IRIS

in anatomy, pigmented muscular curtain at the front of the eye, between the cornea and the lens and perforated by an opening called the pupil. The iris consists of two sheets of smooth muscle with contrary actions, expansion and contraction. These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina. The sphincter pupillae is a circular muscle that constricts the pupil in bright light; the dilator pupillae expands the opening when it contracts. The amount of pigment contained in the iris determines eye colour. When there is very little pigment, the eye appears blue; with increased pigment, the shade becomes deep brown to black. genus of about 300 species of plants in the family Iridaceae including some of the world's most popular and varied garden flowers, centred in the North Temperate Zones. Some of its most handsome species, however, are native to the Mediterranean and central Asian areas. The iris is the fleur-de-lis of the French royalist standard. It is a popular subject of Japanese flower arrangement, and it is also the source of orrisroot, from which essence of violet perfume is made. Irises are either bulbous or rhizomatous (with thick, creeping underground stems) and have six petallike floral segments, the more erect inner ones called standards and the usually drooping outer ones called falls. Best known are the bearded, or German, groupthe common garden irises. These are hybrids of pale-blue I. pallida, yellow I. variegata, purple-blue I. germanica, and perhaps other southern European species. They are hardy, rhizomatous types with sturdy, swordlike leaves and tall stems (to 90 cm ) of three to many flowers. With the introduction in 1900 of taller, heavier, larger-flowered I. mesopotamica, even larger hybrids were created, many of them fragrant, in a full range of colours and combinations, often with brightly contrasting beards on the falls. Dwarf bearded irises, most of which flower in early spring, are for the most part varieties of the almost stemless I. pumila and the taller I. chamaeiris, both from dry, rocky places in southern Europe. Best known of the beardless, rhizomatous group is perhaps the water-loving Japanese iris (I. kaempferi), frequently featured in Japanese watercolours. Its almost flat flowers consist of long, somewhat drooping falls, surrounding narrower, shorter standards. The Siberian iris (I. sibirica), from grasslands in central and eastern Europe, has slender, straight stalks with clustered heads of violet-blue or white blooms. Similar but shorter and more sturdy, I. spuria has round falls, short standards, and rather lax foliage. The yellow, or water, flag (I. pseudacorus) is a swamp plant native to Eurasia and North Africa; the blue flag (I. versicolor) occupies similar habitats in North America. Two outstanding bulbous irises are both from mountains of Spain. They have narrow standards, somewhat broader falls, and spiky, linear foliage. Spanish iris (I. xiphium), violet with yellow or yellow-spotted falls, grows in damp, sandy places. English iris (I. xiphioides), so named because of its popularity in British horticulture, bears bright-blue flowers. Dutch irises are sturdier, earlier-flowering hybrids created in The Netherlands.