Studies on the ultrastructure and histochemistry of plant cuticles: the cuticular membrane of Agave americana L. in situ

The outer epidermal wall of Agave americana leaves was examined in order to gain more information about the location and chemical constitution of the structural components. In middle aged leaves the wall comprised six layers which were designated epicuticular wax, cuticle proper, exterior and interior cuticular layer, exterior and interior cellin wall. A lamellated structure, consisting of a series of electron translucent lamellae of uniform thickness alternating with opaque ones of variable thickness, was observed in the thin cuticle proper on the outside of the cuticular membrane, even without heavy metal treatment. The cuticular layers underneath formed the bulk of the cuticular membrane and they also had two components, an amorphous matrix permeated by a reticulum of fibrillae. Cutin, detected with osmium and with iodine/iodine-sulphuric acid–silver proteinate, was a major component of the opaque lamellae of the cuticle proper and the matrix of the cuticular layer. Carbohydrates were absent from the cuticle proper but were detected specifically in the fibrillae of the cuticular layer and in the cellin wall. Pectic material seemed to be present on both sides of the junction between cuticular membrane and cellin wall, but no discrete zone corresponding to light microscopical observations was detected in the electron microscope. Although the lucent lamellae of the cuticle proper were tentatively ascribed to wax there was no structural or ultrahistochemical evidence for the wax component of the cuticular layer. The various ultrahistochemical reactions are discussed in relation to the known chemical composition of the membrane.