Iridaceae - Description

Description

In the Iridaceae the perianth is formed of two whorls of three tepals, all similar in structure, shape, and often color. Such a corolla differentiates Liliales and Asparagales from other monocots, where the number or size of the tepals are reduced, or where at least one whorl is papyraceous (firm and dry like a papyrus), and usually green or brown. The characters that differentiate Iridaceae, however, are its three stamens (related families such as Alliaceae and Amaryllidaceae have six) and an inferior ovary. Sword-like leaves parallel to the stem and with normally undifferentiated sides (termed "isobilateral" and "unifacial") are another distinctive feature.

Members of Iridaceae are herbaceous plants or, in a few cases, shrubs with woody caudex. They are almost all perennial (three Sisyrinchium species are annuals) that may be either evergreen or seasonal. The rootstock is a rhizome, bulb, or corm. The leaves are found both at the base and on the stem, usually alternate, with the blade oriented parallel to the stem and thus sheathing it at the base. This results in the characteristic fan-like arrangement found in genera like Iris. This type of leaf lacks distinct upper and lower leaf surfaces. In many South African species the leaf has a thickened midrib and often variously thickened or winged margins that may also be crisped. In some species the leaves are needle-like with narrow longitudinal grooves. Species of Moraea are unusual in the family in having channeled leaves with a distinct upper and lower surface.

Flowers may be either actinomorphic or zygomorphic. Almost all the parts are in threes, starting with two equal whorls of three usually large and showy petal-like tepals, distinct or fused in a tube. There are three stamens (rarely two), and their filaments are often partly to completely fused. Anthers have two pollen sacs opening toward the outside, or from their side, and usually along their length. The ovary is located below the tepals (except in Isophysis) with axile (rarely parietal) placentation in three locules. There is a single style branching into three at the top. Iridaceae do not present unisexual flowers, and all flowers have both a style and stamen. Most members other than Sisyrinchium produce nectar from nectaries at the base of the tepals, or on the gynoecium. Iridaceae species are usually pollinated by insects or birds. The flowers are collectively arranged in two different types of inflorescences. Simple or branched spikes occur in all Crocoideae. In other subfamilies the basic inflorescence unit is a type of zig-zagging cyme called rhipidium, which is enclosed in enlarged, opposed, bracts called spathes.

The fruit is a dry capsule, usually splitting along three sides spontaneously at maturity. It is very variable in shape and texture, from firm to cartilaginous, occasionally woody. In most genera they are tetrahedral or variously angled and without obvious adaptations for dispersal. Seeds are also varied in shape. Winged seeds adapted to wind dispersal characterize Gladiolus and Tritoniopsis and also occur in some species of Hesperantha. Globular seeds with shiny coats that are relatively long-lived occur in several genera of Crocoideae. Chasmanthe aethiopica has fleshy seeds adapted to dispersal by birds and several other species that grow in more wooded places – like Chasmanthe and some freesias – have reddish or black seeds that mimic fleshy seeds. They have a hard endosperm, with reserves of hemicellulose, oil, and protein, and a small embryo.