The genus is widely distributed throughout the
north temperate zone. Their habitats are very varied, ranging
from cold regions into the grassy slopes, meadowlands, stream
banks and deserts of Europe, the Middle East and northern Africa,
Asia and across North America.
They are perennial herbs, growing from creeping
rhizomes (rhizomatous irises), or, in drier climates, from bulbs
(bulbous irises). They have long, erect, flowering stems, which
may be simple or branched, solid or hollow, and flattened or have
a circular cross-section. The rhizomatous species usually have
3-10 basal, sword-shaped leaves growing in dense clumps. The bulbous
species have cylindrical basal leaves.
The inflorescences are fan-shaped and contain
one or more symmetrical, six-lobed, slightly fragrant flowers.
These grow on a pedicel or lack a footstalk. The three sepals
are spreading or droop downwards. They expand from their narrow
base into a broader limb (= expanded portion), often adorned with
lines or dots. The three, sometimes reduced, petals stand upright,
partly behind the sepal bases. Some smaller iris species have
all six lobes pointing straight outwards. The sepals and the petals
differ from each other. They are united at their base into a floral
tube, that lies above the ovary. The styles divide towards the
apex into petaloid branches (see pollination, below).
The iris flower is of special interest as an example
of the relation between flowering plants and pollinating insects.
The shape of the flower and the position of the pollen-receiving
and stigmatic surfaces on the outer petals form a landing-stage
for a flying insect, which in probing the perianth for nectar,
will first come in contact of perianth, three with the stigmatic
stamens in one whorl surface which is borne and an ovary formed
of three carpels. The shelf-like transverse projection on the
inner whorl under side of the stamens, which is beneath the over-arching
style arm below the stigma, so that the insect comes in contact
with its pollen-covered surface only after passing the stigma,
while in backing out of the flower it will come in contact only
with the non-receptive lower face of the stigma. Thus, an insect
bearing pollen from one flower, will in entering a second, deposit
the pollen on the stigma, while in backing out of a flower, the
pollen which it bears will not be rubbed off on the stigma of
the same flower.
