Valentine’s Day, just past, might just as well be called Rose Day, so popular is that particular flower that day. But few people reflect, as they give or receive these beautiful blooms, on the science associated with them. Allow me to rectify that oversight.

The term “rose” is applied to flowering plants that are members of the family Rosacae and the genus Rosa–about 150 species in all, which grow all over the northern hemisphere, as far north as the Arctic and as far south as the tropics (at high elevations).

Rose shrubs can be erect, climbing, or trailing, and their stems are covered with thorns (which keep animals from eating them). The ovary of the flower is known as the “rose hip”; from its rim grows five sepals, which alternate with five petals. This arrangement is common to all roses, from Alberta’s famous wild rose to the most elaborate hot-house bloom, but in many types of the latter, the stamens in the middle of the flower have become petal-like, too, creating a double flower.

Most roses also have a distinct fragrance, although many modern rose cultivars, bred primarily for appearance, have almost none. (In recent years, rose societies have restored fragrance as one of the important characteristics of a prize-winning rose, so that may be changing.) Although we think of “rose” as a particular fragrance, there are roses that don’t have that smell. Some, in fact, downright stink–“like sewer gas,” as one disappointed breeder said of a bloom he had to discard. The scent, intended to attract pollinating insects, not humans, is the result of a complex interaction of alcohols, sugars and enzymes and is still far from completely understood.

That scent begins in the chloroplasts, the structures in the green parts of roses that use chlorophyll to harness the energy of sunlight to turn carbon dioxide and water into food and other products the plant needs. The scent’s basic compounds, in a non-fragrant form called glucosides, are transported to the petals. There, an enzyme turns the glucosides into glucose and alcohol. Tiny cones on the inside of the petals, called paillae, expose these alcohols to the air, where they combine with oxygen to form aldehydes, which are 200 times as fragrant as the original alcohols–and don’t smell the same, either. That’s why the scent of a rose changes with the weather: on a cold, cloudy day, there’s very little fragrance, while on a hot, dry day, the fragrance is strong but short-lived.

The fragrance of roses and their appearance have made them a favorite of humans for all of history. The Egyptians cultivated rose gardens almost 6,000 years ago, and it was the Chinese, a few centuries later, who began the process of breeding them. Chinese breeders created roses that bloomed several times a year, but they weren’t as fragrant as Western roses.

Fragrance was much more important to the Romans, who decorated their houses with roses, threw rose petals around for decoration, and even laced their baths with rose water. Islam, too, prized roses.

Roses from West and East eventually mingled; Western gardeners were particularly taken with the Chinese roses’ ability to bloom several times a year. Extensive cross-breeding of China roses with European roses resulted, in the early 19th century, in an explosion of new varieties: 4,000 or more types of what were called “hybrid perpetuals,” combining Western hardiness with Chinese frequency of blooming.

Later in the century, these hybrid perpetuals were crossed with “tea-scented Chinas,” which were ever-blooming, elegant, but not very hardy. The end result was the “hybrid teas,” which combined the teas’ elegance and distinctive odor (which really isn’t anything like tea) with the hardiness of the hybrid perpetuals.

The hybrid teas have dominated rose breeding ever since. To date, there are more than 11,000 varieties of hybrid teas, and in all, there are more than 20,000 varieties of roses, with hundreds more introduced every year. A popular new variety of rose can be worth a million dollars to its creator.

More than 20 million roses are sold as cut flowers and twice that many as garden flowers in North America every year. Our civilization, it appears, is every bit as rose-obsessed as the Romans. Roses are the living emblem of romance, an observation concerning which I believe I’ll let writer Dorothy Parker have the last word (as she almost always did):

Why is it no one ever sent me yet
One perfect limousine, do you suppose?
Ah no, it’s always just my luck to get
One perfect rose.

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