Final answer:
The origin of wood, with an increase in circumference, is due to the activity of lateral meristems, particularly the vascular cambium, which produces secondary xylem containing lignin.
Step-by-step explanation:
The origin of wood, characterized by an increase in the circumference of a tree, is best described by the activity of the lateral meristems, specifically the vascular cambium. The vascular cambium is responsible for the production of secondary xylem and secondary phloem. During secondary growth, the vascular cambium adds layers of vascular tissue, contributing to the continuous increases in the stem's girth. The cells of secondary xylem, which accumulate to form wood, contain lignin that provides hardness and strength to the plant structure.
Annual rings in a tree provide evidence of this growth process. The vascular cambium creates light, large-celled early wood in the spring and denser, small-celled late wood in the fall. Observing these rings can provide information about the tree's age and the climatic conditions during its growth periods.