Cabbage, broccoli, kale and cauliflower are all one species wearing different outfits
The mustard family gives us cabbage, kale, cauliflower, broccoli and collards, and all of them are cultivated forms of the single species Brassica oleracea. The family's sharp bite is chemical armour: when the plant is damaged, enzymes turn stored compounds into substances toxic to many attackers.
Brassicaceae, also called crucifers after the cross formed by their four petals, number over 350 genera. Beyond the cabbage clan, the family includes turnips and Chinese cabbage, rapeseed, radish, horseradish, the garden flower stock, and thale cress, Arabidopsis thaliana, one of biology's most studied lab plants. Most members are herbs, a few are shrubs, and a handful, such as water awlwort, live underwater in fresh water.
The flowers follow a recognisable plan: four sepals, four petals alternating with them, and usually six stamens, two short outer ones and four taller inner ones. The fruit is a capsule divided lengthwise by a thin false wall, with seeds lined up in rows. Botanists call it a silique when it is at least three times longer than wide and a silicle when it is stubbier. That internal wall is one of the traits separating crucifers from their closest relatives, the Cleomaceae, from which they split around 41 million years ago.
The defence system is the family's signature. Plants store dozens of glucosinolates alongside enzymes called myrosinases; when tissue is crushed, the two meet and produce isothiocyanates and related compounds that deter many herbivores. Some insects have beaten it. The cabbage white butterfly and its relatives are notorious crop pests, and the cabbage looper moth is increasingly troublesome because it resists common controls. Invasive garlic mustard in the United States can poison the larvae of native butterflies that rely on local mustards.
Genomes in the family are small by flowering-plant standards, from about 150 million base pairs in thale cress to 2,375 million in Bunias orientalis, though duplication has left some species with up to 256 chromosomes. Carl Linnaeus already treated the group as natural in 1753. Nearly all members use the standard C3 photosynthesis; a few Moricandia species run a hybrid system that works better in heat and drought.
Source: Brassicaceae
