Cruciferae

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Cruciferae

Description

Herbs, sometimes subshrubs. Leaves spirally arranged, basal ones often in a rosette, exstipulate, petiolate to sessile and amplexicaul, entire to variously divided. Inflorescences terminal or sometimes axillary racemes, in flower mostly condensed and often corymbose, in fruit elongate, usually ebracteate. Flowers bisexual, actinomorphic or slightly zygomorphic, hypogynous, cyclic, tetra-merous, heterochlamydous. Sepals 4, free, usually equal, spathulate to clawed, imbricate or contorted. Stamens 6, tetradynamous (rarely 4 or 2), episepalous usually free; Ovary superior, sessile or stipitate, of seemingly two united carpels, secondarily divided into two locules by a thin membranous septum (sometimes transversely locular by intrusions from the fruit wall); Fruit a bivalved dehiscent siliqua or silicula (see key), sometimes a nutlet, lomentaceous or otherwise constricted. Seeds virtually devoid of endosperm, with cotyledons incumbent, accumbent or variously folded.

Distribution

Africa: Asia-Tropical:, Jawa (Jawapresent); New Guineapresent Australasia: Irano-Turanian regions: present New Zealand: present North America: present montane South America: present the Mediterranean: present
A cosmopolitan family with about 380 genera and more than 3000 species, especially diversified in the Mediterranean and the Irano-Turanian regions as well as in parts of Southern Africa, North America and montane South America. The family is comparatively sparse in the tropics, mainly confined to montane and arid areas.
The family includes a number of important crops and spices, notably in the genera Brassica, Sinapis, Raphanus, Crambe, Lepidium, Rorippa, some now cultivated in montane parts of the tropics.
Several cosmopolitan weeds belong to the family (within Lepidium, Coronopus, Capsella, Thlaspi, Arabis, Sisymbrium, Rorippa, Camelina, and others) and may appear as casual aliens virtually anywhere.
In Malesia there are 6 genera with a total of 24 species. The genus Papuzilla (3 spp.), regarded as a New Guinean endemic, is here included within Lepidium. In each of the genera Cardamine and Rorippa 3 spp. are endemic to New Guinea and some adjacent areas, and Rorippa backeri is an endemic of Java.
The affinities of the endemic Rorippa species is with Australia and New Zealand, while the Cardamine species belong to an alliance of tropical montane species around the widespread C. africana.
The remaining 14 species may all have been introduced by man and partly naturalized.

Dispersal

All Malesian species (except Raphanus sativus) disperse by small seeds without special devices. Some seeds are mucilaginous (get sticky with water), which may facilitate transport with birds and other agents.

Morphology

In spite of its size, the family is remarkably uniform. Lignification is rarely substantial except in basal parts, the hypocotylar region and roots. The herbaceous life-form seems to be primary, at least so far as many woody groups seem to have evolved from herbaceous forms. Annuals are common in many genera.

In floral parts little specialization is to be found, but not rarely reductions as to stamens, petals and nectaries. The position of the nectaries varies in relation to the filament bases (and is of taxo-nomic importance) but no particular elaboration is found.

Variation and specialization are important and often drastic as to fruits and diaspores, which means that without developed fruits many species are not determinable.

The seeds are variously ornamented, often winged. The testa is rather uniformly built from one-cell-thick layers: epidermis (which may contain mucilage), subepidermis (which may be absent), palissade layer (usually with pigmented wall thickenings) and a parenchymatous pigmented layer. The ripe seed is almost filled up by the embryo; endosperm is reduced to a one-cell-thick 'aleurone' layer adjacent to the testa. The folding of the cotyledons and their position in relation to the radicle is of taxonomic importance.

Anatomy

Wood anatomy is quite variable within the family and largely associated with stem width. The xylem in thin-stemmed species forms a closed cylinder and the cambium forms a continuous ring. In thicker stems sclerenchymatous tissue separates the xylem bundles and the cambium cylinder is mostly discontinuous. In woody species medullary rays may also separate the xylem bundles. The variation largely reflects degrees of adaptations to arid conditions and occurs within genera and species as well as intra-individually according to age. Even in strongly lignified species the wood elements are of little advanced types, making it probable that woodiness has evolved comparatively recently in the family.

Secretory cells containing myrosinase (myrosin cells) are widely distributed in the family, but in various organs in various genera. Their distribution has been regarded to be of some taxonomic value.

Hairs are always unicellular, but shape varies from unbranched to forked, stellate, T-shaped, etc. and is of considerable taxonomic interest at various levels.

The stomata are mostly of a characteristic type with one small and two larger subsidiary cells: so-called anisocytic or cruciferous stomata. This type is, however, by no means restricted to the Cruciferae.

Taxonomy

The uniformity of this large family makes subdivision difficult. A number of tribal classifications have been proposed, most of them ending up with 15-20 tribes, 9 of those distinct and largely restricted to limited parts of the southern hemisphere. They include few genera and mostly also few species. Among widespread tribes, rich in genera and species, Brassiceae (with Brassica and Raphanus in the Malesian flora) is the only one really distinct, and not been disagreed upon as to its circumscription. Most of the other tribes have been rather schematically delimited and may contain a nucleus of closely related genera to which others seem to have been more arbitrarily added. According to SCHULZ'S system the Malesian genera should be included within Lepidieae (Lepidium and Capsella) and Arabideae (Cardamine and Rorippa).

Cytology

By far the most widespread basic number is x = 8, but dysploidy occurs within several genera (e.g. Brassica) and may account for × = 7, which prevails in a few genera (e.g. Thlaspi, Sisymbrium); x = 5 is known from Arabidopsis, the southern hemispheric tribes Stenopetaleae and Heliophileae, and on polyploid level in Crambe. Polyploidy, often combined with aneuploidy, is extensive in most of the large genera, e.g. Cardamine. The chromosomes are small and do not readily lend themselves to structural studies.

Phytochemo

The family is rather uniform and highly characteristic also from the chemical point of view. Particularly the seeds but also other organs contain glucosides with sulphur and nitrogen in their molecules, so-called mustard oil glucosides, or glucosinolates, compounds unique to the family. The enzyme myrosinase, localized in particular cells, will split the glucosinolates when cell walls are crushed in the presence of water into three compounds, among those the pungent mustard oils. They are either isothiocyanates of usually pleasant flavour or thiocyanates with a strong, often garlic-like odour (e.g. in Lepidium and Thlaspi).

Very important in the seeds are also lipid acids, particularly unsaturated ones with 18, 20 or 22 carbon atoms. Particular for the family and very widespread is the erucic acid, which because of unliked properties should be kept at a minimum in strains of e.g. Brassica cultivated as oil seed crops. Others of those fatty acids (oleic, linolenic and linoleic acids) are of utmost economic importance. Among alkaloid-like compounds sinapin, a protoalkaloid of bitter taste, is very common in the family and concentrated in the seeds. Proteins are of importance in the seeds, while starch is lacking.