Brassicaceae (Cruciferae)
Brassicaceae, commonly known as the mustard, cabbage, or cress family (and alternatively named Cruciferae), is a large, economically important family of eudicots in the order Brassicales (Rosid clade). With about 370 genera and over 4,000 species, it consists predominantly of herbaceous plants. The family is readily recognized by its characteristic flowers with 4 petals arranged in a cross (cruciform), 6 stamens (4 long, 2 short - tetradynamous), and distinctive fruit types called siliques or silicles. Most members possess pungent compounds (glucosinolates) giving them a characteristic sharp taste.
Overview
The Brassicaceae family is cosmopolitan, with its greatest diversity centered in temperate regions of the Northern Hemisphere, particularly the Mediterranean basin and southwestern/central Asia. It holds enormous economic significance for humans. Countless cultivars derived from Brassica oleracea provide staple vegetables including cabbage, kale, broccoli, cauliflower, Brussels sprouts, and kohlrabi. Other species yield turnips and Asian greens (Brassica rapa), radishes (Raphanus sativus), and important oilseed crops like canola/rapeseed (Brassica napus, Brassica juncea, Brassica rapa). Condiments like mustard are derived from the seeds of Brassica and Sinapis species. Additionally, many genera like Alyssum (sweet alyssum), Iberis (candytuft), and Lunaria (honesty/money plant) are popular garden ornamentals.
A defining feature of the family is its unique sulfur-containing secondary metabolites called glucosinolates (mustard oil glycosides). When plant tissues are damaged (e.g., by chewing), the enzyme myrosinase mixes with glucosinolates, hydrolyzing them to produce volatile and pungent isothiocyanates (mustard oils), thiocyanates, and nitriles. These compounds provide defense against herbivores and pathogens and are responsible for the characteristic sharp or peppery taste associated with mustard, horseradish, radishes, arugula, and other family members.
Many Brassicaceae species are common wildflowers, especially in spring, or occur as weeds of disturbed ground, lawns, and agricultural fields. In Oklahoma (current location as of March 27, 2025), numerous native and introduced species are found, including various bittercresses (Cardamine), shepherd's purse (Capsella), pepperweeds (Lepidium), and whitlow-grasses (Draba), often among the earliest plants to flower in spring.
Quick Facts
- Scientific Name: Brassicaceae Burnett (alt. Cruciferae Juss.)
- Common Name: Mustard family, Cabbage family, Cress family
- Number of Genera: Approximately 330-370
- Number of Species: Approximately 3,700-4,100
- Distribution: Cosmopolitan, especially diverse in Northern Hemisphere temperate regions.
- Evolutionary Group: Eudicots - Rosids (Malvids)
- Order: Brassicales
Key Characteristics
Growth Form and Habit
Predominantly annual, biennial, or perennial herbs, rarely subshrubs or small shrubs. Often possess a taproot. Plants are frequently glabrous (smooth) or covered with simple, branched, or star-shaped (stellate) hairs. Contain watery sap, often pungent or peppery due to glucosinolates.
Leaves
Leaves are usually arranged alternately along the stem, though basal leaves often form a rosette which may wither before flowering. Leaves are typically simple, but often pinnately lobed, dissected, or sometimes compound. Margins vary from entire to toothed or lobed. Stipules are absent (exstipulate).
Inflorescence
The characteristic inflorescence is an indeterminate raceme (flowers opening from bottom to top), which often lacks bracts subtending the individual flower pedicels. The raceme frequently elongates significantly as the fruits develop. Sometimes flowers are arranged in corymbs (flat-topped racemes where lower pedicels are longer) or panicles.
Typical Brassicaceae raceme, often bractless and elongating as siliques/silicles develop below the apical flowers.
Flowers
Flowers are typically bisexual, actinomorphic (radially symmetric), and hypogynous (parts attached below the superior ovary). The perianth structure is highly characteristic:
- Calyx: Composed of 4 distinct (free) sepals, typically arranged in two whorls of two (outer and inner pair). The inner pair is sometimes slightly pouched (saccate) at the base, often associated with nectar glands. Sepals are usually erect.
- Corolla: Composed of 4 distinct (free) petals, arranged diagonally to form a cross shape – hence the alternative family name Cruciferae (cross-bearing). Petals are typically clawed (having a narrow basal stalk) and limbate (with a broader, spreading upper blade). Colors are most commonly white or yellow, but also pink, purple, or orange.
- Androecium: Stamens characteristically number 6 and are tetradynamous: 4 inner stamens are long, and the 2 outer stamens (associated with the outer sepals) are shorter. Nectar glands are usually present at the base of the stamen filaments (often surrounding the bases of the short stamens).
