Brassicales

▪ plant order

Introduction

 order of flowering plants that includes cabbages (cabbage) and capers, as well as mignonette, mustard, and nasturtiums (nasturtium). Brassicales includes 17 families, 398 genera, and 4,450 species. There are five family groups: Brassicaceae, Capparidaceae, and Cleomaceae; Akaniaceae and Tropaeolaceae; Caricaceae and Moringaceae; Bataceae, Salvadoraceae, and Koeberliniaceae; and Resedaceae, Gyrostemonaceae, Tovariaceae, and Pentadiplandraceae. Not clearly placed are Limnanthaceae, Setchellanthaceae, and Emblingiaceae.

      The order is very distinct anatomically, ultrastructurally, and chemically, as well as being recognizable easily in molecular comparisons. Indeed, the smell and taste of the plants in Brassicales result from the presence of glucosinolates—sulfur-containing compounds that are also known as mustard oils. These compounds are found in nearly every member of the order and can deter the depredations of everything from bacteria to mammals. However, these same compounds may attract other species. Butterflies of the genus Pieris and its relatives (cabbage whites and orange tips) are attracted to members of Brassicaceae in particular, and they can be very serious pests of cultivated Brassica (Brassicaceae). Indeed, Brassicaceae is often heavily attacked by dermestid beetles (dermestid beetle) and other herbarium pests, while the related Resedaceae, for example, is largely pest-free. Only one family outside the Brassicales order, Putranjivaceae of the order Malpighiales, is known to have glucosinolates. One family in Brassicales, Koeberliniaceae, does not have glucosinolates, though other evidence firmly places it in that order.

      Most members of Brassicales have racemose inflorescences (inflorescence), and the leaves have small stipules. The flowers often do not have the regular n sepals, n petals, 2n stamens, and n carpels arrangement so common in other Rosids. In many Brassicales, the nectary is found between the petals and the stamens, the usual position being between the stamens and the ovary. Green embryos are also common.

      Most of the members of Brassicales have long been recognized as being related, although some botanists are still inclined to place Akaniaceae with Sapindaceae, the two being superficially (though not chemically) similar. Limnanthaceae is sometimes associated with the Asterid I group, and Gyrostemonaceae has been placed with Caryophyllales. Families like Brassicaceae that have ovules borne on the walls of the ovary have often been linked to Papaveraceae (order Ranunculales) and other families with similar ovaries; however, the relationship is not at all close.

Brassicaceae, Capparaceae, and Cleomaceae

  Brassicaceae (often called Cruciferae), the mustard family, is by far the largest family in Brassicales, having 338 genera and 3,710 species found throughout the world. The family includes many common vegetable plants such as broccoli, cabbage, cauliflower, turnips (turnip), and radishes (radish), as well as gardening plants such as sweet alyssum, wallflower, and rock cress. Brassicaceae have flowers with four sepals and petals, and the stamens are typically about as long as the petals. The flowers are more or less zygomorphic, and the nectary is a flap to a ring outside the stamens. The ovary is borne on a long stalk, and there is only a short style. Within the ovary, the ovules are borne on the walls, and there are no partitions. True blue or red flowers are very rare in this whole group.

 Brassicaceae species are annual to perennial herbs found mostly in temperate and tropical montane areas; they are especially abundant in north temperate areas, although less so in eastern North America. The old generic differences, often based on fruit type, have proved unsatisfactory for delineating the family. Thus, Raphanus (the radish genus) and brassica (including broccoli and many other cruciferous vegetables) apparently have very different fruits. In the former, they split transversely into one-seeded segments, and in the latter they open in an ordinary fashion to release the individual seeds. Nonetheless, the two hybridize. Draba (a genus of whitlow grass; 365 species) grows in north temperate to boreal regions and down the Andes. Cardamine (200 species) is also temperate, and it also grows on African and New Guinean mountains. Erysimum (225 species, including Cheiranthus) and Alyssum (195 species) both grow from Europe to East Asia. Arabis ( rock cress; 120 species) and Thlaspi ( pennycress; 55 species) are both north temperate. Lepidium ( peppergrass; 230 species) and Rorippa ( marsh cress; 85 species) are more or less worldwide. Heliophila (75 species) is South African. Aethionema ( stonecress; 70 species) is largely Mediterranean. Matthiola ( stock; 50 species) grows in Macaronesia (islands in the North Atlantic, west of Gibraltar), around the Mediterranean, and in Western Europe.

