The structure and formation of the Hartig net was examined in four different types of spruce ectomycorrhizas from the field. The investigations confirmed that in the case of mycorrhizas with a fully developed sheath enclosing the root apex, the Hartig net is formed by a highly branched fingerlike or puzzlelike hyphal system.
It is shown that the hyphae penetrate and grow mainly in a transversal direction to the axis of the root, resulting in typical indentation patterns on some tangential walls and a fountainlike separation of hyphae when growing from the radial to the tangential intercellular spaces. Septae formation within the branched hyphal system of the Hartig net is rare. Structures which, in sections, appear to be incomplete septae are explained as the result of the branching pattern.
The alignement and the typical branching pattern of the Hartig net is also found around the outermost cortical cells. A description of the changing structure in the different layers of a pseudoparenchymatous hyphal mantle is given. Finally, details of the three-dimensional structure of the mantle and the Hartig net are represented in a block diagram.