Abstract
A study of natural ectomycorrhiza of P. menziesii (Mirb.) by EM has allowed us to ascertain some cytological points concerning the host-symbiot relationships. Most hyphae of the mantle are alive. The hyphus situated on the periphery are loose; those which are deeply set are gathered into a pseudoparenchyma by an interstitial layer that shows clearly under the electrons. The Hartig net is uniseriate and does not penetrate the root cells. The symbiotic fungus is basidiomycetous, characterized by dolipore septum; electron dense grains can be noticed in the vacuoles. The root cells in contact with the Hartig net show starch in the plastids.