This section is from the book "The Gardener V2", 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.
What is the theory of grafting, at least in so far as the nature and affinity of the stock and scion in relation to each other are concerned?
We know there are some families of trees, including a goodly number of species, and even genera, which take readily on each other, - for instance, Plum stocks of various sorts; yet all Plums are used for Apricots, Peaches, Nectarines, and many others, including all the Almond tribe; while Crab and White Thorn are used as stocks for large numbers of trees in the natural order Rosacea. In all these cases there is more or less relationship or affinity between stock and scion, and it does not seem at all surprising that union should take place. But in the case of ordinary forest-trees, such as Beech and Elm, we fail to trace affinity, - the nature and grain of the timber are so very different, Beech being so close and compact with a smooth bark, whilst Elm is of a coarse grain with an exceedingly rough bark, generally ribbed and cracked even to the liber. Yet divergent as these two trees are, we lately found them growing on each other. The Beech was the stock, and Elm the top. We confess we had to see these trees before we could believe such a case possible. But see them we did, two or three dozens of them forming a small avenue in a rather thick plantation. The trees are perhaps fifty years old. The object in working them would appear to be simply curiosity.
They are worked from 2 to 3 feet from the ground. There was an unusually large rough ring at the point of junction, in which in many cases could be traced the tongue of the graft. These trees have grown freely, and had they been allowed room for development, would no doubt have been large trees. We see no practical utility in the practice; we record it simply as a curiosity in arboriculture.
Whilst on the subject of grafting allow me to mention another case. In the kitchen-garden where we live is a pond, on the bank of which grows a large bushy Quince tree now partially blown on one side. On one of the limbs of this Quince a graft of Jargonelle Pear had been put some years ago. It grows freely, and produces perhaps as fine fruit as one often sees of this favourite variety. What we wish to draw attention to is not the fact of the Pear on the Quince - that is common enough - but the fact of the Pear growing so vigorously on the Quince, and in its turn causing the particular limb on which it is worked to increase in proportion. Here we have a vigorous growth, which in its turn reacts on the stock, each acting to the advantage of the other. S. X.