The cabbage and mustard family
Brassicaceae feeds more of the world by weight of vegetable than any other family, and it does it with a single chemical trick that also explains every sharp taste in the family and the one thing florists complain about.
around 370 genera
roughly 4,000 species
Worldwide, richest around the Mediterranean and through central Asia
Everything in this guide that belongs to Brassicaceae
This family is unusually easy to recognize, which is why it kept its old name so long. Four petals, arranged in a cross. The older name was Cruciferae, the cross bearers, and it describes the flower better than the modern one does.
Behind the petals are six stamens, and they are not equal: four are long and two are short, an arrangement almost unique to this family. The fruit is the other giveaway. It is a pod that splits down two sides and leaves behind a papery partition standing in the middle like a frame with the glass gone. Long pods and short round ones are different words in botany but the same structure.
Anyone who has seen the silver pennies of honesty on a mantelpiece has handled that partition. The two outer walls have fallen away with the seeds and what is left, the translucent disc, is the divider from the middle of the pod.
Cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi and collards are not related species. They are one species, selected in different directions from the same wild sea cabbage of the Atlantic coast. Breeders leaned on the leaves and got cabbage and kale; on the flower buds and got broccoli and cauliflower; on the side buds and got Brussels sprouts; on the stem and got kohlrabi.
Nothing else has been reshaped that far. It is the standard demonstration of what selection can do to a single species given a few thousand years, and it is why they all cross with each other and why they all taste faintly of the same thing.
The family did not stop there. Turnip, rapeseed and canola, mustard, radish, horseradish, wasabi, watercress and arugula are all in here as well.
Every plant in this family carries a class of sulfur containing compounds called glucosinolates, held harmlessly in the tissue, alongside an enzyme kept carefully apart from them. Nothing happens while the plant is intact. The moment tissue is crushed, chewed or cut, the two meet and react, and the products are the sharp volatile compounds that define the family on the tongue.
That reaction is the heat of mustard, the sinus clearing lift of horseradish and wasabi, the bite of a radish and the pepperiness of arugula. It is a defense system, and it is armed by the act of being eaten.
It also explains the family's reputation in a bucket. Damaged brassica tissue standing in water goes on releasing sulfur compounds, and sulfur compounds are what the nose is most sensitive to of anything in nature. The smell is chemistry doing exactly what it evolved to do, in the wrong place. Every ornamental member of the family behaves this way and it is not a sign of a bad bunch.
A small, dull, hairy weed in this family called thale cress became the laboratory animal of plant biology. It is tiny, it completes a generation in about six weeks, it self pollinates, it produces thousands of seeds and it has one of the smallest genomes of any flowering plant. In 2000 it became the first plant to have its genome fully sequenced, and a very large share of what is known about how any plant works was learned from it first.
The ornamental side of the family is quieter but familiar: honesty for its silver discs, sweet alyssum spilling over an edge, wallflowers, aubrieta and candytuft. Almost all of them share the family's habit of flowering hard and early on poor ground, which is what a plant does when its strategy is to get in fast rather than to last.