Bark is the tough, protective outer layer of a woody stem, serving as a critical barrier between the inner vascular tissues and the external environment. Its primary function is physical protection against injury, pathogens, and fluctuating temperatures. Structurally, the bark is typically composed of multiple layers, including outer protective tissues and deeper layers containing phloem, which facilitates the downward transport of sugars produced during photosynthesis. The continual shedding and replacement of the outer bark are vital processes that allow the plant to maintain this defense system over its lifespan. The composition and thickness of the bark vary significantly across different species, often correlating with the tree's ecological niche and the stresses it must withstand.