Intercellular space system in xylem rays of pneumatophores in Sonneratia alba (Sonneratiaceae) and its possible functional significance

Sonneratia alba J. Smith is a fast-growing pioneer mangrove tree species, and woody pneumatophores are one of the main morphological characters distinguishing this species from other mangrove species. Xylem rays of the pneumatophores in S. alba were exclusively uni-seriate and homocellular with procumbent cells. Intercellular spaces developed mainly between rows of procumbent ray cells to form continuous canals along the pneumatophore radius. Short axial intercellular spaces were present at the end wall sites of spindle-like procumbent ray cells within the same row, and interconnected with the neighboring radial intercellular canals into an intercellular space network within the xylem rays. The radial intercellular canals of xylem rays were larger in the outer secondary xylem than in the inner secondary xylem of a pneumatophore, and in the underground part than in the aboveground part. Blind pits (unilateral pits) towards the radial intercellular canals developed in the radial walls of vessels and the transverse walls of ray cells. The blind pits of vessels were bordered and vestured, and arranged radially in two regular rows in larger radial intercellular canals, but in one row or diffusely in narrower canals. The structural characters of the xylem rays and the intercellular space system of the pneumatophore suggest their possible involvement in water transport in the secondary xylem. An intercellular space system did not occur in xylem rays of the cable roots and stems of this species.