Final answer:
Sutures in the skull, which are immobile fibrous joints, may over time be replaced by plane joints that allow for slight movements to adapt to stress and changes in the bones.
Step-by-step explanation:
Sutures are replaced by plane joints because sutures are fibrous joints found only in the skull, characterized by immobility or very limited movement. As aging occurs, some sutures may ossify and be replaced by plane joints, particularly in the cranial area, to allow for slight movements that can relieve stress or adapt to changes in the bones over time. Plane joints, such as those between the carpal or tarsal bones, allow for small sliding or gliding movements providing limited flexibility which can be important for the minor adaptations and movements required in response to various stresses.