The investigations here described were carried out in connection with afforestation experiments at Wareham Forest, Dorset. The thesis consists of three papers. The first paper of the thesis deals with the mycelial and physiological characters of Boletus bovinus Linn, in pure culture, and with its growth reactions on such organic composts as are of interest in forestry practice. In connection with this, two types of mycorrhizal associations formed by Boletus bovinus with Pinus spp. at the experimental area, are described. The second paper is concerned with the cultural behaviour of several forms and strains of Mycelium radicis Melin, and with the description of various types and degrees of pseudo-myeorrhiza formation by this mycelium. Discussing the pathogenicity, attention is drawn to the fact that it is the proportion of pseudo-mycorrhizas and mycorrhizas formed by any individual tree which is of significance in respect to its general health. The third paper of the thesis deals with experiments and observations connected with analysis of the factors responsible for the suppression of growth in the experimental area. The anomalous features in the humus constituent of the experimental soil were studied by the effect of addition of cellulosic materials to this soil. The results obtained support the hypothesis that changes in the biological activities of the soil through addition of certain organic products have a direct effect on tree growth.<p
