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Rapid Flower-Opening in Iris pseudacorus
Bernd Heinrich*
Abstract. Iris pseudacorus (Yellow Flag Iris), normally produces 1 flower, or exceptionally, it can
produce more than 1 flower per stalk at any one time. Flower buds and opened flowers are available
at all times every day, but the transition stages between buds and expanded flowers are not. I here
report that the flowers open explosively and discuss the proximate cause and the potential significance
of this behavior.
Iris (iris) flowers consist of 3 basic parts: 3 large, drooping leafy sepals; 3 vertical,
often nearly equally showy petals (the banner); and a leafy style. The sepals have a greatly
enlarged hanging lip with markings that serve as a nectar guide that help orient pollinators
into a chamber under the style that serves as the roof of a tube formed by the style of the
flower, with its stigma at the tube entrance, and the anther with its support closely adjoined.
Pollen is both received and delivered as the pollinator enters the flower at one of 3 similar
points. In most flowers, the style is a simple rod, but in irises it is flattened and flanged at
the sides. The flower bud is round and spike-like, with the petals tightly wrapped around
the other parts and each other.
I observed a small Iris pseudacorus L. (Yellow Flag Iris, hereafter Yellow Flag), a nonnative
plant, growing directly along the Atlantic shoreline of Star Island, near the border
of Maine and New Hampshire. Replanted, it eventually flowered next to my doorstep in
Maine. I watched it daily throughout June 2014, observing how the conical flower buds
metamorphosed into resplendent flowers. Here, I reconstruct that transition based on observations
from photographs and sketches.
Early in the flower-opening cycle, the stem holding the Yellow Flag flower bud extended
in length so that the bud reached above the 2 leaf bracts that had surrounded it the
day before. Subsequently, the flower bud expanded near the base several hours before
opening. A top view showed the 3 sepals curled into a swirl near their tips (Fig. 1). The
opening of all sepals to nearly full extension occurred literally from one glance to the next
in ~1 second. However, as in other irises, the 3 (tiny in this species) banner petals stayed
upright. The flower remained fresh for 2 days, after which the petals again coalesced into
a coil around each other and then shrank. The ovary grew, and the remains of the petals
dried and dehisced.
Flower opening and closing, as well as some other movement in plants, (e.g., Mimosa
leaf movements for predator defense; Volkov et al. 2010), involve volume changes in different
structures that expand or shrink as water is shifted in or out of expanding tissue. Water
shifts by osmosis result in part from uptake of sugars and conversion from polysacharides
to monosacharides (van Doorn and Kamdee 2014, van Doorn and van Meeteren 2003). Iris
flower opening has been reported to require elongation of the flower pedicel and ovary,
which is largely controlled by plant-growth hormones (Celikel and van Doorn 2012, van
Doorn et al. 2013).
Growth and osmotic pressure are involved in iris-flower expansion, but such gradual
processes might not explain the mechanics of the sudden movement I observed in Yellow
Flag. The growth of the pedicel beneath the flower is rapid and apparently allows the
*PO Box 153, Weld, ME 04285; bheinrich153@gmail.com.
Manuscript Editor: Greg Spyreas
Notes of the Northeastern Naturalist, Issue 22/3, 2015
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flowers to open very quickly. But the snap of petal extension, as observed here, would
require prior storage of energy, followed by a trigger mechanism for sudden release,
perhaps following the same pathway that causes the forcible ejection of seeds by a number
of plant species (Stamp and Lucas 1983), including the common Impatiens capensis
Meerb. (Orange Jewelweed) that can throw its seeds several meters from the fruit (Smitt
et al. 1985).
Sudden movements resulting from release of stored energy are common in arthropods
(Heinrich 1993) and involve a variety of physical arrangements in which muscles may contract
slowly, while the energy of their contraction is stored by a hold-fast. Release of the
stored energy occurs in a burst, but only after a specific threshold of force is reached that
overwhelms the hold-fast. Presumably, the iris’s stored energy would derive from increased
water pressures in the flower tissues which would cause them to bend under tension; the
banner tissue might become inflated relative to the sepal tissue that covers it in bud. This
scenario would require a holding mechanism. In this case, just prior to opening, the petals,
which are wrapped around each other in the unfolding bud, appear to be held in place by
their contact near the sepal tips (Fig.1). This stable position might then persist until sufficient
outward force builds up to release the hold of the petal s around each other.
