Neotropical Brassicaceae

Ihsan A. Al-Shehbaz

Missouri Botanical Garden, St.Louis, MO, U.S.A.

Description

Herbs, annual, biennial, or perennial, sometimes subshrubs or shrubs, very rarely lianas; taprooted or with few to many-branched and slender or woody caudex, sometimes with slender or tuberous rhizomes, rarely stoloniferous; terrestrial or rarely submerged aquatics, with a pungent watery juice, without or rarely with multicellular glandular papillae or tubercles.  Trichomes unicellular, simple, stalked or sessile, 2-many forked, stellate, dendritic or malpighiaceous (medifixed, bifid, appressed), rarely peltate and scalelike, eglandular.  Stems erect, ascending or prostrate, herbaceous or rarely woody, leafy or leafless, sometimes absent.  Leaves exstipulate or rarely with tiny, stipule-like glands at base of petioles and pedicels, simple, entire or variously dissected, rarely trifoliolate or pinnately compound; basal leaf rosette present or absent; cauline leaves almost always alternate or rarely opposite, petiolate or sessile, sometimes absent.  Inflorescences racemes, corymbs, or panicles, ebracteate or less frequently bracteate, sometimes flowers solitary on pedicels originating from axils of rosette leaves; pedicels persistent with fruit, variously oriented.  Flowers hypogynous, actinomorphic, bisexual; sepals 4, in 2 decussate pairs (1 pair lateral and 1 median), free or rarely united, not saccate or lateral (inner) pair saccate, caducous or rarely persistent, erect, ascending, spreading or reflexed, forming tubular, campanulate or urceolate calyx; petals 4, alternate with sepals, arranged in the form of a cross (cruciform; hence the earlier family name Cruciferae), rarely rudimentary or absent, differentiated or not into blade and claw, basally unappendaged; blade entire or emarginate, sometimes reduced and much smaller than well-developed claws; stamens 6, in 2 whorls,  tetradynamous (lateral (outer) pair shorter than median (inner) 2 pairs), rarely equal in length, sometimes stamens 2 or 4; filaments slender, median pairs free; anthers dithecal, dehiscing by longitudinal slits; pollen grains 3-colpate, trinucleate; nectar glands receptacular, highly diversified in number, shape, size, and disposition around filament bases, always present opposite bases of lateral filaments, median glands present or absent; pistil 1, 2-carpelled; ovary superior, sessile or borne on a distinct gynophore, 2-locular and with a false septum connecting 2 placentae, rarely unilocular and eseptate; placentation parietal, rarely apical; ovules 1-300 per ovary, anatropous or campylotropous, bitegmic, crassinucellate or rarely tenuinucellate.  Fruits typically 2-valved capsules, arbitrarily termed silique (siliqua) when length at least 3  width, or silicle (silicula) when length less than 3  width, dehiscent or indehiscent, sometimes nutletlike, lomentaceous, samaroid, or schizocarpic, segmented or not, terete, angled, latiseptate (flattened parallel to septum), or angustiseptate (flattened at a right angle to septum); gynophore absent or sometimes distinct; valves dehiscing acropetally or rarely basipetally; replum (persistent placenta) rounded or flattened; septum complete, perforated, or reduced to a rim; style 1, persistent, sometimes obsolete or absent; stigma capitate or conical, entire or 2-lobed, lobes spreading or connivent, sometimes decurrent, free or connateSeeds usually yellow or brown, flattened or plump, ovoid, oblong, globose, orbicular or ovate, uniseriately or biseriately arranged in each locule, rarely aseriate, winged or wingless, mucilaginous or not when wetted; embryo strongly curved; cotyledons entire or emarginate, variously oriented in relation to radicle: incumbent (embryo notorrhizal: radicle lying along back of 1 cotyledon), accumbent (embryo pleurorrhizal: radicle applied to margins of both cotyledons), conduplicate (embryo orthoplocal: cotyledons folded longitudinally around radicle); endosperm absent; germination epigeal.

