Brassicaceae

Based on Wikipedia: Brassicaceae

There's a family of plants so influential that it shaped both human civilization and the evolution of butterflies. Its members have fed armies, sparked scientific revolutions, and waged an invisible chemical war against insects for millions of years. You've almost certainly eaten one of its children today.

This is the story of the Brassicaceae, better known as the mustard family—or, if you prefer the older Latin, the Cruciferae, meaning "cross-bearers." That poetic name describes their flowers: four petals arranged like a tiny cross.

A Family Portrait

The mustard family is enormous. As of late 2025, botanists recognize over 350 genera and more than 4,300 species. The largest genus alone, Draba, contains 440 species. To put that in perspective, there are roughly 5,400 mammal species on Earth. This single plant family rivals the entire class Mammalia in sheer variety.

Most of these plants are humble herbs—the kinds of green things you might walk past without noticing. Some are shrubs. A handful are vines. One particularly adventurous species, water awlwort, lives completely submerged in fresh water, having abandoned land entirely.

What unites them? A distinctive body plan. Their flowers always have four petals and four sepals, arranged alternately like dancers at a country ball. Inside each flower sit six stamens—four long ones in the center, two shorter ones at the sides—a pattern so consistent that Carl Linnaeus, the father of modern taxonomy, used it to define the entire group back in 1753. He called them "Tetradynamia," meaning "four powers," referring to those four dominant stamens.

Their fruits are equally distinctive: long pods called siliques, or shorter ones called silicles, divided down the middle by a thin wall. When you've eaten a radish pod raw from the garden, you've encountered this architecture firsthand.

Your Refrigerator Is Full of Them

The mustard family punches far above its weight in human nutrition. Consider what you'd lose if it vanished: cabbage, kale, cauliflower, broccoli, Brussels sprouts, and collard greens. All of these are actually the same species—Brassica oleracea—bred into wildly different forms over millennia. It's as if we'd taken wolves and bred them into creatures as different as a Chihuahua and a Great Dane, except we did it with a single weedy European plant.

But wait, there's more. Turnips and Chinese cabbage come from Brassica rapa. Rapeseed, the source of canola oil, is Brassica napus. The common radish is Raphanus sativus. Horseradish—that sinus-clearing condiment—is Armoracia rusticana. Watercress, arugula, wasabi: all mustards.

The family also includes ornamental plants like stock (Matthiola), grown for its fragrant flowers, and some of the most noxious weeds on Earth, like garlic mustard (Alliaria petiolata), which has invaded North American forests with alarming success.

The Tiny Plant That Changed Science

Among all these edible and ornamental cousins sits one unassuming weed that transformed our understanding of life itself: Arabidopsis thaliana, commonly called thale cress or mouse-ear cress.

If you've ever walked past a disturbed patch of soil in Europe or western Asia, you've probably stepped on some. It's small, unprepossessing, and produces tiny white flowers that most people never notice. It's also the single most studied plant on Earth.

Why? Because it's the perfect laboratory organism. Its genome is remarkably compact—just 150 million base pairs, compared to the roughly 2,375 million base pairs in the related Bunias orientalis. That's a sixteenfold difference within the same family. Arabidopsis grows fast, from seed to seed in about six weeks. It self-pollinates easily. A single plant fits in a small pot and produces thousands of seeds.

When scientists needed a "model organism" for plant genetics—the way fruit flies and mice serve for animal genetics—Arabidopsis was the obvious choice. In 2000, it became the first plant to have its entire genome sequenced. Nearly every fundamental discovery in plant molecular biology over the past forty years has involved this humble weed.

Chemical Warfare in Your Salad

Ever wonder why mustard burns your tongue? Or why broccoli has a slightly bitter edge? That sensation is the plant fighting back.

