As a rule, however, the number in a whorl is irregular.
There is no satisfactory explanation of this break in the regular alternation of successive whorls; the outer whorl' of stamens arises in course of development before the inner, so that there is no question of subsequent displacement.
In the staminal whorl especially it is common to find additional rows.
When there is more than one row or whorl in a flower, those on the outside are sometimes longest, as in many Rosaceae; at other times those in the interior are longest, as in Luhea.
This structure was formerly regarded as pointing to the fusion of two organs, and the pale was considered by Robert Brown to represent two portions soldered together of a trimerous perianth - whorl, the third portion being the " lower pale."