Final answer:
The lengthwise growth of the diaphysis is facilitated by the division and maturation of chondrocytes in the epiphyseal plate, which is followed by the replacement of mature cartilage cells with bone.
Step-by-step explanation:
The activities of the epiphyseal plate that account for the lengthwise growth of the diaphysis involve the continuous division and maturation of chondrocytes, which are cartilage cells. On the side of the epiphyseal plate nearest the epiphysis, new cartilage is formed through the process of cell division. Then, these cells move towards the diaphysis, enlarge and mature in what is called the zone of maturation and hypertrophy. Subsequently, the older, matured chondrocytes are replaced by bone through a process of calcification on the diaphyseal side of the plate. This ossification process contributes to the lengthening of the bone as the diaphysis elongates.