SUMMARYThe structure of the roots of Avicennia marina (Forsk.) Vierb., a salt excretor, and Bruguiera gymnorrhiza (L.) Lam., an excluder (non-excretor), differed in tbe lengtb and tbickness of tbe root cap, pbenolic content, tbe development of tbe Casparian strip in tbe endodermis and in the differentation of tbe vascular tissues.The uptake and distribution of lantbanum, used as a marker for ion movement differed in the two species. Tbougb botb bad similar root concentrations, tbe amount of lantbanum in tbe leaves of Avicennia marina was significantly bigber tban tbat in stem and root, wbereas in Bruguiera gymnorrhiza it was significantly lower. Tbe barrier to lantbanum transport, probably located in tbe bypocotyl, is discussed in relation to tbe anatomy of tbe root and tbe uptake of salt under natural conditions.

Features of the epidermis such as stomata, hairs, cork and silica cells are described from both light and electron microscope studies. The stomatal complex consists of two guard cells and two subsidiary cells. After division of the guard mother cell a pore is left at each end of the dividing wall. The cork and silica cells arise from a single another cell and develop differentially. The silica cell enlarges more than the cork cell and finally becomes filled with solidified silica. The outer tangential and radial walls of the cork cells become very thick‐walled, whereas the inner tangential and radial walls of the silica cells become thickened. The outer tangential wall of the silica cell remains thin and is covered with a thin layer‐ of cuticle. This wall frequently collapses in old cells leaving a depression in the surface of the stem. The change in the ultrastructure of the cork and silica cells are described and the possible functions of these cells discussed.

Tubers in all five species develop from the hypocotyl region of the seedlingS. A perivascular cambium arises cutting off mainly starch‐storing parenchyma and collateral vascular bundles to the inside. A phellogen gives rise to cork on the outside. Between the two cambial layers there may or may not be layers of parenchyma, not storing starch but containing raphides. The vascular bundles consist of xylem with vessels, scalariform tracheids and parenchyma; and phloem, with sieve tubes and parenchyma.

Each salt-excreting gland of the mangrove Avicennia marina (Forsskål) Vierh. consists of two to four collecting cells, one stalk cell, and eight to twelve excretory cells. Differential membrane staining by zinc iodide-osmium tetroxide (as a post-fixative) or phosphotungstic acid (as a section-stain) was used to characterise the ultrastructure of the glands. A large amount of tubular endoplasmic reticulum was found in the stalk and excretory cells of the gland, but not in the collecting cells. The ultrastructural arrangement of the endoplasmic reticulum indicates that salt is loaded from the apoplasm into the endoplasmic reticulum of the symplasm at the base of the stalk cell, traverses both cell types in the endoplasmic reticulum, and is excreted at the outer edge of the gland by an eccrine-type mechanism. Increasing development of the tubular endoplasmic reticulum accompanied differentiation of the gland cells.

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