A Closer Look At Wood and Bark AP Biology Spring 2011

Formation of Bark As 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 Bark The rupturing causes the cortex and epidermis to split away Ongoing cell division at cork cambium gives rise to periderm

Formation of Bark Bark: the periderm and secondary phloem Consist of living cells and dead tissues on the outside of vascular cambium

Formation of Bark Cork: 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 Bark Lenticels: localized areas where the packing of cork cells is loosened up a bit Allow passage of gases through bark

Heartwood and Sapwood Heartwood: lies at the center of older stems and roots It is a depositary for resins, oils, gums, and tannins It is clogged and no longer functions in transport Helps tree defy gravity

Heartwood and Sapwood Sapwood: secondary growth located between heartwood and vascular cambium It is wet, pale in colour, and not as strong It is rich in a sugar-rich fluid transported from the roots Used in the production of maple syrup

Early Wood, Late Wood, and Tree Rings In regions with cool winters or dry spells, the vascular cambium is inactive during part of the year

Early Wood, Late Wood, and Tree Rings Early Wood: contains xylem with large diameters and thin walls Start of growing season

Early Wood, Late Wood, and Tree Rings Late Wood: contains xylem with small diameters and thick walls

Early Wood, Late Wood, and Tree Rings Growth Rings: appear as alternating light bands of early wood and dark bands of late wood

Early Wood, Late Wood, and Tree Rings Hardwoods: 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 Rings Softwoods: Pines, redwoods, other conifer species No vessels or fibers Their xylem has tracheids and rays of parenchyma Weaker and less dense than hardwoods