The mustard family has evolved an elegant chemical defense system. Their cells contain two key ingredients, stored separately: glucosinolates (sulfur-containing compounds) and myrosinases (enzymes). When an insect—or a human—damages the plant's cells by chewing, these two ingredients mix. The myrosinases convert glucosinolates into isothiocyanates, thiocyanates, and nitriles.

These compounds are toxic to most animals, fungi, and bacteria. The system works like a biological bomb: harmless until triggered, then suddenly releasing chemical weapons.

Different mustard species produce different cocktails. An individual plant might manufacture more than fifty distinct glucosinolates. The energy cost is staggering—up to fifteen percent of everything the plant invests in growing a leaf goes toward making these defensive chemicals. Evolution clearly decided the expense was worth it.

This is why mustard tastes pungent. It's why wasabi clears your sinuses. You're experiencing a defense mechanism designed to make herbivores reconsider their life choices.

An Evolutionary Arms Race

But here's where it gets interesting. Some insects called the plants' bluff.

The Pieridae—a family of butterflies that includes the common white butterflies you see flitting around gardens—evolved countermeasures. One strategy involves glucosinolate sulfatase, an enzyme that modifies glucosinolates before they can be converted into toxins. Another rapidly breaks down the dangerous compounds into harmless nitriles. These butterflies essentially learned to disarm the bomb before it could explode.

The most familiar of these adapted butterflies is Pieris rapae, the small white or cabbage white. It's one of the most successful agricultural pests in the world, munching through cruciferous crops on every continent except Antarctica. Its caterpillars can devastate a cabbage patch in days.

The diamondback moth, Plutella xylostella, pulled off the same trick independently. It too converts isothiocyanates into less problematic nitriles. In recent years, some populations have even developed resistance to Bacillus thuringiensis toxin, a biological pesticide that farmers use against them. This moth is now one of the most difficult pests to control worldwide.

This back-and-forth—plants evolving new chemical defenses, insects evolving ways around them—has probably driven much of the diversity we see in both groups today. The mustard family and its butterfly nemeses have been locked in an evolutionary arms race for millions of years.

A Clever Trap

Farmers have discovered they can sometimes use this ancient war to their advantage. One species, Barbarea vulgaris (bittercress), produces not just glucosinolates but also triterpenoid saponins—a second line of chemical defense that the butterflies never evolved to counter.

The volatile glucosinolates in bittercress still attract egg-laying butterflies. The plant smells right; the chemical signature says "lay your eggs here." So the butterflies do. But when the caterpillars hatch and start eating, the saponins kill them.

Planting bittercress near crop brassicas creates a deadly bait. The butterflies are lured to their doom, leaving the actual harvest alone. It's biological pest control using the insects' own evolutionary history against them.

Where They Grow

The mustard family has conquered most of the planet. You can find its members across every continent except Antarctica. They thrive from sea level to mountaintops, from Mediterranean shores to Arctic tundra.

But they have a homeland. The family likely originated in the Irano-Turanian region—roughly, the area spanning from Turkey through Iran and into Central Asia. This zone still hosts about 900 species in 150 genera, with around 530 species found nowhere else on Earth. It's a mustard diversity hotspot.

The Mediterranean comes second, with about 630 species. North America hosts roughly 780 species, of which an impressive 600 are endemic—found only there. South America has 340 native species, Australia and New Zealand share 114 between them, and Southern Africa has over 100.

Some mustards have become remarkable specialists. Over a hundred species accumulate heavy metals, particularly zinc and nickel, to concentrations that would kill most plants. Several Alyssum species can store nickel at up to 0.3 percent of their dry weight. Scientists have explored using these hyperaccumulators for "phytoremediation"—cleaning contaminated soils by growing plants that soak up the pollution—or even "phytomining," extracting valuable metals by harvesting and processing the plants.

The Question of Relationships

Where does this family fit in the tree of life? The question has puzzled botanists for centuries.

Linnaeus recognized them as a natural group in 1753. Later taxonomists placed them variously in orders called Rhoeadales, Parietales, and eventually Capparales or Brassicales. Some thought they descended from poppy relatives. Others disagreed.

