Root cell ultrastructure in developing aerenchyma tissue of three wetland species

Patterns of root cortex cell development and ultrastructure were analysed i n Sagittaria lancifolia L., Thalia genieulata L. and Pontederia cordata L, using scanning and transmission electron microscopy (SEM, TEM). In all thre e species, cortex cells were arranged in radial columns extending from the endodermis to the hypodermis/epidennis. During gas space formation, the cor tex cells elongated parallel to the root radius and shrank in the plane per pendicular to the radius leaving long and thin rows of cortex cells extendi ng from the endodermis to the epidermis. Although the cortex cells appeared collapsed in tissue with well-developed gas spaces, TEM revealed that the cortical cells as well as the epidermal cells maintained intact membranes a nd many normal organelles. Formation of root cortex tissue with well-develo ped gas spaces does not require cell death in these species. Living cortex cells in root tissue with mature gas spaces could provide a symplastic path way for transport between the root stele and the living epidermal (C) 2000 Annals of Botany Company.