Basal meristem and root development in Isoetes asiatica and Isoetes japonica

We studied the anatomy of the basal meristem and root development in Isoetes asiatica and Isoetes japonica to better understand the Isoetes rhizomorph, an enigmatic organ. The basal (root-producing) meristem of the rhizomorph is continuous with the lateral meristem, and both comprise a single meristem. The basal meristem with a layer of thin initials has both organogenetic and thickening meristem attributes. A combination of these attributes indicates that the basal meristem is distinctive among the vascular plant meristems, including cambium. The root primordia develop from young derivative cells organized in three to four radial fines, and their main body is composed of a three-layered apical meristem. The primordia, unlike the roots, produce a small number of derivative cells as well as a procambium and future root cap, and they do not form a cortex and epidermis. Prior to developing into roots, the primordia are shifted closer to the corm surface through further development of cortical tissues from the basal meristem and sloughing of surface cells. This unique developmental pattern is established through the subendogenous origin of the second embryonic root and the endogenous origin of the third root. The meristem and pattern of root development in Isoetes is compared with that indicated for the lepidodendrid and isoetalean rhizomorphs and contrasted with that in the Selaginella rhizophore. The comparison seems not to support the interpretation of the roots as leaf homologues.