ISA Certified Arborist Practice Exam

Which layer of a tree is primarily responsible for growth?

1. Bark

2. Cambium layer

3. Heartwood

4. Phloem

The correct answer is: Cambium layer

The cambium layer is primarily responsible for a tree's growth. This thin layer of actively dividing cells is located between the bark and the wood. It plays a crucial role in the secondary growth of the tree, which allows it to increase in diameter each year. Through the process of cell division, the cambium produces new cells that develop into both xylem (wood) and phloem (the outer layer that transports nutrients). This growth is essential for the tree to gain strength, support, and access to resources like water and nutrients over its lifespan. In contrast, bark mostly provides protection and insulation for the tree, while heartwood is composed of older, non-living cells that no longer participate in growth or nutrient transport. Phloem, while involved in the transport of nutrients, does not contribute to growth in the same way as the cambium does, as it mainly contains living cells responsible for transporting carbohydrates produced during photosynthesis from the leaves to the rest of the tree.