This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
Fringing our rivers, ditches and lakes, the Yellow Iris appears to be defending them with drawn sword. Everybody knows the sharp-edged leaves of this species, that may cut the hands of the gatherer if he be not careful. Equally well-known are the bright blossoms that begin to appear in May and keep up a succession until late in July; but probably most of the unscientific readers who have honoured me with their company thus far - and who have learned, I trust, to know the parts of a flower at sight - would be incorrect in their description of this common flower. Anyway, it will be worth their while dissecting a flower. The parts of the flower are in threes, but the sepals are more petal-like than the petals, and so are the styles. The sepals are in fact the most striking organs; they are broad, and reflexed to form convenient alighting platforms for a heavy humble-bee. The petals are narrow, erect, or curved towards the centre of the flower, to be out of the way of the broader, arching style, which is spread out and coloured like a petal, with the stigmatic surface near the upturned tips. Beneath this arching style lies the anther, similarly curved, and opening away from the stigma.
Note the why and wherefore of this departure from orthodox arrangements of floral organs. At the bottom of the flower-tube honey is secreted, and to obtain this the flower is visited by humble-bees. In order that his long tongue may reach the honey, the bee has to push his head and back against the stigma and the anther. If he has previously visited a flag-flower his back will be covered with pollen, some of which will adhere to the stigma. He will also take away on his head and back some of the pollen from the flower he is now visiting, and will fertilize other flags with. it.
There is another British species, The Stinking Iris, Gladdon, or Roast-beef plant (Iris foetidissima), with purple sepals, yellow petals and stigmas. Flowers not quite so large as the last. Woods and copses. May to July.
Yellow Iris. Flag.