The first two entries for this week discussed weird ways that plants clone themselves in the wild. The last two will deal with weird ways that plants can be cloned in cultivation, and with the effects -- good and bad -- of the ways we use them. The phenomena for Thursday and Friday don't really happen without human intervention, and you'll soon see why.
In the case of grafting, the reason is simply that plants don't usually come into natural contact that way. To graft two plants, a gardener cuts off a stem from one of them -- the scion -- at a sharp angle. He or she then slices a notch into the stem of another one -- the stock -- and sticks the scion's cut stem into the notch. If the layers inside the two stems are lined up properly, and they're tied into place and given time to heal, the two plants will actually fuse together and keep growing. All the parts of the stock plant will grow as is typical for its species, but the scion stem will grow as typical for its species.
Gardeners frequently use this technique to combine characteristics from different species into a single plant. Say that you graft a scion from a cherry tree onto a peach sapling, for instance. Once the hybrid tree matures, you can end up with a tree half of whose branches produce peaches while the other half produces cherries. Even better, graft the stem of a tomato onto a potato's roots: you'll get edible tomatoes above ground and edible potatoes below.
Alternately, if you're the European wine industry, you could use grafting to save your very livelihood.
The aphid Dakytlosphaira vitifoliae, AKA the grape phylloxera, is native to the eastern USA and is a parasite on grape vines. To the grape species native to the same places as phylloxera -- the riverbank grape Vitis riparia, the fox grape Vitis labrusca, and the summer grape Vitis aestivalis -- the aphid is a fairly minor pest. To the European grapevine, Vitis vinifera, though, phylloxera infestation is lethal. Early attempts at starting vineyards in America all failed spectacularly as soon as the aphid found them. The cause was unknown until much later: grape growers knew only that America was hostile to their vines.
Imagine the devastation, then, when phylloxera found its way to Europe. After 1863, when its effects were first recorded in France, it spread like wildfire and devastated the French wine industry. No one knew exactly what was happening. Chemical pesticides and fertilizers were of no use. Nothing worked at all until, in 1868, French botanists hypothesized phylloxera as a cause. Their hypothesis was confirmed in 1870. Now that the cause was known, all that was needed was some solution. Pesticides still didn't work. What was it, people wondered, that let the native American grapevines survive the blight that was massacring V. vinifera?
Actually, they decided, they didn't need to know precisely what it was, as long as the resistance transferred across a graft. By grafting scions from wine-producing V. vinifera cultivars onto rootstocks from resistant American V. aestivalis, viticulturists were able to develop vines that produced familiar European grapes but would not succumb to phylloxera. The wine industry was saved! The fiercer French purists disdained the inferior American stocks, fearing that their wine grapes would be contaminated, but it quickly became clear that there was no choice. It was either embrace the resistant species or be wiped out.
To this day, any viticulturist who wants to grow V. vinifera without the threat of massive phylloxera losses must use only grafted plants. Pesticides are still no use, and no resistant strain of pure V. vinifera has been developed. Winemakers still debate whether an un-grafted V. vinifera produces better wines than one with an American rootstock, but the point cannot be tested: phylloxera is all but ubiquitous now.
For all that their fruit is less popular than V. vinifera's, then, the humble native grapevines of America -- riverbank, fox, and summer grapes -- are responsible for virtually every vintage produced since the Great French Wine Blight. If you like your wine, thank a native grape!