- Gynoecium: Ovary is superior, typically composed of 2 fused carpels. It usually forms 2 locules (chambers) separated by a thin, papery false partition called the replum, which connects the two parietal placentas where the ovules are attached. A single style (short or long) terminates in a stigma that is usually capitate (head-like) or 2-lobed.
Characteristic Brassicaceae flower showing 4 cruciform petals, 6 tetradynamous stamens (4 long, 2 short), and superior ovary.
Fruits and Seeds
The fruit is a specialized type of dry capsule derived from the 2-carpellate ovary with its replum, typically dehiscing by two valves peeling away from the persistent replum, leaving the seeds attached to its margins. There are two main types based on shape:
- Silique: Elongated capsule, typically more than 3 times as long as it is wide (e.g., mustards, wallflowers, toothworts).
- Silicle: Short and broad capsule, less than 3 times as long as it is wide. Silicles can be flattened parallel to the replum (latiseptate, e.g., Alyssum, Lunaria) or perpendicular to the replum (angustiseptate, e.g., Capsella, Lepidium), or sometimes globose or inflated.
Seeds are usually numerous, arranged in 1 or 2 rows in each locule. They lack endosperm or have very little; the embryo is often large and contains oils.
Silique: elongated capsule opening by 2 valves, leaving seeds on the replum.
Silicle: short capsule, can be flattened parallel (left) or perpendicular (right) to replum.
Chemical Characteristics
The defining chemical feature is the presence of glucosinolates (mustard oil glycosides). When tissues are crushed, the enzyme myrosinase (stored separately) hydrolyzes these compounds into pungent and volatile isothiocyanates (mustard oils), thiocyanates, and nitriles. This "mustard oil bomb" serves as a defense against herbivores and pathogens and gives the family its characteristic sharp flavor. Seeds are often rich in fatty oils (e.g., canola oil).
Identification Basics
Brassicaceae is usually readily identifiable by its unique combination of floral and fruit characteristics, often accompanied by a pungent taste or smell.
Primary Identification Features
- 4 Cruciform Petals: Four distinct petals arranged in a cross shape.
- 6 Tetradynamous Stamens: Six stamens, four long and two short.
- Superior Ovary with Replum: Ovary typically 2-carpellate with a false partition (replum) visible in fruit.
- Silique or Silicle Fruit: Dry capsule (long or short) opening by 2 valves separating from the replum.
- Herbaceous Habit (Mostly): Predominantly non-woody herbs.
- Pungent Sap/Taste: Often a sharp, peppery taste or smell when leaves or stems are crushed (due to glucosinolates).
- Alternate Leaves (Often): Leaves usually alternate, often lobed or dissected, frequently forming a basal rosette.
Secondary Identification Features
- Racemes Usually Lacking Bracts: Individual flowers in the main part of the raceme typically lack subtending bracts.
- Silicle Shape/Flattening: Details of short fruit shape (round, heart-shaped, winged, flattened parallel or perpendicular to replum).
- Hair Type: Presence and type of hairs (simple, branched, stellate) on leaves/stems.
- Basal Rosette: Presence and shape of basal leaves.
Seasonal Identification Tips
- Spring: Peak flowering season for many native wildflowers and weeds in temperate regions like Oklahoma.
- Summer/Fall: Fruits (siliques/silicles) develop and persist, often providing the best characters for genus/species identification. Some species flower later in the season.
- Winter: Basal rosettes of biennials or perennials are often visible. Dried fruit stalks may persist.
- *(Current Time Context: Late March in Oklahoma - Many weedy mustards like Capsella and Cardamine hirsuta, and native Cardamine concatenata or Draba species, are likely flowering or setting fruit).
Common Confusion Points
- Papaveraceae (Poppy Family): Also often herbaceous with alternate dissected leaves and capsule fruit. Key differences: Usually has milky/colored latex; flowers typically have 2-3 caducous sepals, 4 or 6 petals (not strictly cruciform), numerous stamens (not tetradynamous); capsule different (e.g., poricidal).
- Ranunculaceae (Buttercup Family): Some herbaceous members have alternate dissected leaves, superior ovaries, 4 petals/sepals. Key differences: Lack pungent chemistry; flowers often have numerous simple pistils (not syncarpous ovary with replum); fruit often achenes or follicles; stamens usually numerous.
- Onagraceae (Evening Primrose Family): Flowers often have 4 petals and may be yellow or white. Key differences: Ovary is inferior; stamens usually 8 (not tetradynamous).
- Some Caryophyllaceae/Apiaceae/etc. with white/yellow flowers: Check for the combination of 4 distinct cruciform petals AND 6 tetradynamous stamens AND the characteristic silique/silicle fruit.