The functional significance of the iris flower’s structure, flowering schedule, and rapidopening
mechanism, may have evolved under the selective pressures of pollinator behavior.
Figure 1. Sketches from life and photographs of the stages in the cycle of a flower. Numbers 1–4 show
flower-bud development. Number 4, with side and top views, show the bud before opening. Number
5 shows side and top views of the flower 1–2 seconds after openi ng.
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Although I observed Archilocus colubris L. (Ruby-throated Hummingbird) visiting the iris,
these flowers likely evolved under the selective pressure of large bees that visit the flowers
in search of nectar. Naïve bees travel and sample widely, but eventually tend to frequent
flowers that provide sufficient rewards (Heinrich 1976). Thus, if a bee keeps encountering
empty flowers that she identifies by their location and floral signals, she will learn to avoid
such flowers. Each iris flower’s flag-like sepals have 2 nectaries at their base. I used 20-μL
disposable glass pipettes to extract the nectar from individual nectaries in the Yellow Flag
flowers I studied and found that they yielded up to 2 μL. This nectar would not be enough
to satiate a large bee, but it is probably enough to stimulate her to search for another flower
of the same kind (Heinrich 1975b).
My isolated Yellow Flag plant produced seeds. The plant was growing in a forest clearing,
with no other plants of its kind; thus, these seeds were unlikely to have been the product of
cross-pollination (except for perhaps some hybridization with other species of iris in the area,
though that is thought to be uncommon; G. Spyreas, Illinois Natural Hitory Survey, Champaign,
IL, pers. comm.). The fruits of this plant eventually dehisced, and the seeds were large
enough (7–8 mm in diameter) that they likely would have dropped straight down. Yellow Flag
also spreads by vegetative growth. Taken together, these observations suggest that clumped or
neighboring Yellow Flag might reflect self-pollination or vegetative reproduction.
In order for this plant to achieve cross-pollination, pollinators may need to travel far.
Plants using nectar as a reward must offer an adequate amount of food energy to retain
flower-constant bees (Heinrich 1975a, b). But, a bee that repeatedly visits the same plant
(or clone) might not outcross it unless it also visits other (perhaps isolated) individuals
of the same species. With its relatively large seeds, which might limit dispersal potential,
neighboring Yellow Flag plants could become genetically inbred without longer distance
outcrossing events. Have these irises evolved mechanisms to encourage cross-pollination
through increased inter-plant pollinator travel, despite the potential costs?
Yellow Flag flowers must be visible from afar to attract naïve bees, and at the same time
its large yellow sign must be an honest advertisement of reward. The complex flower, however,
could be a misleading signal if nectar availability was delayed by slow, gradual flower
expansion. Instant flower opening would preserve honesty of advertising because naïve bees
will not be fruitlessly attempting to enter it before nectar is present. Furthermore, the briefer
the duration of the flower’s availability for fertilization, the less likely it will be revisited
by the same bee. Comparative data with other species’ natural history might help to bracket
potential selective pressures for the observed explosive flower opening, and differentiate it
as either an adaptation for increasing the percentage of out-crossed versus selfed flowers,
or as an incidental by-product of chance.
Acknowledgments. I greatly appreciate the critical reviews and helpful suggestions by
John Alcock and 2 anonymous reviewers.
Literature Cited
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iris flowers but strongly inhibits it in water-stressed flowers. Journal of Plant Physiology
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Heinrich, B. 1975a. Bee flowers: A hypothesis on flower variety and blooming times. Evolution
29:325–334.
Heinrich, B. 1975b. Energetics of pollination. Annual Review of Ecology and Systematics 6:139–170.
Heinrich, B. 1976. Foraging specializations of individual bumblebees. Ecological Monographs
46:105–128.
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Heinrich, B. 1993. The Hot-Blooded Insects. Harvard University Press, Cambridge, M A. 600 pp.
Smitt, J., D. Ehrhardt, and D. Swartz. 1985. Differential dispersal of self-fertilized and outcrossed
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Stamp, N.E., and J.R. Lucas. 1983. Ecological correlates of explosive seed dispersal. Oecologia
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van Doorn, W.G., and C. Kamdee 2014. Flower opening and closure: An update. Journal of Experimental
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van Doorn, W.G., I. Dole, F.G. Celikel, and H. Harkema. 2013. Opening of iris flowers is regulated
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