AbstractIn Long‐Evans rats the area of the cerebellum was irradiated with multiple doses of low‐level X‐ray beginning on day 12 after the bulk of stellate cells were acquired. The treatment spared basket, stellate and early‐forming granule cells but led to a substantial reduction in the total granule cell population and a correlated miniaturization of the cerebellar cortex. Nevertheless most Purkinje cells had normally shaped planar dendritic arbors, with an upward directed stem dendrite, several smooth branches and a multitude of spiny branchlets. The frequent piling up of spiny branchlets near the surface was attributed to the truncation of the bed of parallel fibers by this radiation schedule.In this last paper of the series the accumulated results are summarized and evaluated. The hypothesis is offered that while the growth of the Purkinje cell perikaryon is an autonomous process, the oriented perpendicular growth of a single stem dendrite depends on the presence of basket cell axons, the outgrowth of smooth branches on the presence of stellate cell axons, and the proliferation of spiny branchlets on interaction with parallel fibers. The parallel fibers are responsible for the orthogonal, planar growth of the dendritic arbor and a hypothesis is offered about the mechanisms i