Most apple trees in modern orchards are two plants joined together.
Grafting fuses the top of one plant onto the roots of another so they grow as one. It lets growers copy a prized variety exactly, make trees fruit years sooner, or keep them small. It has been practised for well over two thousand years, and you can grow several kinds of fruit on one tree.
A grafted plant has two parts: the scion, chosen for its fruit, flowers or leaves, and the rootstock, chosen for its roots. The trick is to press together the thin layer of growing tissue under the bark, the vascular cambium, of both parts and keep them alive until they knit, usually a few weeks. Studies in a small lab plant found the food-carrying tissue connects within about three days and the water-carrying tissue within a week. The joint stays a weak point, because only newly grown tissue fuses.
Why bother? Because the scion carries the genes you want to reproduce, grafting copies a variety exactly. It also skips a tree's juvenile years: many fruit trees take five to nine years to flower from seed, but a mature scion on a rootstock can fruit in as little as two. Dwarfing rootstocks keep modern orchard apples small, so they can be planted densely and picked without tall ladders. Plant the graft below soil level, though, and the scion may grow its own roots and reach full size. Resistant rootstocks also protect crops from soil pests, a famous example being rootstocks used against the vine pest phylloxera.
The technique is old. By 500 BCE, according to the Mishnah, it was a commonplace way to grow grapevines in the Near East; a Greek medical text of 424 BCE mentions it, and Cato the Elder gave detailed instructions in Rome around 160 BCE. A Chinese agricultural treatise of the 6th century CE describes grafting pears onto other rootstocks. In nature, touching roots or branches of the same species sometimes fuse on their own.
Grafting usually works only between close relatives, and plants such as grasses, which lack a cambium, generally can't be grafted. Within those limits it allows some playful results: a 'family tree' bearing several apple varieties, a 'fruit salad tree' carrying several different stone fruits, or a pomato, with tomatoes above ground and potatoes below.
Source: Wikipedia — Grafting · Text summarised from Wikipedia (CC BY-SA 4.0)
