Final answer:
The metaphysis, especially the epiphyseal plate, is the site of growth in long bones. It is where new cartilage cells are created, ultimately leading to growth in bone length until early adulthood. Appositional growth also contributes to the widening of bones during this period.
Step-by-step explanation:
The site of growth in long bones is the metaphysis, specifically at the epiphyseal plate. This area is the narrow region that joins the end of the bone (epiphysis) with the shaft (diaphysis). The epiphyseal plate is a layer of hyaline cartilage where new cells are produced for longitudinal bone growth during a person's developmental years. Once the bone has ceased to grow, usually in early adulthood, the epiphyseal plate is replaced by bone and becomes the epiphyseal line.
Growth in the diameter of long bones occurs through a process known as appositional growth, where osteoclasts break down old bone tissue inside the medullary cavity and osteoblasts create new bone beneath the periosteum. This dual action of resorption and formation allows bones to become wider as they elongate.