Abstract

The annulus of Tetraplodon angustatus B. S. G. has been studied with transmission electron microscopy and of Tayloria tenuis (With.) Schimp. with scanning electron microscopy. Both species have an annulus which serves as a dehiscence mechanism of the capsule. Dehiscence takes place through cell wall separation at the middle lamella. The annulus remains attached to the operculum. Longitudinal section of the capsule of T. angustatus shows an annulus consisting of three cells in two layers (outer with two cells, inner with one cell). Ultrastructural peculiarities of the annulus cells are: 1) alternating electron-dense and more electron-translucent areas in the cell walls, representing a system of radial canals in the cell wall; 2) dense cytoplasm lacking vacuoles and occupying only a part of cell volume; 3) extraplasmic space and cell wall canals filled with a substance that appears to be mucilage; 4) outer parts of outer cell walls especially electron-dense and stratified. The annulus cells are considered to produce mucilage and hydrolytic enzymes in the course of their development.