The gametophyte of the anthocerotophyte Phaeoceros laevis is associated with an aseptate fungus that forms extracellular thick-walled hyphae, intercellular thin-walled hyphae, and intracellular arbuscules. Vesicles are formed by swelling of inter- or intracellular hyphae and are common in summer. The fungus colonizes the parenchyma of the host, except at the growing apices, and is always absent from the epidermal cells. The intracellular hyphae are enclosed by an interfacial region that consists of the invaginated host plasmalemma, a light zone extending from the host periplasm, and a matrix continuous with the host wall. Bacterium-like organelles and two types of electron-dense granules are found in the fungal hyphae. Infected host cells show increased cytoplasmic contents; the single chloroplast loses starch and the pyrenoid and forms numerous convoluted branches that are intermingled with the arbuscular hyphae. The arbuscule eventually degenerates, producing intracellular clumps of collapsed hyphae. Ce...