Final answer:
The area that separates the diaphysis and epiphysis on long bones and is responsible for longitudinal growth is called the epiphyseal plate or growth plate.
Step-by-step explanation:
The tissue found separating the diaphysis and epiphysis on long bones, where longitudinal growth occurs, is the epiphyseal plate. The epiphyseal plate, also known as the growth plate, is a layer of hyaline cartilage located in the metaphysis at each end of a long bone. During development, the epiphyseal plate allows for the growth of the bone in length through the process of endochondral ossification. This growth involves a series of zones where chondrocytes undergo division, maturation, and hypertrophy. Growth in diameter, called appositional growth, is due to osteoblasts depositing new bone tissue beneath the periosteum, while osteoclasts resorb old bone lining the medullary cavity.
Once a bone has stopped growing in early adulthood (around 18-21 years), the epiphyseal plate is replaced by osseous tissue and becomes known as the epiphyseal line.