Notes on delimitation

  • The Brassicaceae (also known as Cruciferae) is type family of the order Brassicales that includes 17 other families, including the sister family Cleomaceae.
  • The tribal classification of the family has been quite controversial, but extensive molecular studies during the past decade have been synthesized to produce a new phylogenetic tribal classification summarized by Al-Shehbaz (2012) into a family-wide generic and tribal synopsis. In this new classification system, the small SW Asian-Mediterranean tribe Aethionemeae (ca. 45 spp.) is sister to the rest of the family which is divided into three major lineages.
  • The small tribes Eudemeae (7 genera, 31 spp.), Cremolobeae (2 genera, 32 spp.), and Schizopetaleae (2 genera, 16 spp.) are endemic to South America, and their centres of diversity are in the Neotropics.
  • The New World tribes Thelypodieae and Halimolobeae, but not Physarieae, are more diversified in the Neotropics than in North America. Draba L.(ca. 390 spp. worldwide) is represented in South America by 70 species, all except four of which are endemic to the Neotropics.

Distribution in the Neotropics

  • The Neotropical Brassicaceae include 274 species in 45 genera, of which the 238 native species belong to 28 genera.
  • The 36 naturalized species belong to 21 genera of which four have both native and introduced species.
  • The most speciose genera are Draba and Lepidium (see below).

Distinguishing characters (always present)

  • The Brassicaceae are easily distinguished by the cruciform corolla and tetradynamous stamens (see family description). The exceptions are in some species of Lepidium that have only two stamens and a rudimentary or absent corolla.
  • The characteristic capsule (often called silique or silicle; see above) is distinctive in the family for all species with dehiscent fruits, in which the fruit valves fall off and the 'false' septum remains.

Other important characters

  • The three distinctive features above collectively occur in the vast majority of species, and when one or two features do not apply then the third would help. For example, in Lepidium spp. with two stamens and apetalous flowers, the fruit type described above is distinctive.
  • In genera such as Isatis and Rapistrum, both with indehiscent fruits and naturalized species in the Neotropics, species are easily placed in the family by the cruciform corolla and tetradynamous stamens.

Key differences from similar families

  • From the sister family Cleomaceae, the Neotropical Brassicaceae are easily distinguished by having actinomorphic corolla, fruit septum, and curved embryos without a gap between the radicle and cotyledons, as well as by lacking the stipules and palmately compound leaves.

Number of genera

There are 274 genera in the Neotropics, of which 36 are naturalised

  • Aschersoniodoxa Gilg & Muschl. (4 spp.) 
  • Barbarea W.T. Aiton (1 sp. naturalised)
  • Brassica L. (5 spp. naturalised)
  • Brayopsis Gilg & Muschl. (6 spp.)
  • Cakile Mill. (1 sp.)
  • Capsella Medik. (1 sp. naturalised)
  • Cardamine L. (14 spp. and 3 spp. naturalised) 
  • Cremolobus DC. (7 spp.) 
  • Dactylocardamum Al-Shehbaz (1 spp.)
  • Descurainia Webb & Berthel. (7 spp.)
  • Dictyophragmus O.E. Schulz (2 spp.)
  • Diplotaxis DC. (1 sp. naturalised)
  • Draba L. (66 spp.) 
  • Englerocharis Muschl. (4 spp.)
  • Eruca Mill. (1 sp. naturalised)
  • Erysimum L.(1 sp.)
  • Eudema Humb. & Bonpl. (4 spp.)
  • Exhalimolobus Al-Shehbaz & C.D. Bailey (4 spp.)
  • Hirschfeldia Moech (1 sp. naturalised)
  • Hornungia Rchb. (1 sp. naturalised) 
  • Iberis L. (2 spp. naturalised)
  • Isatis L. (1 sp. naturalised)
  • Lepidium L. (29 spp. & 1 naturalised)
  • Lobularia Desv. (1 sp. naturalised)
  • Mancoa Wedd. (3 spp.)
  • Mathewsia Hook. & Arn. (4 spp.)
  • Matthiola W.T. Aiton (2 spp. naturalised)
  • Menonvillea DC. (6 spp.)
  • Mostacillastrum O.E. Schulz (13 spp.)
  • Nasturtium W.T. Aiton (1 sp. & 1 sp naturalised)
  • Neuontobotrys O.E. Schulz (9 spp.)
  • Pennellia Nieuwl. (4 spp.)
  • Petroravenia Al-Shehbaz (1 sp.)
  • Physaria (Nutt. ex Torr. & A. Gray) A. Gray (1 sp.)
  • Polypsecadium O.E. Schulz (8 spp.)
  • Raphanus L. (2 spp. naturalised)
  • Rapistrum Crantz (1 sp. naturalised)
  • Romanschulzia O.E. Schulz (4 spp.)
  • Rorippa Scop. (12 spp. and 3 spp. naturalised)
  • Sibara Greene (5 spp.)
  • Sinapis L. (2 spp. naturalised)
  • Sisymbrium L. (4 spp. naturalised)
  • Thlaspi L. (1 sp. naturalised)
  • Weberbauera Gilg & Muschl. (16 spp.)