      Members of Capparaceae, the caper family, are trees, shrubs, or lianas, sometimes herbs, that are usually found in the tropics. The family may contain up to 16 genera and 480 species, although some genera currently included may not belong there. Capparis (about 250 species) is pantropical but also grows in warm temperate areas. Boscia (37 species) is found in Africa and Arabia. Cadaba (30 species) is Old World, especially Africa. There are often many stamens, and the fruit is usually a berry.

      Members of Cleomaceae and Brassicaceae are mostly herbs. Their inflorescence is more or less flat-topped, elongating only after the open flowers have faded. The petals are typically narrowed strongly at the base. Six is the common number for the stamens. The ovary is made up of two parts, and when ripe the two sides fall off, leaving the thickened and hardened placental portion, or replum (the part where the seeds were attached) behind.

 Cleomaceae contains 10 genera and about 300 species, which grow in tropical to warm temperate regions, especially in the New World. Cleome (spiderflower) (including Podandrogyne) contains about 275 species that grow in tropical and warm temperate areas. There are quite often prickles on the stem, and these may be in the position of the stipules. The inflorescence often has very leafy bracts; both stamens and ovary may be borne on a stalk together; and the seeds are sometimes arillate.

 Species of Capparis and Cleome may be pollinated by bats, hawkmoths, and other large insects. Capparis spinosa produces capers, while several species of Cleome are ornamentals. Species of Crateva have several minor uses. Brassicaceae includes a number of ornamentals—for example, Matthiola incana (Brompton stock (stock)), Hesperis matronalis ( dame's rocket), and Erysimum cheiri (formerly Cheiranthus cheiri, the wallflower). Cress seedlings are often Lepidium sativum, Brassica nigra, and Sinapis alba, while watercress (of cucumber and watercress sandwiches) is usually Rorippa nasturtium-aquaticum. In nearly all cases it is the pungent sulfur-containing compounds characteristic of the order that make the plants so tasty. Arabidopsis thaliana is one of the most intensely studied flowering plants, particularly important because of the detailed developmental work that has been carried out on it.

Akaniaceae and Tropaeolaceae

      Akaniaceae and Tropaeolaceae both have large zygomorphic flowers with eight stamens and an ovary with three compartments, with the ovules at the apex of each. Geographically and morphologically they might otherwise seem an unlikely pair.

      Members of the Akaniaceae, or Akania, family, are deciduous or evergreen trees that grow in southwestern China, adjacent Vietnam, and Taiwan (Bretschneidera sinensis) or eastern Australia (Akania bidwillii). The leaves are pinnate, with a leaflet at the end. The flowers are only weakly zygomorphic. There are two ovules in each ovary compartment, and the fruit is a capsule with rather large seeds in Bretschneidera.

 Members of Tropaeolaceae, or the nasturtium family, are rather fleshy vines or herbs with a single genus, Tropaeolum (including Magallana and Tropheastrum), with 95 species distributed from Mexico to Patagonia. The petioles are twining, and the broadleaf blades have palmate venation and may be palmately lobed or divided, or peltate. The flowers are large, are strongly zygomorphic, and have a nectary spur on the upper side, and the petals, which may be fringed on the margins, are strongly narrowed at the base. The fruit usually separates into three one-seeded units. Pollination is by birds and large insects. Tropaeolum furnishes a number of popular garden ornamentals, with the commonly cultivated T. majus being a complex hybrid. T. tuberosum has edible tubers.

