When Cabbages Wear Disguises

What do the following all have in common: cabbage, broccoli, cauliflower, kale, collard greens, Brussels sprouts, kohlrabi, and savoy? Yes, they are all vegetables — but what else? You might have guessed that they are all in the same botanical family, which is true, but they are much more closely related than that. The surprising answer is that all of these vegetables are varieties of a single species of plant. Just as humans domesticated the wolf, ultimately producing an astounding variety of dogs, humans long ago domesticated the humble wild cabbage and subsequently created an amazing selection of distinct vegetables.

The original wild ancestor of all these vegetables, Brassica oleracea, still grows wild in Europe. It’s a tough, hardy plant, capable of withstanding harsh conditions. It is typically found growing on seaside limestone cliffs, where most other species of plants find it hard to survive. It’s not that wild cabbage actually prefers such difficult conditions. It is perfectly happy to grow in rich deep soil, such as that found in a garden. Its problem is that it does not tolerate competition — situations where other plants can crowd it out. By growing where other plants cannot survive, it has carved out a successful niche in the wild. But as a garden plant, the wily cabbage has enlisted humans to control the weeds, thereby eliminating the competition.

Suppose you wanted to see an original wild cabbage for yourself. The best place to look would be a limestone cliff overlooking the ocean on the coast of Great Britain, France, or northern Spain. However, the wild plant does not look much like garden cabbage, for the simple reason that the leaves don’t curl into a tight head. Instead, wild cabbage looks more like a variety of kale. The species has been cultivated in Europe for at least 2,000 years, and probably much longer. During this time a large number of cultivars (cultivated varieties) have been developed.

The fascinating thing about these cultivars of Brassica oleracea is that many of them look so very different from each other. The reason for this variation is fairly simple. As people grew these plants year after year in their vegetable gardens, they selected the best specimens to reproduce and grow again the next year. Over time this resulted in dramatic enhancements of the favored attributes. However, all of the above-ground parts of this species are edible, and therefore there are many choices regarding what parts of the plant to enhance through selective reproduction. In different localities across Europe, people selected different parts of the plant for gradual improvement:

1) Large, loose, abundant leaves

The oldest group of cultivars includes kale and collard greens. Plants in this group bear the greatest resemblance to wild cabbage, except that these cultivars tend to have much larger and more plentiful leaves. In addition to being useful garden vegetables in their own right, these crops also served as ancestors to most of the other cultivars listed below.

2) A large, terminal “head” of leaves

In this group of cultivars, the terminal bud (the growing point at the end of the main stem) has become a large round “head” of tightly packed, curved leaves. Members of this group include cabbage, savoy cabbage, and red cabbage. Cabbage was under cultivation in Germany at least as far back as the 1100s, nearly a thousand years ago.

3) Dense heads of flower buds

Two different groups of cultivars were developed to have compact heads of flower buds. In these varieties the dense bud-heads are harvested and eaten. (Caution: Be careful when using the phrase “dense bud-head” in polite company.) One of these groups includes broccoli, and the other group includes cauliflower and similar vegetables.

4) Large, prolific lateral buds

In certain cultivars, the lateral buds are the focus of interest. All along the main stem of the plant, these lateral buds grow into dense little heads, like tiny cabbages. From this description, you can probably guess that we are talking about Brussels sprouts. True to the name, these cultivars were originally developed in Belgium.

5) The central stalk

In still other cultivars, the central stalk of the plant is the part that is eaten. Over generations of selection by growers, this central stalk has become large and round, looking much like a turnip — except that is grows above the ground, instead of below. The common name for these cultivars is kohlrabi, and they first appeared in Europe in the 1400s.

6) The entire above-ground plant, as a tender shoot

In this final category of cultivars, the entire above-ground shoot is harvested for eating while it is still tender. This shoot includes the leaves, the flower buds, and the central stalk — no waste! There are two distinct groups that fit this category. One group is called “sprouting broccoli”, and is popular in southern Europe. The other group includes gai lan (also called Chinese broccoli), along with other varieties popular in China, Vietnam, and Thailand.

The wild cabbage is not the only plant in the genus Brassica that has been domesticated as a food crop. In fact, at least a half dozen species have been domesticated, several of which are called “mustards”. Two of the most important species are Brassica rapa and Brassica napus.

The species Brassica rapa, in its wild form, goes by many names, including “field mustard”. Although native to Eurasia, it has spread to the rest of the world, and can be found growing wild in every U.S. state and all over Canada. A field that has become overrun with field mustard can look quite pretty in the early spring, as the profusion of small 4-petaled flowers turns the landscape a lemon-yellow.

Field mustard bears a strong family resemblance to wild cabbage, and like its sister species, it has been domesticated as a food plant for a very long time. In Europe and North America, the best known group of cultivars is the turnip. These cultivars have been bred to have a large round root, which is harvested as a vegetable. But turnip greens are also a popular vegetable, especially in parts of the southeastern U.S. A second group of cultivars is popular in southern Europe, and is best known as “rapini”.

