Forestry Sciences Laboratory, Pacific Northwest Forest and Range Experiment Station, USDA, Forest Service, Corvallis, Oregon 97331

SUMMARY Pacific madrone (Arbutus menziesii) ectendomycorrhizae synthesized in pure culture with Thelephora terrestris, Corticium bicolor, Cenococcum graniforme, and Pisolithus tinctorius are described. It is suggested that most arbutoid mycorrhizae of Pacific madrone and other ericaceous plants are formed by the same fungi which are associated with ectomycorrhizae of forest trees. Harley (1969) has pointed out the close morphological similarity of arbutoid mycorrhizae of Arbutus, Arctostaphylos, and other ericaceous plants to ectomycorrhizae of forest trees. In both, a closely similar fungal sheath or mantle is formed so that they appear quite alike externally. And both possess a Hartig net although that of the arbutoid mycorrhiza is restricted to the outer tier of cells. However, in contrast to the ectomycorrhiza, in the arbutoid mycorrhiza the symbiotic fungus penetrates outermost cells, filling their lumina with ramifying hyphae. Harley, thus, regards the arbutoid mycorrhiza as a connecting link between the ectomycorrhiza and the endomycorrhiza. Fungi which form the arbutoid mycorrhiza are yet but little known. Francke (1934) regarded a fungus which he isolated from Monotropa mycorrhizae as a Boletus sp. Later, Bjorkman (1960) demonstrated that Monotropa plants were connected to nearby pine and spruce trees by a common mycorrhizal fungus. A fungus isolated from Monotropa mycorrhizae, and morphologically similar to the one obtained by Francke, formed ectomycorrhizae with pine in pure culture. An arbutoid mycorrhiza of Arctostaphylos uva-ursi (L.) Spreng. from the Oregon coast was linked to Cortinarius aureifolius var. hesperius