Caricaceae and Moringaceae

      Caricaceae and Moringaceae form a very distinctive group with many anatomical features in common. Their stems are stout; the venation of the leaves is palmate; and there are tiny glands at the base of the petiole or on the blade; the stipules too are glandular. The numerous ovules are borne on the walls of the ovary, and the seed coat is notably thick.

Caricaceae

 Caricaceae, or the papaya family, contains stout-stemmed trees or, rarely, vines in 4 genera with 34 species. The family is mostly Neotropical, and Carica (23 species, including papaya) occurs throughout the warmer parts of this area. There are three genera in Mexico alone, while in tropical Africa there is only the genus Cylicomorpha, with two species. Caricaceae have flowing latexlike exudate and palmately compound or lobed leaves. The inflorescences (inflorescence) are axillary and cymose, and the flowers—always of a single sex—are moderate in size and usually have fused petals. The fruit is a sometimes sizable berry and contains numerous seeds clearly borne on the walls, with each surrounded by mucilage.

      Species of Jacaratia have female flowers with white spreading stigmas that apparently mimic male flowers; the former lack nectar and have rather free petals, whereas the latter have nectar and the petals are fused. Such variation within a species (Carica is similar) is unusual.

      Papaya fruits come from Carica papaya (papaya), a native of Central and South America. The green fruits yield a milk sap, which is dried and from which papain, an enzyme that breaks down proteins, is obtained. It is used as a meat tenderizer.

Moringaceae

      Members of Moringaceae, or the horseradish tree family, are woody, often quite stout-stemmed shrubs or trees containing one genus, Moringa, with 12 species growing in Madagascar, northeast and southwest Africa, and Arabia, with three species spreading to India. Foliage of Moringaceae often smells unpleasant when crushed. The family is recognizable by its spirally arranged, deciduous, up to three times compound leaves that have conspicuous swellings or pulvini where the parts join. The flowers look like pea flowers, but they are constructed in a different way, as there are only five stamens, which are held to one side of the flower. The fruit is long and explosively dehiscent, consisting of three parts and containing often winged seeds.

      Moringaceae grow in drier parts of the world, and some are bottle trees, or have a large underground portion that withstands periods of drought. Pollination is by bees or birds, and several species have wind-dispersed seeds. Moringa oleifera (horseradish tree) has a number of uses: it yields a nutritious oil, edible fruits, and ingredients for various traditional medicines, and it has been processed to create biofuel.

Bataceae, Salvadoraceae, and Koeberliniaceae

      Bataceae, Salvadoraceae, and Koeberliniaceae have in common ultrastructural features, the same base chromosome number, and flowers that lack a nectary and have only two carpels. They, and many other Brassicales, have a curved embryo.

      Bataceae and Salvadoraceae are close anatomically and have opposite leaves with secondary veins ascending from or near the base. The pollen is smooth; there are two basal ovules in each compartment of the ovary; and there is no style in Bataceae. Bataceae are fleshy shrublets, with a single genus, Batis, and two species, one from Australia and south New Guinea and the other in the Neotropics (to Florida), including the Galapagos Islands. The plant is shrubby, and the leaves are fleshy and have tiny stipules. The flowers are either male or female and are aggregated into dense inflorescences. The female flowers lack bracts, sepals, and petals and are fused together to form a compound fruit; this may be either a capsule or a drupe. This is a curious little family, both members of which are plants of salt marshes. The flowers in the two species are, however, rather differently constructed.

      Salvadoraceae includes 3 genera and 11 species of shrubs that grow in drier and sometimes saline places in the area from Africa (including Madagascar) to Southeast Asia and western Malesia. The flowers have the same number of sepals, petals, and stamens, and there are sometimes nectar glands alternating with the stamens. The fruit is fleshy, containing either seeds or a stone. Twigs of Salvadora persica, a species that grows from Africa to India, make a bristly chewing stick, and the plant has valuable antiseptic properties that make it useful in toothpastes. The foliage can be eaten by domesticated animals, and the fruit is edible (some scholars think that it might be the “mustard seed” described in the Bible).

