The endotrophic mycorrhiza of four heterotrophic and four autotrophic orchid species have been investigated by light and electron microscopy. Contrary to what has been found in some other plants, the cell wall of the cortex cells of the orchids lacks lignin which might preclude the invasion of fungi. Cellulose and pectin are attacked by the mycorrhizal fungi; these were all found to produce cellulase and pectinase. Within the cells of the plant, the hyphae of the fungus were found always to be enveloped within the invaginated plasmalemma which continues forming pectin and cellulose and, in the case of exodermal cells, may also form suberin. These pectin and cellulose layers are rather thin or even seem to be lacking in cells with young and active mycorrhizal fungi; on the other hand they are clearly visible in cells with decaying hyphae. It may be inferred that the active fungus digests pectin and cellulose about as fast as they are formed by the plant cell plasmalemma. Decaying hyphae and the pectin and cellulose layer around them are usually found in the central part of the fungus-digesting cells and form a clot, which is also enveloped by a pectincellulose layer. During the process of digestion the plasmalemma shows pinocytosis.