Fig. 9.

The endothelium. (A) Basal part of two endothelial cells. The cell wall carries densely packed wall protuberances. The labyrinthine wall complex extends into the cell lumen. The radial wall between the endothelial cells becomes thinner towards the inner parenchyma (upper left). Whereas the primary cell wall of the endothelium is thin, the adjacent perisperm cell wall is thicker. (B) The middle lamella between endothelial cells is interrupted where wall protuberances are continuous in adjacent cells (arrows). The space between the protuberances is occupied by a granular substance. (C) Tangential section through the labyrinthine wall complex, showing horizontally interconnected protuberances. (D) Densely packed labyrinthine wall enclosing an electron-dense substance at the base of endothelial cells (arrows). The other part of the cells is occupied by a granular and fibrillar substance (asterisk). Abbreviations: Et, endothelial cell; LW, labyrinthine wall complex; Ps, perisperm cell wall; WT, wall protuberances.