The problem of root meristems, previously considered to exchange cells between the stele and the cap as well as between cortex and cap, has been solved by using DNA-labelling and stathmokinetic methods to observe the change in behaviour of the meristem as the primary root ages in Cucurbita pepo L.
The stelar pole is quiescent and can contribute few, if any, cells to the cap. During early growth the tiers of cells of the columella meristem are intermittently quiescent and active. This causes the highest average rate of mitosis in the cap to occur in a tier of the columella that is not adjacent to the stelar pole. It also interrupts the patterns of cells in the columella to form up to four complexes of cells along its length. As the root ages, rates of mitosis become generally lower, except in the quiescent stelar pole, and fewer tiers of the columella stay meristematic. The proximal tier of the columella, adjacent to the stelar pole, now acquires the highest rate of mitosis in the meristem. This simplifies the columella pattern, but the meristem never becomes closed because no quiescent buffer establishes itself between the stele and the central cap meristem and so a boundary between the cortex and peripheral cap meristems never becomes stabilized.