This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Perennial often tuberous-rooted herbs with usually glabrous equitant distichous leaves and terminal bracteate spikes, umbels, corymbs or panicles of showy flowers. Perianth superior, composed of six divisions in two series, equal or unequal, the inner sometimes smallest. Stamens 3. Stigmas often petaloid. Fruit an inferior 3-celled many - seeded capsule dehiscing loculicidally. Seeds spheroid, angular or winged, albuminous. This order comprises about 50 genera and 500 species, dispersed throughout the temperate regions of the whole world. The British species are few and rare, with the exception of Iris Pseudacorus, the Yellow Flag.