Root morphology in Ceratopteris richardii sporelings exhibits heteroblastic variation. Growth analysis of the first six roots borne along the shoot shows that root 5 has a predictable pattern of development that is well suited for detailed study. The developmental anatomy of this fifth root, which is located near the base of sporeling leaf 5, is described using histological and statistical techniques. Three proximal division faces of the tetrahedral root apical cell divide in a strict sequence to produce three merophyte orthostichies in the body of the root. Formative divisions in merophytes behind the apical cell produce initials that give rise to a characteristic number of cell files in each merophyte. These formative divisions occur in a relatively regular order during merophyte ontogeny, and there are no significant differences among plants. However, the formative division pattern among merophytes within a root is not strictly the same. Predictable inter-merophyte differences arise because a two-fold anatomical symmetry that is characteristic of mature roots is superimposed on a three-fold radial symmetry that originates behind the apical cell. As formative divisions are completed, extensive proliferative divisions subsequently increase cell numbers within each cell file of a merophyte. The cellular parameters of fifth root development observed here are reminiscent of patterns previously described for the heterosporous fern Azolla. However, C. richardii should prove to be a more tractable model to further investigate the genetic regulation of root development in a non-seed plant.

Key words: Ceratopteris richardii, root development