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Ectomycorrhiza formation between Dryas integrifolia and Hebeloma cylindrosporum is initiated by changes in the growth pattern of hyphae when they first contact root epidermal cells in the mycorrhizal infection zone. Fungal hyphae then grow within the middle lamella of the plant root. Fungal growth is modified and branching patterns similar to those found in the Hartig net of most ectomycorrhizal systems result. Inter-cellular hyphal growth is impeded by the deposition of wall material around plasmodesmata, the latter maintaining symplastic continuity between root cells. The middle lamella in these regions is, however, eventually occupied by growing hyphae that disrupt plasmodesmata. Transfer cell-like wall ingrowths occur in cortical cells in the Hartig net region. Ultrastructural changes occur in Hartig net hyphae and contiguous root cells during the ontogeny of the ectomycorrhiza.