signal [ SIG-nl ] noun: an area of contrasting color, usually yellow, on the fall of an iris in the place of a beard
Some flowers have specialized names for their flower parts. For irises, the petals are called standards and the sepals falls, which can be remembered with the following mnemonic: “standards stand up and falls fall down.” In the center of their falls, some irises, like cultivars of Iris germanica (German iris), have a hairy tuft called a beard. Others, like bulbous perennial Iris reticulata (dwarf iris) and rhizomatous Iris ensata (Japanese iris), Iris sibirica (Siberian iris), and natives Iris versicolor and Iris virginica (northern and southern blue flags), have a signal, which is a patch of contrasting color–usually yellow or white. The signal patch of some irises, like native Iris cristata (dwarf crested iris), has ridges or cockscombs called a crest.
The primary pollinators of irises are bees although hummingbirds and butterflies also visit the flowers. While humans see a color spectrum from red to purple, bees see a spectrum from orange to ultraviolet and are especially attracted to white, yellow, blue, and ultraviolet, the latter, which is invisible to the human eye but forms patterns visible to bees on many of the flowers they pollinate. (To see what bees see, visit Flowers in Ultraviolet Arranged by Plant Family.)
Photo © Mary Free
Iris signals attract bees and show them exactly where to land on the flower and, along with the fall’s patterned venation, guides them directly to the nectaries located at the base of the petals. As a bee follows this nectar guide along the fall, pollen already on its back rubs off onto the stigmatic lip. Continuing under the low-hanging style arm, the bee acquires new pollen as it passes beneath a large, pollen-producing stamen pressed against the arm and hidden from view. The color and patterns of signals and nectar guides allow bees to forage efficiently, gathering the most nectar and pollen in the least amount of time.
References
Leonard AS, PaPaj, DR. 2011. ’X’ marks the spot: The possible benefits of nectar guides to bees and plants. Functional Ecology. doi: 10.1111/j.1365-2435.2011.01885.x.
Riddle S. 2016. How Bees See And Why It Matters. Bee Culture.
Roguz K, Gallagher MK, Senden E, Bar-Lev Y, Lebel M, Heliczer R, Sapir Y. 2020. All the Colors of the Rainbow: Diversification of Flower Color and Intraspecific Color Variation in the Genus Iris. Frontiers in Plant Science 11: 569811. doi: 10.3389/fpls.2020.569811.
Vogelpohl S. Know Your Natives – Crested Iris. Arkansas Native Plant Society. [10 April 2019].