 

Useful tips for generic identification

Interactive key to the genera of the world:

http://flora.huh.harvard.edu/Brassicaceae/navikey/Brassicaceae_Genera_World/Brassicaceae_Genera_World_NaviKey.html

Notable genera and distinguishing features

Only native ones: see terminology in family description

  • Aschersoniodoxa: scapose plants with large, oblanceolate, flattened fruits.
  • Brayopsis: flowers and fruits on solitary pedicels originating from a perennial rosette.
  • Cakile: seashore plants with corky, 2-segmented fruits.
  • Cardamine: often compound leaves and linear fruits with flattened replum.
  • Descurainia: 2- or 3-pinnatisect leaves, dendritic trichomes, dehiscent siliques.
  • Lepidium: flattened, angustiseptate, 2-seeded dehiscent silicles.
  • Mancoa: angustiseptate, many-seeded fruits and branched trichomes.
  • Mathewsia: subshrubs with yellow flowers, dendritic trichomes, and broadly flattened
    large fruits.
  • Romanschulzia: filaments spreading, equal in length, strongly enlarged at base; sepals spreading, soon falling.

Status

  • Genera Aschersoniodoxa, Brayopsis, Cremolobus, Dactylocardamum, Englerocharis, and Eudema are endemic. The vast majority of native species (ca. 210 spp.) are endemic to the Neotropics.
  • Three species of Brassica (cabbage, cauliflower, turnip) and one of Raphanus (radish) are cultivated crops and also naturalized.
  • Lobularia maritima (L.) Desv. is a cultivated ornamental widely naturalized. Less commonly naturalized are Malcolmia maritima (L.) W.T. Aiton, Matthiola incana (L.) W.T. Aiton, and Iberis species.

General notes

  • The Brassicaceae include several important crop plants grown in the Neotropics as vegetables (e.g., species of Brassica and Raphanus).
  • The only native Neotropical crop is Lepidium meyenii (maca) cultivated in the high Peruvian Andes and consumed locally.
  • Vegetable oils of various Brassica species, especially B. napus L. (canola), probably rank first in terms of the world's tonnage production, but have not been grown in our area.
  • Several ornamentals are listed above, and 27 of the 36 naturalized species are weedy elsewhere. Lepidium didymum L. and L. bonariense L. are among that notable native South American weeds that have become naturalized elsewhere in the world.
  • The native species of our area grow (Neotropics?) primarily at high altitudes, and only some species of Rorippa and Cardamine grow in typical tropical lowlands.

Important literature

Al-Shehbaz, I.A. 2012. A generic and tribal synopsis of the Brassicaceae (Cruciferae). Taxon
(in press).

Al-Shehbaz, I.A., M. A. Beilstein & E. A. Kellogg. 2006. Systematics and phylogeny of the Brassicaceae: an overview. Pl. Syst. Evol. 259: 89 120.

Al-Shehbaz, I.A. and co-workers. 2010. Brassicaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America vol. 7, pp. 224-746. Oxford University Press, New York.

Appel, O. & I.A. Al-Shehbaz. 2003. Cruciferae. In: Kubitzki, K. & Bayer, C. (eds.), Families and Genera of Vascular Plants, vol. 5, pp. 75 174. Springer Verlag, Heidelberg, Berlin.

Schulz, O.E. 1936. Cruciferae. In: Engler, A. & Harms, H. (eds.), Die natürlichen Pflanzenfamilien, vol. 17B, pp. 227 658. Verlag von Wilhelm Englemann, Leipzig.

Warwick, S.I., Mummenhoff, K., Sauder, C.A., Koch, M.A. & Al-Shehbaz, I.A. 2010. Closing the gaps: phylogenetic relationships in the Brassicaceae based on DNA sequence data of nuclear ribosomal ITS region. Pl. Syst. Evol. 285: 209-232.

How to cite

Al-Shehbaz, I.A. (2012). Neotropical Brassicaceae. In: Milliken, W., Klitgård, B. & Baracat, A. (2009 onwards), Neotropikey - Interactive key and information resources for flowering plants of the Neotropics. http://www.kew.org/science/tropamerica/neotropikey/families/Brassicaceae.htm.