BARK

INNER BARK -- made by the same vascular cambium that produces wood. Many of the cells that develop are totally different.

Another difference is that in softwood tracheids (for example), the protoplasm is lost prior to fluid conduction while in bark, the fluid is conducted only while the protoplasm fills the lumen.

Only the most recent layer conducts fluids. 0.2 -0.3 mm thick (total thickness of inner bark is 0.5 to 15 mm)

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Softwood Inner Bark:

Longitudinal & Ray Parenchyma as well as Longitudinal & Ray Epithelium cells ---    short, thin walled and unlignified.

ray tracheids -- albuminous cells

NO longitudinal tracheids

SIEVE CELLS -- primary avenue of conduction -- similar to longitudinal tracheids but shorter -- no structural function because cell walls seldom lignify or form secondary cell walls -- therefore, no pits  -- sieve areas appear in wall allowing interconnection of protoplasm.

PHLOEM FIBERS -- long slender, thick-walled and often heavily lignified -- structural component -- 10% by volume and not present in some pines.

STONE CELLS -- irregular shaped, extremely thick, multi-layered cell walls that are highly lignified -- usually found in clusters -- absent in pines.

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Hardwood Inner Bark

longitudinal & ray parenchyma and phloem fibers -- similar to softwoods

SIEVE TUBE ELEMENTS -- more definite structure -- arranged end to end with pore marked end walls called sieve plates which permit unbroken strands of cytoplasm to extend from cell to cell

COMPANION CELL --  longitudinal parenchyma that is always paired with a sieve tube element -- produced by the same cambium initial and appears to regulate the formation of the sieve tube element

Outer Bark (Periderm)

PHELLODERM (inner layer) -- flattened cells -- additional moisture protection -- other cells are thin-walled and greatly expanded and thought to provide thermal protection

PHELLOGEN (cork cambium) -- meristematic layer one cell wide

PHELLEM (outer layer) -- thin-walled cells heavily wax impregnated, thick-walled lignified element -- serves as moisture barrier and shock absorber

there are several layer of outer bark with each layer consisting of the three layers just mentioned

See Figure 7.3 on Page 151 in the Textbook

Bark Formation

As new inner bark is formed, it forces the older layers out -- at the same time, very thin-walled sieve cells or sieve tube elements are crushed. Parenchyma cells expand greatly in diameter.

A re-arrangement of bark elements -- some parenchyma cells unite into tangential bands and become meristematic producing a new 3 layer periderm inside the old periderm.

Ray contact is cut off by each new layer of periderm causing all outer bark except the last formed periderm to be dead.