Crocus sativus - Köhler–s Medizinal-Pflanzen-194.jpg

''Crocus'' (; plural: crocuses or croci) is a genus of seasonal flowering plants in the family Iridaceae (iris family) comprising about 100 species of perennials growing from corms. They are low growing plants, whose flower stems remain underground, that bear relatively large white, yellow, orange or purple flowers and then become dormant after flowering. Many are cultivated for their flowers, appearing in autumn, winter, or spring. The flowers close at night and in overcast weather conditions. The crocus has been known throughout recorded history, mainly as the source of saffron. Saffron is obtained from the dried stigma of ''Crocus sativus'', an autumn-blooming species. It is valued as a spice and dyestuff, and is one of the most expensive spices in the world. Iran is the center of saffron production. Crocuses are native to woodland, scrub, and meadows from sea level to alpine tundra from the Mediterranean, through North Africa, central and southern Europe, the islands of the Aegean, the Middle East and across Central Asia to Xinjiang in western China. Crocuses may be propagated from seed or from daughter cormels formed on the corm, that eventually produce mature plants. They arrived in Europe from Turkey in the 16th century and became valued as an ornamental flowering plant.

Description

;General
''Crocus'' display the general characteristics of family Iridaceae, which include basal cauline (arising from the aerial stem) leaves that sheath the stem base, hermaphrodite flowers that are relatively large and showy, the perianth petaloid with 2 whorls of 3 tepals each and septal nectaries. The flowers have 3 stamens and a gynoecium of 3 united carpels and an inferior ovary, 3 locules and axile placentation with fruit that is a loculicidal capsule.

''Crocus'' is an acaulescent (lacking a visible lower stem above ground) diminutive seasonal cormous (growing from corms) herbaceous perennial geophytic genus. The corms are symmetrical and globose or oblate (round in shape with flatted tops and bottoms), and are covered with tunic leaves that are fibrous, membranous or coriaceous (leathery). The corms produce fibrous roots, and contractile roots which adjust the corms depth in the soil, which maybe pulled as deep as into the soil.The roots appear randomly from the lower part of the corm, but in a few species, from a basal ridge.

;Leaves
Plants produce several basal linear bifacial green leaves that arise from the corms. These are adaxially (upper surface facing axis) flat or channelled with pale median stripes, while the opposite (abaxial) surface is strongly keeled, with two grooves on either side. The leaves have a distinctive shape in cross-section, being boat-shaped with two lateral arms with margins recurved inwardly towards the central riged keel, forming the sides of the "boat". The keel may be square or rectangular, but is lacking in ''C. carpetanus''. The pale central stripe is caused by parenchymatous cells which lack chloroplasts and may contain air spaces. The leaves are from wide and long. The leaf-like bracts are membranous, while the smaller bracteoles are either membranous or absent. The leaf bases are surrounded by up to 5 membranous sheaths called cataphylls, a specialised leaf. The bases of the cataphylls form the corm tunic, and their number varies from 3 to 6, and enclose the true leaves (euphylls), bracts, bracteoles and flowering stalk.

;Flowers
The number of peduncles (flower stems) vary from one to several and remain underground, emerging only at the fruiting stage, bearing flowers that are solitary or several, so that a true scape is absent. The flowers are pedicellate (attached to the peduncle by a short subterranean pedicel stalk). The pedicel is sometimes subtended (below pedicel) by a membranous, sheathing prophyll (leaf-like structure).

The showy, salver to cup-shaped, single or clustered actinomorphic flowers taper off into a narrow tube; the flowers emerge from the ground, and can be white, yellow, lilac to dark purple, or variegated in cultivars. The flower tube is long, cylindrical and slender, expanding apically. The floral tube is long and narrow with 6 lobes in 2 whorls. The perianth is 3+3 (3 sepals+3petals) and gamophyllous (with fused segments). The tepal whorls are similar, equal or subequal with a smaller inner whorl, and cupped to outspread. The bracts are membranous, but the inner ones are sometimes lacking.

The 3 stamens are erect and linear and inserted in the throat of the perianth tube, with anthers shorter than the filaments. Pollen grains are inaperturate (apertures absent) but sometimes spiraperturate (spiral shaped). Each flower has a single style which is exserted (projecting beyond the corolla tube) and slender distally with three to many branches. The branches are highly variable, being short or long, and simple, bifurcate (dividing in two) or multifid and sometimes distally flattened. The inferior ovary has 3 carpels with axile placentation. It remains underground, and as the seeds ripen, the pedicel (stem of the flower) grows longer - so the fruit is above the soil surface.

