SUMMARY Mycorrhizas of Pterospora andromedea Nuttall and Sarcodes sanguinea Torrey were examined with light and electron microscopes. Both plants were found to have a multi‐layered fungal sheath covering the surface of their roots and a Hartig net surrounding the epidermal cells but not penetrating the underlying cortex. Dolipore septa are present in the fungi of both plants. Individual determinate hyphae enter each epidermal cell through the radial wall which is oriented toward the tip of the root. As they penetrate, the epidermal cell wall appears to invaginate forming a fungal peg. The wall of the peg produces numerous protuberances similar to those found in transfer cells. In the combined fungal‐epidermal cell wall of the fungal peg the two walls do not remain distinct and it is impossible to be certain if both continue to the tip. Eventually, the tip of the fungal peg appears to open and form a mebranous sac, delimited by the epidermal cell plasmalemma, containing granular material with fibrillar inclusions which are approximately the size of microtubules. The relationship of the contents of the membranous sac to the contents of the fungal peg is discussed.

The mycorrhizae of six species of Pyrola were examined with light and electron microscopes. The hyphae on the root surface varied from a loose weft to an abundant mass with numerous strands, but no organized sheath was observed. Infection began with the formation of a Hartig net several millimetres behind the root tip. Hyphae from this net subsequently grew into each epidermal cell, forming masses of intracellular hyphae. These hyphae were surrounded by the host plasmalemma and a matrix material, presumably of host origin. During the stage of mature infection the host cytoplasm was finely granular and filled with organelles. The host vacuoles often had tanninlike deposits along their tonoplasts. Senescence of the symbiosis began with the gradual degeneration of the host cytoplasm, which became dark and vesiculated with loss of its organelles. The fungal hyphae and matrix material appeared essentially unchanged at this stage but eventually degenerated and collapsed. The fungal partners were normally basidiomycetes with dolipore septa, but one ascomycetous infection (distinguished by simple septa and Woronin bodies) was found to have a similar mycorrhizal organization. It differed in having an intermittent Hartig net. The presence of both the Hartig net and intracellular hyphae indicates that these are arbutoid mycorrhizae.