However, it is in Asia where this species has developed into the widest variety of distinct vegetables. In the U.S., the most common of these Asian varieties are bok choy and napa cabbage. (Other examples include tatsoi and certain kinds of mizuna.) In bok choy the bases of the leaves form a tight cluster, but otherwise the leaves are loose. But in napa, the leaves form a tight head of curled leaves, very similar to cabbage. And yet napa is not the same species as cabbage — it is the same species as turnip. Keep in mind that the wild ancestors of both species produce loose, individual leaves, without forming heads. In both species, selection by centuries of growers has resulted in varieties that form dense heads. This is not too surprising, because humans appreciate the convenience of such a solid, dense source of food. We have also done the same thing with lettuce, which is a completely unrelated species.

Even though wild cabbage (Brassica oleracea) and field mustard (Brassica rapa) are closely related, there is a barrier that makes it hard for these two species to cross. One of them has 9 pairs of chromosomes, while the other has 10 pairs of chromosomes. (Each chromosome contains many genes.) So if one parent contributes 9 chromosomes, and the other contributes 10 chromosomes, then it is impossible for the donated chromosomes to form a matched set of pairs. Therefore a normal cross between these two species does not produce viable seeds. However, many kinds of plants — including our Brassica friends — have found an intriguing workaround for this issue. Once in a while a parent will contribute not just one set of chromosomes, but a paired set of chromosomes. This usually doesn’t work. But if both parents contribute a double set, then all the chromosomes in the resulting embryo can line up in neat pairs.

This process, called polyploidy, occasionally happens in the wild, yielding a cross between Brassica oleracea and Brassica rapa. The resulting offspring have 19 pairs of chromosomes — 9 pairs from one parent, and 10 pairs from the other. These crosses have been frequent enough to launch a brand new species, Brassica napus, with 19 pairs of chromosomes. Note that this species, commonly called “rape” or “rapeseed”, arose on its own in the wild, without direct assistance from humans, and continues to grow wild in Europe.

This natural cross provided yet another tasty species of Brassica for humans to domesticate. The seeds of domesticated Brassica napus are the principal source of canola oil. Another variety of Brassica napus has been bred to have a large turnip-like root. This variety goes by the name rutabaga in parts of the English-speaking world, and by the name “swede” in other English-speaking areas. Still other varieties of Brassica napus are grown for their greens, which bear a strong resemblance to kale, and are sometimes called “Russian kale”.

Brassica napus is not the only species of Brassica to have a high oil content in the seeds. Oil can also be commercially expressed from Brassica rapa and Brassica juncea. Furthermore, the seeds of several species of Brassica are used for making mustard. In Europe and Asia, the seeds of Black Mustard (Brassica nigra) are typically used for making mustard. However, until less than a century ago, Europeans mostly used the seeds of Indian Mustard (Brassica juncea) to prepare mustard. Several varieties of Indian Mustard are also grown as leafy greens, primarily in Asia.

Most of the mustard sold in North America is made from a closely related species called Sinapis alba. This plant was once classified as another species of Brassica, but now it has been put into a different genus. This species yields a milder flavored mustard than the other two species, without the intense bite. For people who enjoy the sharp bite, an alternative to mustard is horseradish sauce, prepared from another related species, Armoracia rusticana. Most Japanese wasabi is made from horseradish, although the original wasabi was made from still another related species, Eutrema japonicum.

These last three species are all in the mustard family, but are not so closely related to be part of the genus Brassica. All botanical families are named after a genus in the family, and thus the mustard family was given the formal name of Brassicaceae. At one time this family was called Cruciferae, and today we still refer to foods from this family as “cruciferous vegetables”. The word Cruciferae is derived from the Latin word for cross, and refers to the four evenly spaced petals in the flowers.

Not all species of Brassicaceae serve as food plants, but a surprising number are indeed eaten by humans. Two others worth mentioning are radish and arugula. Radishes, including the Asian “daikon”, are familiar root vegetables bred from the species Raphanus raphanistrum. Arugula, called “rocket” in many English-speaking countries, is a leafy salad green derived from the species Eruca sativa.

We’ve now mentioned quite a long list of food plants, all in the mustard family (Brassicaceae). So let’s summarize these foods, grouped by species:

  • Brassica oleracea = cabbage, broccoli, cauliflower, kale, collard greens, Brussels sprouts, kohlrabi, savoy, gai lan
  • Brassica rapa = turnip, bok choy, napa cabbage, tatsoi, rapini, mizuna (some types), oil
  • Brassica napus = canola oil, rutabaga (swede), Russian kale
  • Brassica nigra = mustard sauce (European style)
  • Brassica juncacea = spicy brown mustard sauce, mizuna (some types), mustard greens, mustard oil
  • Sinapis alba = mustard sauce (North America style)
  • Armoracia rusticana = horseradish, wasabi (usually)
  • Eutrema japonicum = wasabi (originally)
  • Raphanus raphanistrum = radish, daikon
  • Eruca sativa = arugula (rocket)

So if you would like to include more cruciferous vegetables in your diet, you certainly have lots of options to choose from!

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