The salt gland of Avicennia marina consists of two to four collecting cells, one stalk cell and usually eight secretory cells. The side wall of the stalk cell is completely impregnated with electron-dense material. An amorphous substance appears between the upper walls of the secretory cells and the cuticle above them. The latter possesses many narrow channels.
The ultrastructure of the secretory cells was studied under various conditions. The protoplast is usually dense and poorly vacuolated. The nucleus is relatively large, and the cytoplasm rich in organelles, especially E.R. elements, Golgi bodies and mitochondria. There are many vesicles which appear to be derived from the Golgi bodies and from E.R. cisterns. Elongated vacuole-like structures, apparently derived from Golgi cisterns, and membranous bands were also observed: it is suggested that both represent different stages of the same structure. In many glands the protoplast appeared to be contracted in one of the secretory cells and the space between it and the cell wall was filled with an amorphous electron-dense substance.
Antimonate precipitation and electron probe analysis were employed to locate ions in the tissues. Both techniques indicated that the salt content of the gland cells was lower than that of the mesophyll. A downhill gradient appeared to exist from cells near the xylem, through the mesophyll to the gland, and was continued through the gland itself. The possible location of ion pumps is discussed in the light of this evidence.