;Fruit and seed
The fruit is a small membranous capsule, ellipsoid or oblong-ellipsoid in shape and the many seeds are globose to ellipsoid. The seed surface is highly variable, including papillate (covered in small protuberances), digitiform (finger-like) and other epidermal cell types. In some species the seeds are arillate, with fleshy appendages. Crocus seeds have both inner and outer integuments and in some species the outer epidermis may display long papillae. Embryo-sac development is Polygonum type. Dehiscence (splitting of the capsule to release the seed) is of the loculicidal type in which it splits through the wall of the locules leaving the septa that separate them intact.

;Karyology
''Crocus'' has extensive aneuploidy (abnormal number of chromosomes), with some uncertainty as to the base number of chromosomes. The chromosome numbers shows extreme variability,ranging from 2n=6 to 2n=70 even within a single species.

;Phytochemistry
The Iridaceae contain a wide range of phenolic compounds. However, 6-Hydroxyflavones are found only in ''Crocus'', which is also characterised by the presence of crocins, water-soluble yellow carotenoids, in the floral tissues. Crocin is a diester of crocetin, responsible for the colour of the styles and stigma of ''C. sativus'', and hence saffron. A few species contain mangiferin, a glucosylxanthone.

While the flowers may vary dramatically between species, there is little variation in the leaves, but sufficient variability in corm tunics that they may be used as an aid in differentiating taxa.

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Taxonomy

History

The crocus was well known to the ancients, being described at least as early as Theophrastus (c. 371c. 287 BC), and was introduced into Britain by the Romans, where the saffron crocus was used as a dyestuff. It was reintroduced into Western Europe by the Crusaders. The crocus is mentioned in mediaeval and later herbals, one of the earliest being the 14th century '' Tractatus de Herbis.'' William Turner (1548) states that the crocus is referred to as saffron in English, implying that only ''C. sativus'' was known at that time. However, by 1597 John Gerard writes of "sundry sorts" and uses the term saffron and crocus as interchangeable. He included both spring and autumn flowering crocus, but distinguished Wild Saffron (Crocus) from Meadow Saaffron (Colchicum). He described eleven forms. Some of his specimens were obtained from Clusius. In the following century, John Parkinson in a more detailed account was more careful to include separate chapters for ''Colchicum'', with the common name of meadow saffron, from ''Crocus'' or saffron. Parkinson (1656) states that there are "divers sorts of saffrons" describing 27 spring flowering plants and 4 autumn flowering ones, pointing out that only one of those was the true saffron crocus, which he called ''Crocus verus sativus autumnalis''. Similar accounts are found in continental European herbals, including those of l'Obel in Flanders (1576) and Besler's '' Hortus Eystettensis'' in Bavaria (1613).

The genus ''Crocus'' was first formally described by Linnaeus in 1753, with three taxa, and two species, ''C. sativus'' (type species), var. ''officinalis'' (now treated as a synonym of ''C. sativus'') and var. ''vernus'' (now ''C. vernus'') and ''C. bulbocodium'' (now ''Romulea bulbocodium''). Thus Linnaeus recognised two taxa that are accepted as separate species in modern classifications, one vernal and one autumnal crocus, but incorrectly assumed they were only varieties of a single species, while his second species was actually from a closely related genus that was only recognised later (1772). However a subsequent re-examination of Linnaeus's specimens suggested the presence of several different species that he did not recognise as being separate. Linnaeus' system, based on sexual characteristics, ''Crocus'' was classified as Triandra Monogynia (Three stamens, Single pistil). Linnaeus's system was supplanted by the "natural" system which used a hierarchy of taxonomic ranks based on weighting of the importance of structural characteristics of the plant. Jussieu (1789) placed the genus Crocus in his ''Ordo'' (family) Irides or ''Les iris'', as a member of the class ''Stamina epigyna'' (stamens inserted above the ovary) as part of the monocotyledons, the first level of the division of the flowering plants.

One of the first monographs of the genus appeared in 1809, by Haworth, followed in 1829 by that of Sabine, and Herbert in 1847. In 1853, Lindley continued the placement of ''Crocus'' as one of 53 genera in Iridaceae, which he included in a higher order of monocotyledons, the Narcissales. Baker published a monograph on the genus in 1874, adopting a very different schema to that of Herbert. In 1883, Bentham and Hooker described the Irideae (Iridaceae) as having more than 700 species, and divided it into 3 tribes and further into subtribes. Tribe Sysyrinchieae as having 2 subtribes, including Ixieae. The latter was circumscribed with four genera, ''Crocus'', ''Syringodea'', ''Galaxia'' (''Moraea'') and ''Romulea