The crab apple tree in the parkway by Wax Trax Records had me baffled. All winter this particular crab has thousands of fruits that cling to the branches through strong winds and ice storms. On days when the red berries are capped with snow and set off against a blue sky, the tree is spectacular.

In past years when it’s come into blossom, most of the shriveled berries are still on the branches. I appreciate seeing it each time I walk by, but it puzzles me. Why would a tree evolve to the point where its apples–and therefore its seeds–never reach the dirt?

Your average deciduous tree drops its leaves and fruit in the fall, forming an abscission layer, a line of pithy cells, at the point where the stem meets the branch to close the openings in the bark where the leaves and fruit were attached. Letting go of the fruit is a normal process, presumably painless, and necessary if the tree wants to pass on its genes. But the Wax Trax tree doesn’t seem to care.

Around 350 million years ago plants got wise to the fact that their immobility was a reproductive disadvantage. The plant that drops seeds in its own shade risks having its offspring run low on sunlight and nutrients. Plants first adapted by manufacturing lightweight spores and seeds that the wind carried for them. Then 250 million years later certain species began to develop a more sophisticated system.

They created the great fruit trick. Trees grow fruit for only one reason: to manipulate animals into spreading their seeds around for them. Oranges, pears, guavas are all elaborate ploys to get an animal to ingest the plant’s seeds and excrete them along with some nice fertilizer some distance away from the parent plant.

The Wax Trax crab apple could drop its fruit as many types of crab apples do so it could be picked up by small mammals. But it doesn’t. So maybe it intended its seeds to be spread by birds. According to the book Attracting Birds: From the Prairies to the Atlantic by Verne Davison, robins, sparrows, starlings, and pigeons will all eat crab apples–though they’d all rather eat something else. The first three will go for insects and worms when they’re available. Pigeons prefer to eat seed or the disgusting loaves of soggy bread people throw on the sidewalk.

Other crabs in my neighborhood have only a few apples on them in late February, so the birds must eat them at least on occasion. But the Wax Trax crab is loaded with literally thousands of apples. Clearly the birds aren’t doing their part here.

When simple ecological sense fails to provide clear answers, it’s usually a good bet that the human influence is at work. The value of the crab apple genus as a food source prompted the Romans to begin meddling with it 2,000 years ago. All crab apples are members of the Malus genus, the same one regular apple trees belong to. The difference between the two is in the size and palatability of the fruit. If it’s greater than two inches in diameter and tastes reasonably good, it’s called an apple. Crab apples are smaller than two inches and have an “unpleasant, bitter, or crabbed taste,” according to Arie F. den Boer, a crab apple fancier who wrote one of the definitive texts on the subject in 1959.

Yet all members of the Malus genus are edible. Eating a crab apple won’t kill you or even make you sick, though some species taste so bad you’d never willingly chew one. There are even recipes for the small fruits.

The Romans were the first to create a way to reliably reproduce the biggest and best apples. Left to their own devices, apple trees hybridize wildly. As a consequence, planting the seeds of a prized apple tree won’t guarantee another generation of good apples. As an alternative, the Romans figured out how to graft branches of the best apple producers onto other trees, which would then produce apples identical to the parent plant.

In grafting, the tip of the plant that’s to be used as the rootstock is carved into a blunt cone. Then the growing end of a branch of another plant–your favorite apple tree, for example–is cut off and a V-shaped opening is carved into its base. The V is pressed down onto the cone so that the two plants’ thin reproductive layers are jammed tightly against each other. Then they’re waxed and tied together.

Rootstock generally is grown from the seed of trees that are particularly vigorous. The only trait it contributes to the species grafted on is that vigor– healthiness, an ability to withstand drought, a strong root system that anchors the tree securely. It doesn’t matter what the apples of the rootstock taste like because it isn’t supposed to produce fruit. It simply nourishes and supports the scion. The plant that grows from the graft is a clone, blooming and producing apples identical to those of the tree the scion was cut from.

You might have seen a filmstrip in junior high that showed an apple tree with a single branch of fruit totally different from all the others on the tree. But most nursery trees are grafted while the rootstock is young so that every branch, flower, and fruit displays traits identical to that of the scion. All the apple and crab apple trees now sold to the public by nurseries are the products of this type of asexual reproduction. (It’s curious how diligently humans eliminate the act of sex in nearly every species we domesticate: fruit trees, lawns, our pets, seedless cucumbers, and on and on. Nobody but us gets to have sex if we can help it.)

Grafting was used almost exclusively on food-producing trees until the 19th century, when civilization could afford the time to concern itself with ornamental plants. In the mid-1800s seeds, fruits, and dormant stock from Asian crab apple species were sent to England. Later in the century the Arnold Arboretum in Boston sent its own expedition to China and Japan to gather material. “Europe seems to have been poorly endowed with members of the apple family,” wrote Arie den Boer. But Asia was extraordinarily blessed. China alone has 35 to 40 naturally occurring species of Malus.

This wealth of genetic material gave rise in the 20th century to a fanatical group of crab apple lovers and breeders, who let the species hybridize like crazy and then grafted the results they liked to keep them true. Unfortunately most of the more than 900 taxa of crab apples that exist today were developed solely with an eye toward improving their floral display. No one was selecting for disease resistance, and the trees grew frail and fussy. They became the antithesis of the tough thorny trees found in the wild. As a consequence, for every person who loves crab apples there’s now someone who professes to detest them.

In the last decade or so crab apple fanciers have improved the stock by weeding out the frail species and by hybridizing cherished species with hardier ones. These new cultivars resist fire blight, cedar-apple rust, and other common crab diseases.

But according to Tom Green, executive director of the International Ornamental Crabapple Society, which was founded in 1985, there’s another reason people tend to hate crab apples. The apples that drop on sidewalks and driveways in late summer are soon squished, producing a slick rotting mess that yellow jackets love. The city of Chicago, worried about the liability it might assume if someone were to slip on the fruit and be injured, won’t plant crab apples as street trees, though the forestry bureau does use them in parks.

In response, nurseries have also been developing crab apple taxa that tend to hang on to their fruit. The Wax Trax tree is one of these. So are the other crabs in the neighborhood that still have a few apples on them. The difference between the Wax Trax tree and the others is probably the taste of the fruit. Crab apples vary greatly in flavor just as regular apples do. While developing trees that keep their fruit for longer periods of time, crab apple producers have inadvertently increased the bitterness of the fruit. After spending centuries trying to create more flavorful Malus taxa, we’re now creating ones that are so bitter even tough Chicago birds won’t touch them.

In any kind of natural ecosystem the unpalatability of the Wax Trax tree would ensure the death of its genes. And Bucktown is hardly a natural ecosystem. Even if birds ate its apples and excreted the seeds in someone’s yard, any seedlings produced would just be mowed down or weeded out. This tree’s only realistic chance of reproducing would be people’s liking it so much that they took grafts from it. So I guess that in an urban ecosystem ruled by sexlessness and liability this tree may be adapted for genetic survival after all.