THE PERIDERM OF FLOODED AND NONFLOODED LUDWIGIA-OCTOVALVIS (ONAGRACEAE)

The periderm of Ludwigia octovalvis swells readilly when immersed in water. In non-flooded individuals the periderm is very compact, composed of typical brick-shaped, thick-walled phellem cells, without intercellular spaces, and a maximal radial length of 40-mu-m. The periderm of flooded individuals is very loose, with sausage-shaped, thin-walled phellem cells reaching a maximum length of 280-mu-m and with long intercellular spaces between them. Phellem cells of flooded plants showed elongation very early in their differentiation from phellogen cells, forming slight protuberances which increased in size with increasing distance from the phellogen. The protuberances grew predominantly in the radial direction, centrifugally. Phellem cells of flooded plants just differentiated from the phellogen had dense cytoplasm and rounded nuclei with a more or less central position; those further away from the phellogen showed sparse cytoplasm and their nuclei were pulled towards either tip of the cells or laid against the cell walls.