Vyvyan's observation that the growth of the grafted apple-tree is controlled more by the rootstock than by the scion implies that apple-tree growth is more limited by a function of the root-system than of the foliage; pruning away half the roots would be expected to reduce growth to a greater extent than pruning half the leaves. Following leaf and/or root pruning of young trees with twin root-systems, growth over two successive three-week periods in late summer was reduced only by leaf pruning; the compensatory adjustments in distribution of growth were insufficient to account for the discrepancy between observation and expectation. The compensatory processes were (1) increase in the growth-rates of the pruned parts relative to the total growth (this was most marked for the roots), (2) increase in the rate of function of the pruned part, as estimated indirectly as plant increment per g functioning part (this rate increased most for pruned leaves). It was concluded that neither leaf nor root was continuously limiting growth, and that both usually operated below their maximum efficiency.