide vessels
Late wood has a low proportion of vessels
These two create an annual ring
Some call them growth rings
Plants that grow equally all year are said to be diffuse pourous
Species restricted to early wood are ring porous
Heartwood and sapwood
Heartwood
Darker wood in the center of the log
Sapwood
Lighter wood around the heartwood
Tylosis plug vessels to prevent fungus growth
A new layer of sapwood is formed annually, as well as a layer of heartwood
Reaction wood
Wood to help with lateral stress on branches
Angiosperms form reaction wood on top of the branch
Conifers form reaction wood on the bottom of the branch
Outer Bark
Cork and Cork Cambium
As phloem matures, the stem/root increases in diameter
Secondary phloem forms the cork cambium (phellogen)
After division, inner cells remain cork cambium
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Outer cells differentiate into cork cells (phellem cells)
Some species can produce phelloderm
Forms periderm
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Provides temporary protection
Typically short-lived
Outer bark
Tissues outside innermost cork cambium
Inner bark
Phloem between vascular cambium and cork cambium
Lenticels and Oxygen Diffusion
Lenticels
Regions of aerenchymatous cork
Allows oxygen to penetrate across the bark
Lenticel-producing regions of cork are more active than ordinary cork
Initiation of cork cambia
In some species, cork cambia first appears before a twig or root is one year old
In other species, cork cambia forms when stems are several years old
Anomalous Forms of Growth
Anomalous Secondary Growth
Secondary bodies different from common bodies
Roots of sweet potatoes
Storage parenchyma is increased dramatically
Numerous vascular cambia arise
Included phloem
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Vascular cambium produces xylem and phloem
After a short period cambium cells differentiate into xylem
Secondary phloem becomes new vascular cambium
The secondary phloem located between two bands of xylem
Unequal activity of the vascular cambium
Woody vines typically grow larger into sectors of the cambium
Secondary growth in monocots
Vascular cambia originates from cortex cells
Makes only parenchyma
Some parenchyma cells differentiate into secondary vascular bundles
Unusual primary growth
Palm trees
A vascular cambium never develops
Each vascular bundle is encased with strong, heavy fibers
Gives a "woody" look
Establishment growth
The increase in width and addition of adventitious roots
Can occur in monocots with extremely broad stems
Secondary Growth in Roots
Root vascular cambium
Contains ray initials
And fusiform initials
Used to store water in dry areas
Significant differences from stem bark
Differing metabolisms
Differing microhabitats
