The thesis consists of four papers of which the first two have already been published. Paper I entitled "A study of Boletus elegans Schum. in pure culture", deals with the morphological characters and the physiological behaviour of this fungus in pure culture. Paper II entitled "The role of the Larch root in the nutrition Boletus elegans Schum.", is concerned with the results of adding excised roots to fungal cultures; these indicate that a water-soluble substance capable of stimulating the growth of the fungus is present in the roots of European and Japanese Larch, but not in those of Scots Pine. Paper III entitled "Mycorrhiza--formation in nature," contains an account of the isolation of three endophytes, detailed descriptions of the structure of seven types of infection and a general survey of mycorrhiza-formation in mature Larch trees. Paper IV entitled "Mycorrhiza-formation in experimental culture", is concerned with four sets of experimental cultures with Larch seedlings in which various methods of inoculation were tested.