The structure of the roots of Avicennia marina (Forsk.) Vierh., a salt excretor, and Bruguiera gymnorrhiza (L.) Lam., an excluder (non-excretor), differed in the length and thickness of the root cap, phenolic content, the development of the Casparian strip in the endodermis and in the differentation of the vascular tissues.

The uptake and distribution of lanthanum, used as a marker for ion movement differed in the two species. Though both had similar root concentrations, the amount of lanthanum in the leaves of Avicennia marina was significantly higher than that in stem and root, whereas in Bruguiera gymnorrhiza it was significantly lower. The barrier to lanthanum transport, probably located in the hypocotyl, is discussed in relation to the anatomy of the root and the uptake of salt under natural conditions.