What is the outermost layer of bark on a tree?

The outermost layer of bark on a tree is known as the outer bark. This layer serves as the tree's protective barrier against physical damage, pests, and environmental factors. It consists of dead cells that form a tough, thick coating, effectively safeguarding the inner tissues of the tree.

In the context of tree biology, it’s important to understand the roles of the other layers: the cambium is the growth layer that produces new bark and wood cells; the phloem, which is just beneath the outer bark, is responsible for transporting nutrients produced by photosynthesis from the leaves down to the rest of the tree; and heartwood is the central core of the tree trunk that provides structural support but is no longer involved in sap conduction. Each of these layers is crucial to the tree's overall health and function, but when considering the immediate outermost portion, it is indeed the outer bark that fulfills this role.