InHoya roots most exodermal cells are elongated and a band of suberin lamellae is formed in all their walls early in development; later on carbohydrate tertiary wall layers are deposited inside the suberin lamellae. Some exodermal cells which are restricted to root hair-bearing areas are short and unsuberized but their outer tangential wall is conspicuously thickened. Combined evidence from light microscopy and transmission and scanning electron microscopy reveals the bulk of this cap-formed thickening as a mosaic structure with two different components forming an extensive labyrinth. Irregular masses of a lignified, amorphous substance are separated by radially oriented, tortuous channels containing a very dense, granular-fibrillar material. The innermost wall layer is fibrillar and shows a texture and density similar to the material in the separating channels. The cap contains prominent pits with plasmodesmatal connections between short cells and the epidermis. In mature and non-functional short cells a band of suberin lamellae and eventually tertiary wall layers are deposited.