Modern DNA analysis has clarified much. The Brassicaceae's closest relatives are the Cleomaceae (spider flower family) and Capparaceae (caper family). All three share a common ancestor and are now placed together in the order Brassicales. The Cleomaceae and Brassicaceae split from each other roughly 41 million years ago—back when mammals were still recovering from the dinosaurs' extinction and the Himalayas were just beginning to rise.

You can tell the families apart without DNA testing. Brassicaceae have bisymmetrical flowers—mirror-imaged both left-to-right and top-to-bottom. Cleomaceae flowers are only bilaterally symmetrical, like a face: left mirrors right, but top differs from bottom. Capparaceae often have a gynophore, a stalk lifting the female flower parts above the rest. These differences trace back to that 41-million-year separation.

Pollination and Reproduction

Almost all mustards rely on insects for pollination. Their four-petaled flowers evolved to attract bees, flies, and butterflies. Many species have chemical mechanisms in their pollen that prevent self-fertilization, encouraging genetic diversity through cross-pollination.

But there are rebels. Cardamine chenopodifolia practices cleistogamy—it self-pollinates in flowers that never open, guaranteeing reproduction even if no pollinators arrive. At the other extreme, Pringlea antiscorbutica—the Kerguelen cabbage, native to windswept subantarctic islands—has given up on insects entirely and relies on wind pollination.

Garlic mustard, that invasive menace of North American forests, hedges its bets. It can cross-pollinate when pollinators are available but is perfectly capable of self-fertilizing when they're not. This flexibility helps explain its success as an invader.

Some mustards have evolved creative ways to spread their seeds. Several Cardamine species have explosive seed pods that catapult seeds several feet away when touched. Many produce seeds with sticky coats that hitch rides on passing animals. This may explain why some genera have achieved near-global distribution despite being rooted in place.

A few species don't bother with seeds at all. Cardamine bulbifera produces gemmae—tiny plant clones that drop off and grow into new individuals. Cardamine pentaphyllos has coral-like roots that break easily into segments, each capable of becoming a new plant. When you can reproduce without sex, you don't need to worry about finding a mate.

The Carbon Question

Plants capture carbon dioxide through photosynthesis, but they don't all do it the same way. Most plants, including almost all mustards, use what's called C3 carbon fixation—the original photosynthetic pathway that evolved billions of years ago.

C3 photosynthesis works well under moderate conditions but becomes inefficient in heat, drought, or low nitrogen. Some plants have evolved C4 carbon fixation, a more complex but more efficient system under those stressful conditions. Corn, sugarcane, and many grasses are C4 plants.

The mustard family is stubbornly C3. Almost every one of its 4,300-plus species uses the ancestral pathway. But a few Moricandia species have evolved something in between—a hybrid C3-C4 system. Scientists study these unusual mustards to understand how the transition from C3 to C4 occurs, hoping the knowledge might someday help breed more drought-tolerant crops.

The Cross-Bearers' Legacy

The Brassicaceae earned their old name, Cruciferae, from those four-petaled flowers shaped like crosses. Medieval Europeans saw religious symbolism in the pattern. Modern scientists see evolutionary constraints: the body plan works, so evolution keeps it.

Either way, this family has shaped human history in ways few plant groups can match. It fed ancient civilizations through their winters, when stored cabbages and turnips might be all that stood between a village and starvation. It provided the sharp flavors of mustard and wasabi that make food interesting. It gave molecular biologists their favorite research subject.

And in gardens and fields around the world, it continues its ancient war with butterflies—a chemical conflict older than the Himalayas, playing out invisibly in every bite of broccoli.

The next time you eat coleslaw or spread mustard on a sandwich, you're participating in an evolutionary drama forty million years in the making. The plants you're eating evolved those sharp flavors to punish insects. The fact that humans came to enjoy the punishment is one of evolution's stranger jokes.