The combination of 4 petals + 6 tetradynamous stamens + silique/silicle fruit is almost exclusively diagnostic for Brassicaceae.
Field Guide Quick Reference (Brassicaceae)
Look For:
- Herbaceous (mostly)
- Flowers with 4 distinct petals (cruciform)
- Stamens 6 (4 long + 2 short = tetradynamous)
- Ovary superior, usually 2-carpellate
- Fruit a silique (long) or silicle (short) with replum
- Leaves usually alternate, often lobed/rosette
- Often pungent taste/smell
Key Variations:
- Flower color (white, yellow, pink, purple)
- Fruit shape (long/short, flat/round, winged)
- Leaf dissection / Basal rosette
- Hair type (simple, branched, stellate)
- Annual, biennial, or perennial
Notable Examples
The Mustard family provides many essential vegetables, oils, condiments, and includes common weeds and wildflowers.
Brassica oleracea
Wild Cabbage & Cultivars
Native to coastal southern and western Europe. Wild ancestor of an incredible variety of cultivars selected for different parts: leaves (kale, collards), terminal bud (cabbage), lateral buds (Brussels sprouts), flower buds (broccoli, cauliflower), stem (kohlrabi). Flowers typically yellow.
Arabidopsis thaliana
Thale Cress
A small, inconspicuous annual weed native to Eurasia and Africa, now cosmopolitan. Forms a basal rosette. Flowers tiny, white. While seemingly insignificant, it is crucially important as a model organism in plant biology and genetics due to its small genome, rapid life cycle, and ease of cultivation.
Capsella bursa-pastoris
Shepherd's Purse
A common annual weed found worldwide in disturbed areas, lawns, and fields (very common in OK). Forms a basal rosette of variable, often lobed leaves. Produces racemes of tiny white flowers followed by highly distinctive, triangular or heart-shaped, flattened silicles (flattened perpendicular to replum).
Cardamine concatenata
Cutleaf Toothwort
A native perennial spring ephemeral wildflower found in deciduous woodlands of eastern and central North America, including eastern Oklahoma. Arises from a segmented rhizome. Features palmately divided stem leaves (usually 3) and racemes of white or pale pink flowers. Fruit is a slender silique. (Cardamine hirsuta, Hairy Bittercress, is a common non-native weed).
Raphanus sativus
Radish
An annual or biennial herb cultivated globally for its fleshy taproot. Native origin uncertain, possibly Asia. Basal leaves often large and lyrate-pinnatifid (lobed with large terminal lobe). Flowers typically pinkish-purple or white. Fruit is a peculiar indehiscent or irregularly breaking silique, often pithy or corky, not splitting cleanly like typical siliques.
Sinapis alba
White Mustard / Yellow Mustard
An annual herb native to the Mediterranean region, cultivated for its seeds used to make table mustard (particularly yellow mustard) and as an oilseed crop or cover crop. Features pinnately lobed leaves and racemes of bright yellow flowers. Fruit is a bristly silique with a prominent, flattened beak.
Phylogeny and Classification
Brassicaceae belongs to the order Brassicales, situated within the Malvid clade of Rosids (Core Eudicots). This order is chemically distinct, characterized by the production of glucosinolates (mustard oil glycosides) – a feature shared with related families like Cleomaceae and Capparaceae. Brassicaceae represents the largest and most economically important family within this order.
The family itself is strongly monophyletic, well-defined by its unique floral structure (4 cruciform petals, 6 tetradynamous stamens) and fruit types (silique/silicle with replum). Relationships within the family are complex and subject to ongoing research, but several major tribes and lineages are recognized, often correlating with variations in fruit morphology, embryo structure, and chromosome numbers. The evolution of glucosinolate diversity is thought to be linked to co-evolutionary arms races with herbivores and pathogens.
Position in Plant Phylogeny
- Kingdom: Plantae
- Clade: Angiosperms (Flowering plants)
- Clade: Eudicots
- Clade: Rosids
- Clade: Malvids
- Order: Brassicales
- Family: Brassicaceae (alt. Cruciferae)
Evolutionary Significance
Brassicaceae is a highly successful and well-defined family, demonstrating rapid diversification possibly linked to its characteristic chemical defense system (glucosinolates). The unique floral structure (cruciform petals, tetradynamous stamens) is remarkably consistent across the family. The evolution of diverse fruit morphologies (siliques and various silicle shapes) reflects adaptations for different seed dispersal strategies. The family's immense economic importance through artificial selection, particularly within the genus Brassica, highlights the evolutionary potential captured during domestication. Furthermore, Arabidopsis thaliana serves as a cornerstone model organism for plant genetics, development, and molecular biology research worldwide.
