Which layer of a tree is critical for growth and is located just beneath the bark?

The cambium layer of a tree plays a crucial role in its growth as it is responsible for the production of new bark and new wood. This thin layer of actively dividing cells is located just beneath the bark. It facilitates the tree's growth in diameter by generating new cells both outward (which become phloem) and inward (which become xylem). The cambium is essential for the tree's ability to transport nutrients and water as well as to support overall growth.

In understanding the other layers mentioned, the phloem is involved in nutrient transport, specifically sugars produced by photosynthesis, but it does not have a direct role in the tree's growth in terms of expanding its diameter. The xylem is primarily responsible for the transport of water and minerals from the roots, and while it adds to the tree's structure, it forms after the cambium and is not responsible for growth. Heartwood is the central, non-living part of the tree that provides strength and support but does not play a role in active growth. Thus, the cambium is the most critical layer directly associated with the growth process in trees.