Reading passage
Long before scientists understood genetics, farmers knew that a branch from one tree could be persuaded to grow on the trunk of another. This technique, known as grafting, joins the tissues of two plants so that they function as one. The upper part, called the scion, determines the fruit or flower that will be produced, while the lower part, the rootstock, provides the roots and much of the plant's resilience. Today grafting is used on an industrial scale for apples, grapes, roses, tomatoes and many other crops, yet its basic principle has changed little in thousands of years.
The earliest written records of grafting come from China around two thousand years ago, and Greek and Roman writers such as Theophrastus and Pliny described the practice in detail. Roman gardeners grafted apple and pear trees, and their methods were preserved in monastery gardens through the Middle Ages. By the sixteenth century, grafting manuals were among the bestselling practical books in Europe, and new varieties of fruit spread along trade routes as bundles of cuttings rather than as seeds.
The biological explanation for grafting's success lies in the cambium, a thin layer of actively dividing cells between the bark and the wood. When the cambium layers of scion and rootstock are held in close contact, their cells multiply and form a bridge of new tissue, through which water and nutrients can eventually pass. Success depends on clean cuts, firm binding, protection from drying out and, crucially, compatibility: closely related species usually graft well, while distant relatives often fail to form a lasting union.
One reason grafting remains essential is that many fruit trees do not grow true from seed. An apple pip carries a shuffled combination of its parents' genes, so a tree grown from the pip of a sweet dessert apple may bear sour or woody fruit. Grafting allows growers to clone a desirable variety indefinitely; every Cox's Orange Pippin apple tree in the world, for example, descends from cuttings taken from a single seedling that grew in an English garden around 1825.
Rootstocks do far more than hold the scion upright. Breeders have selected rootstocks that resist soil-borne diseases, tolerate waterlogged or alkaline soils, and control the size of the tree. Most modern apple orchards are planted on dwarfing rootstocks, which produce short trees that are easier to prune, spray and harvest, and that begin bearing fruit years earlier than full-sized trees. The choice of rootstock can alter yield, fruit size and even flavour.
Grafting is not without drawbacks. The operation is labour-intensive and requires skill, which adds to the cost of planting material, and the union between scion and rootstock can remain a weak point, liable to snap in strong winds or fail years later. Some combinations grow together poorly, a problem known as incompatibility, which may not become visible until the trees are mature. Growers must also remove any shoots that sprout from below the graft, since these belong to the rootstock and can overwhelm the scion.
Grafting has also saved entire industries. In the late nineteenth century, an insect pest called phylloxera devastated European vineyards, destroying millions of hectares of vines. Wine grapes of the classic European species, Vitis vinifera, had no resistance to the pest, but wild American vines did. The solution was to graft European scions onto American rootstocks, a remedy so successful that almost every bottle of wine produced today comes from grafted vines.
In recent decades the technique has found a new frontier in vegetables. Grafted tomato, aubergine and watermelon plants, grown on vigorous hybrid rootstocks, tolerate soil diseases that once forced farmers to abandon fields or fumigate them with chemicals. In Japan and South Korea, machines now graft thousands of seedlings per hour, and researchers are exploring grafting as a way to transfer traits such as salt tolerance between related species. What began as a gardener's trick has become one of agriculture's most sophisticated tools.