This section is from the book "The Gardener V3", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
We would make the same subdivision here as among trees which are to be grown in orchards, in quarters by themselves, or in rows in the kitchen-garden. For villa or cottage gardens, we think that medium-sized trees on walls as well as in the open are most suitable. However, we, in all cases, are guided by circumstances, and it frequently happens that there are walls in even small gardens which require the largest-growing kinds; in other cases only the very smallest are suitable. We remember being shown by a villa-owner the most fruitful Jargonelle Pear we ever saw. It was planted against, and wholly covered, the gable of a two-storey warehouse. It had a great depth of soil to revel in, although a macadamised road lay right over its root. The training and pruning had been of the most free-and-easy kind, and would have disgusted a gardener with trim-training ideas; but the result was a very large healthy tree, bearing an enormous quantity of splendid fruit. Hundreds, thousands of gables and other high walls are covered with fruit-trees which are highly ornamental, and profitable into the bargain, and better ornaments need not be wished.