FURTHER OBSERVATIONS ON NATHORSTIANA (ISOETACEAE)

The patterns of root arrangement and stelar morphology in the Lower Cretaceous fossil Nathorstiana are essentially identical with those of extant Isoetes. These structural similarities imply a similar mode of development in both plants and are used to infer an Isoetes-like, linear, root-producing meristem along the lower end of the stele in Nathorstiana. The root-bearing base of Nathorstiana differs from that of Isoetes principally in two respects. In earlier developmental stages, the rooting base of Nathorstiana was unlobed and radially symmetrical, whereas Isoetes is originally bilobed. Unlike Isoetes, which lacks stelar elongation and sheds its root-bearing cortex at the periphery of the cortical lobes, the downward growth of the root-bearing base of Nathorstiana resulted in the accumulation of a substantial extent of axial root-bearing surface.