What is the term used to describe the layer of actively dividing cells responsible for the secondary growth of plant stems and roots?

The term used to describe the layer of actively dividing cells responsible for the secondary growth of plant stems and roots is cambium. Cambium is a type of lateral meristem that forms a thin layer between the xylem (wood) and phloem (bark) tissues in vascular plants. This layer is crucial for the plant's ability to increase its girth, allowing stems and roots to grow thicker over time. The cambium produces new layers of xylem on the inside and phloem on the outside, which is essential for the ongoing growth and health of the plant.

The other choices relate to different concepts in botany. Caliper refers to the measurement of the diameter of a tree trunk, clump refers to a growth form of plants growing closely together, and chlorophyll is the green pigment found in plants that is essential for photosynthesis. Each of these terms has its significance in plant biology but does not pertain to the layer responsible for secondary growth in stems and roots.