      Koeberliniaceae includes just one species, Koeberlinia spinosa, a woody, thorny plant that grows in the drier areas of central and southwestern North America and in Bolivia in South America. It is vegetatively very like other nondescript thorny desert shrubs, and the leaves are very reduced, but its flowers often have fewer than twice as many stamens as petals. The fruit is a berry.

The Resedaceae group

      Resedaceae, Gyrostemonaceae, Tovariaceae, and Pentadiplandraceae have flowers in which the sepals and petals often do not tightly surround the flower as it develops, and they have embryos that are curved in the seeds. Their interrelationships are poorly understood, with little known about the basic morphology and anatomy of the smaller families.

      Resedaceae contains 3 genera and 75 species of annual to perennial herbs and shrubs, which grow mostly in drier and warmer north temperate or subtropical regions. The plants are especially common in the Mediterranean, the Near East, and the Sahara, but they are also scattered in suitable places through much of Africa. Resedaceae can become weedy, although rarely seriously so. Pollination is by short-tongued bees. Seeds are shaken out of the capsules by wind or else fall out.

      Reseda (mignonette) (68 species) grows from Europe to Central Asia. The flowers are zygomorphic, and the petals are unequal, the largest usually having more or less fringed appendages on their backs. The nectary disc is especially developed on the upper part of the flower (as is quite common in this group of families). The carpels are quite distinctive, since they often do not really close. The fruit is dry, rarely a berry, with the seeds being thrown from or simply falling out of the capsule. Reseda luteola yields a yellow dye that was much used in antiquity, and R. odorata yields an oil used in making scent.

      Gyrostemonaceae is a small family of trees and shrubs, with 5 genera and at least 18 species, all native to Australia. Gyrostemon has 12 species. The flowers are of different sexes and are usually small. The stamens, which have at most short stalks, are borne in one or more whorls around the central axis of the flower, as are the carpels. The fruit is very variable, and the seeds have fleshy appendages or arils. Gyrostemonaceae species are wind-pollinated. Once the seeds have fallen to the ground, they may be dispersed by ants.

      Tovariaceae contains one genus, Tovaria, and two species of annual herbs that grow in the Neotropics. The species have trifoliate leaves with stipules, terminal, racemose inflorescences, and flowers with parts in sixes to nines that have a short style and spreading stigma. The fruit is a berry.

      Pentadiplandraceae is a small family (one genus with one or two species) of shrubs or lianas from Western Africa. The expanded petal bases are concave and coherent, forming a cavity, and each petal has a thin, free, more conventional-looking petal lobe with a very narrow base. The fruit is a berry.

Other families

      Limnanthaceae, or the meadowfoam family, includes one or two genera and eight species growing in temperate North America. They are rather soft-stemmed herbs with deeply lobed or compound leaves and rather widely open flowers, and there may be one style coming from the base of the ovary. The fruits separate into rather spiny single-seeded portions.

      Setchellanthaceae contains only one species, Setchellanthus caeruleus, a shrub found in Mexico. It may be recognized by its large blue flowers, with their parts usually in sixes that are borne in the axils of leaves. Vegetatively, the plant is rather undistinguished, although it has T-shaped hairs and rather small leaves without teeth that have secondary veins arising from near the base. The three-locular fruit separates along the partitions, leaving a persistent central column. Setchellanthus used to be included in Capparaceae.

      Emblingiaceae also contains only one species, Emblingia calceoliflora, which is native to western Australia. It is a rather coarsely hairy subshrub, with very curious flowers borne in the leaf axils. There is some controversy over the morphology of these flowers, which are zygomorphic and held upside down. The sepals are fused, though the tube is divided down one side, and there are only two petals. The stamens and ovary are borne on a common stalk; only four of the eight stamens are fertile. The dry fruit does not dehisce, but the seeds have a fleshy aril, which is a rather unusual combination.

Paul E. Berry

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