The dry weight increment of all parts, especially the rootstock, was less in fruiting than in non-fruiting maiden Worcester Pearmain apple trees; the fruit accounted for one-third of the total increment on M.IX. Fewer shoots grew out, and they had decreased rates of extension from the end of June, especially the later-formed shoots. The leaves were smaller and this was the earliest observed difference between fruiting and deblossomed trees. The rate of leaf production was lower after late July. While fruiting affected the shoots on the tree as a whole, the proximity of fruit affected individual shoots. The influence of fruit was less in August and growth of the trees increased relative to deblossomed trees.
Growth rates of leaves and shoots were similar until late June on deblossomed trees on dwarfing M.IX and on non-fruiting trees on invigorating M.XVI, but many more shoots extended on M.XVI in June. Shoots on M.IX extended at a much decreased rate in July and early August and the internodes were shorter but thicker than on M.XVI. In trees on M.IX a smaller proportion of the dry weight increment was in the rootstoek. The efficiency of leaves on M.XVI was greater than of leaves of deblossomed trees on M.IX, but less than on fruiting trees. These data, and additional facts published by other workers, have been integrated in an attempt to explain the dwarfing mechanism of M.IX.
Trees on M.IX are smaller because they are restricted to fewer growing points which continue extension for a shorter period. It is believed that the even smaller rate of growth of the root system limits the trees so that they are unable fully to utilize the products of photosynthesis in growth.