A Closer Look At Wood and Bark

AP BiologySpring 2011

Formation of BarkAs tree ages inner core of xylem continues its

outward expansion Resulting pressure is directed toward the

stem or root surface Will rupture the cortex and outer part of

secondary phloem When that occurs, parenchyma cells in this

region start dividing Give rise to cork cambium

Formation of BarkThe rupturing causes the cortex and

epidermis to split away Ongoing cell division at cork cambium gives

rise to periderm

Formation of BarkBark: the periderm and secondary phloem

Consist of living cells and dead tissues on the outside of vascular cambium

Formation of BarkCork: densley packed rows of cells

Each with a wall thickened by a fatty substance called suberin

Innermost cells are alive- have access to nourishment from xylem and phloem

Can protect, insulate, and waterproof the stem or root surface, forms over wounded tissues

When leaves are about to drop, cork forms at the place where petioles attach to stems

Formation of BarkLenticels: localized areas where the packing

of cork cells is loosened up a bitAllow passage of gases through bark

Heartwood and SapwoodHeartwood: lies at the center of older stems

and rootsIt is a depositary for resins, oils, gums, and

tannins It is clogged and no longer functions in

transportHelps tree defy gravity

Heartwood and SapwoodSapwood: secondary growth located

between heartwood and vascular cambiumIt is wet, pale in colour, and not as strong It is rich in a sugar-rich fluid transported from

the rootsUsed in the production of maple syrup

Early Wood, Late Wood, and Tree RingsIn regions with cool winters or dry spells, the

vascular cambium is inactive during part of the year

Early Wood, Late Wood, and Tree RingsEarly Wood: contains xylem with large

diameters and thin wallsStart of growing season

Early Wood, Late Wood, and Tree RingsLate Wood: contains xylem with small

diameters and thick walls

Early Wood, Late Wood, and Tree RingsGrowth Rings: appear as alternating light

bands of early wood and dark bands of late wood

Early Wood, Late Wood, and Tree RingsHardwoods:

Oak, hickory, other eudicot trees

Evolved in temperate and tropical zones

Has vessels, tracheids, and fibers in its xylem

Early Wood, Late Wood, and Tree RingsSoftwoods:

Pines, redwoods, other conifer species

No vessels or fibersTheir xylem has

tracheids and rays of parenchyma

Weaker and less dense than hardwoods