Specific intercalary growth of rhizophores and roots in Selaginella kraussiana (Selaginellaceae) is related to unique dichotomous branching

Rhizophores - leafless axial organs, which apices are the only sites for root formation - are unique to the Selaginella genus. Both rhizophores and roots are dichotomously branched, forming a multibranch rhizophore-root system. In the examined Selaginella kraussiana, the first dichotomous division of a rhizophore results in the initiation of the twin root meristems, giving rise to the root primordia. These primordia are temporarily arrested in growth, but at the same time they are elevated, hidden inside the rhizophore apex due to the activity of a meristematic zone at the rhizophore axis. This meristematic region, located basally to the root meristems, apart from the rhizophore tissues consists of the derivative cells of root primordia and may be considered as a specific intercalary meristem.The growth mode of the roots is similar to that of the rhizophore, including dichotomous branching of the meristem and a temporary developmental arrest of the subsequent root primordia. The reiteration of the developmental program also involves the formation of new intercalary zones, thus through divisional activity, the root with dichotomized apex grows for some time as a single axis and, then the young roots emerge. At each step of the rhizophore-root branch system formation, tissues of the parental axis contribute significantly to the growth and emergence of the next axes. Therefore, dichotomous branching in Selaginella appears to be exceptional in comparison with other known dichotomies of axial organs.