Ultrastructural observations indicate that, during development, the single-celled trichomes on the young leaves, petioles, and stipules become isolated from symplastic and apoplastic continuity with other leaf cells. This occurs with the development of a suberin layer internal to the primary wall of the trichome. With maturation, a massive, internal wall layer develops. Subsequently, the trichomes senesce and collapse, resulting in the extrusion of the degraded cellular material in the lumen of the trichomes through pores onto the surface of the leaves. Extrusion of material from the trichomes primarily occurs on young organs. Apparently no extrusion occurs from the trichomes on the older leaves since they are all senescent, collapsed, and have a much reduced luminal content. Changes in the ultrastructure of the fluid, sticky resin on the young organs, compared with that of the dry, hard resin on the older leaves, indicate that chemical modification of the resin occurs during and after extrusion. The exte...