A Flower’s Reproductive Antics

For two hours on a wet, summer day, my friend and colleague Grace Glynn and I did little more than sit in one place to discover, ponder, discuss and enjoy the antics of Fen Grass-of-Parnassus (Parnassia glauca). More to the point: we spent two hours understanding how this plant’s flowers have sex with one another.

Two hours. One place. One plant. I consider that an act of rebellion.

To be everywhere at once is to be nowhere forever. — Edward Abbey

Diversion and distraction are now human afflictions. Blame the internet in part. FOMO, fear of missing out, is defined as the feeling of anxiety owing to the belief that our friends, posting to social media from somewhere else, are doing something better than we are. “To be everywhere at once is to be nowhere forever,” Edward Abbey once wrote. Even nature can offer a kind of FOMO, particularly among birdwatchers, those listers who turn their gaze away from their bird in view toward notions of one better up ahead, toward the “what’s next.”

For Grace and me there was no what’s next. Fen Grass-of-Parnassus was enough. That’s because this plant is calculating and methodical in its game of reproduction (including how it welcomes that rare bee to its nectar rewards each autumn). So I will distill for you two hours of flower observation into about two minutes of reading:

Image 1 – The first stamen begins its rise to the occasion.

Have a look at this first flower (Image 1 – click for a bigger view). I’ve labelled key parts: a pollen-producing anther sits atop a filament — together they constitute a stamen, five of which encircle a green pistil (barely visible), where fertilization happens. Those “staminodes” are modified stamens advertising nectar to potential pollinators.

Also know that most flowers do not mate with themselves. One way flowers prevent that kind of self-pollination is for their pollen and pistil to mature at different times. By contrast, in cross-pollination, one flower’s pollen fertilizes a mature pistil on another plant’s flower, which maximizes genetic diversity — a good thing in the struggle for existence.

Like many plants, Fen Grass-of-Parnassus relies on insects to carry its pollen from one flower to the next, luring bees and flies with the promise of nectar, with those green stripes (nectar guides) pointing the way. As an insect finds nectar at the base of each flower, it gets dusted with pollen (or gathers it in the case of some bees) before flying off to another bloom.

But Fen Grass-of-Parnassus plays a long game with its insects. Instead of producing pollen from all five stamens simultaneously, it issues pollen sequentially, one anther at a time (and probably one anther per day). Better yet, each anther goes to great lengths — literally — to touch its pollinators.

Note how the anther I’ve labelled above in Image 1 is elevated on its filament and arched over the pistil. It is the first of the five to make this extension — moving into position to more easily address a visiting insect with pollen (which has yet to emerge from that first anther). After a day or so in place, dishing out its pollen, the stamen will swing outward to make room for one of its brothers to take its turn directly over the pistil. This goes on sequentially until all five stamens have made the maneuver.

Image 2 – The last of five stamens in place.

In Image 2 note how the fifth and final stamen (at about 4 o’clock) has arched itself over the pistil, with pollen still visible on its anther. The other four anthers, having already completed their mission, are brown and spent, their filaments extended and retracted toward the outer edges of the petals. All the while, that green pistil has not yet developed its stigma — the sticky portion that receives pollen before germination. No stigma, no self-pollination from those visiting anthers.

Image 3 – The pale, four-parted stigma rises.

Finally, Image 3 shows a flower, late in the game. Its five stamens have completed their journeys. The staminodes remain, luring insects. But note that the green pistil — dead center — now sports a well-developed, four-parted, pale stigma, which will capture, at long last, pollen from a fly or bee visiting from some other Fen Grass-of-Parnassus plant.

Grace

So there you have it: a plant that methodically rearranges and synchronizes its reproductive parts — a kind of dance that Grace and I could not see in action, but could infer and understand over time.

No distractions: no email, no politics, no tweeting, no pandemic. Instead, only our hand lenses, our notebooks, a botany manual (Fernald 1950) and a few photos taken now and then so that we could bring this tale to you. Well, not only that. We also had the curiosity, exuberance, temerity and patience to do little more than sit beside a trail watching the flowers of Fen Grass-of-Parnassus going about their business of reproduction.

I can think of few better ways to spend two hours on a late-summer day.

Postscript: I had waited a year to watch this Parnassus performance. One year ago I visited with these same plants, but only to seek out one of their pollinators — a rare bee called Parnassia Miner (Andrena parnassiae), which I’m told had been rarely photographed alive on its favorite flower. Only when I wrote about the bee a year go did I learn from friend and botanist Steven Daniel about these anther antics. If you care to read last year’s post, you will not only learn about the bee and the flower, but also about a beetle that, having been swallowed by a frog, proceeds to crawl out the frog’s butt unscathed. (Okay, now THAT’S a worthy distraction.)

The rare Parnassia Miner (Andrea parnassiae) bee on its flower. / © Bryan Pfeiffer

References and Notes for Botanists

• Armbruster, W. Scott, et al. 2013. In the right place at the right time: Parnassia resolves the herkogamy dilemma by accurate repositioning of stamens and stigmas.  Annals of Botany. 2014 Jan; 113(1): 97–103. Published online 2013 Nov 20. doi: 10.1093/aob/mct261
• Fernald, M.L. 1950. Gray’s Manual of Botany: Eighth (Centennial) Edition — Illustrated.
• That paper by Armbruster above really helped us figure out these flowers (thanks, Steven!). Even so, Grace and I left the Parnassia patch with questions: Does each anther really position itself over the pistil for only one day? Does each stamen extend and position itself in a consistent, sequential pattern? Clockwise? Counterclockwise? We’ll be back next year to find out!
• Those staminodes are also intriguing. Fernald says they are nectariferous. It would seem that the yellow-green “globules” would be so. But we’ve read that their nectar source lies at their base. Another thing about the staminodes: there are five of them, equal to the petal and stamen count (and not 15 as might seem to be the case from my photos); it turns out each one of the five staminodes are three-parted, connate at their base, and extending into “little tridents whose prongs are topped with tennis balls” (my term, not official botanical parlance).