Development and ultrastructure of the haustorium of Viscum minimum. I. The adhesive disk

Abstract

The adhesive disk of the haustorium of Viscum minimum Harvey develops an epithelium of uni- or bi-cellular secretory trichomes. Before contact with the host is obtained, the trichomes secrete adhesive cutinaceous material. This secretion contains fragments of the lamellar cuticle proper, the reticulate cuticular layer, and the noncutinized part of the cuticularized wall, in additions to pockets of granular material. The volume of the adhesive product per trichome may be several times the volume of the secretory cell itself except in front of the future intrusive organ where secretion is reduced. Formation of cutin cystoliths was observed in sections of wall of different ages within single trichomes. The trichomes are rich in mitochondria, plastids with starch, spherosomes, ribosomes, and ER that are associated with the strongly convoluted plasmalemma. A special type of tubular ER is described. Zones of collapsed cells are formed in the cortex as the apex of the wedge-shaped intrusive organ emerges. The cuticularized wall of the host then disintegrates, and the intrusive organ forms a fissure within the host. This becomes lined, to the depth of a few cells, partly with cuticularized and finally compressed cells originating from the adhesive epithelium, and partly with new cells produced by the intrusive organ. Key words: Viscum minimum, adhesive disk, adhesive epithelium, cuticle ultrastructure, cutin cystolith, haustorium, secretion.