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Because of the vertical orientation of the dominant anatomical feature of wood (vessels and fibers), sap within the stem of maple trees moves primarily in a vertical direction, either upward during as the tree is freezing/uptake phase or downward (mostly) during the thawing/exudation phase. When a tree is tapped, a zone of impermeable wood forms around the wood.
The purpose of this chapter is to describe a conceptual framework for understanding how trees grow and how they and other perennial plants defend themselves. The concept of compartmentalization has developed over many years, a synthesis of ideas from a number of investigators. It is founded on observations of trees injured in the field by wind, snow, ice, fire, animals, and insects, as well as during pruning, coppicing, sugaring, and other forest and orchard management practices. It is based on experimental studies of natural and artificial wounds with and without controlled inoculations with selected pathogenic and saprophytic microorganisms.