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Giant Whitetop, Broadleaf Painted-Sedge

19 Jul

Rhynchospora latifolia

(The name means wide-leaved beak-seed.)

Cyperaceae, the Sedge Family

Today’s wildflower is the chosen topic because it is so pretty and so eye-catching now in mid summer.   Rhynchspora latifolia decorates wet meadows and similar habitats standing 3 feet tall, waving its white and green flower heads above its neighbors.    Its smaller cousin Rhynchospora colorata has fewer white lobes, has the lobes not narrow abruptly at the green parts,  often grows more crowded, and prefers slightly less wet sites. According to reports R. colorata prefers alkaline as opposed to R. latifolia in acid locales, although I suspect that habitat difference is a bit overstated.  I’ve seen the two growing within a few yards of each other. Checking that out with a pH meter would make a great class research project.

rhyn lat flower head

The white “petals” are leaves (bracts).

What’s famously odd about the two rhynchosporas, additional Rhynchospora species not represented locally, and a smattering of other sedges and grasses is that they are fully or partially insect-pollinated species in huge plant families the textbooks generalize as wind-pollinated.  These are plants whose ancestors turned away from insect-pollination to wind, then the painted-sedges reverted to insects..

That’s like going back to golf after you throw away the clubs.  They had to re-evolve all the insect-related pollination apparatus starting from scratch.   Colorful petals were long-gone so the replacements are false petals made of white leaves (bracts) green at the tips.   Nectar is gone too, but there’s bright yellow pollen to attract and feed visitors.   What’s dismaying is that these flowers have re-evolved sweet floral perfume, released it seems only briefly in the morning. Most of the day they are without fragrance, but at perfume time they are surprisingly potent.

Morning seems to be the main time for action.  That is when all or most of the pollen-bearing anthers come forth, although they can persist all day.   That is also when most of the pollen-receptive stigmas come forth, although some may be apparent all day.

Rhyn lat stigmas showing 2

Spikelets (each thick white unit is a flower cluster).  Anthers are yellow. Stigmas are smaller, finer, and white.   Each spikelet is wrapped in bracts and contains several crowded flowers.

In each tiny flower the pollen-making anthers emerge from the bracts enclosing them and start releasing pollen before the pollen-receiving stigmas mature and emerge.  The anthers are bright yellow and prominent.  The stigmas are white, small, and held close to the flower head. Male-first is called protandry (PRO-tand-ry).  However, the flower head is made of hundreds of flowers, so that even if one flower is “male,” adjacent flowers may be in the female phase, or transitional.    Thus the entire head can be a mass of male and female flowers, although late in the day the balance shifts strongly to persistent stamens as most of the stigmas wither.

Rhynchospora latifolia anther tips and good stigmas 731AM 2

Spikelet in early morning. Anthers just peeking forth. Stigmas (white)  are from different flowers in same spikelet.

The species needs a preponderance of anthers as opposed to stigmas, because the roles the anthers have in attracting and feeding pollinators, and in manufacturing massive quantities of pollen.

If functionally male and female flowers can be mixed close together in the same spikelet, does self-pollination occur?   Evidence that effective self-pollination is unusual or nonexistent is that  many flower heads mature no fruits, or very few, while some others are productive.  Fruit productivity reflects luck in cross pollination.  No visitors from other flower heads carrying pollen from afar, no fruits.

If thwarting self-pollination does not seem a  key reason for  protandry in individual flowers, what is the benefit of a flowering making stamens first?   I don’t know, but have a speculative notion:

As the stamens and stigmas mature they have to pass through a narrow bottleneck of tiny leafy bracts in order to see the light of day.  If the soft and delicate stigmas matured first they might be crushed in the bottleneck by the large, firm, pollen-filled anther bullies.   Like letting the mice off of Noah’s Ark before the horses.    Once the anthers have “cleared the door,” it is safe for the more delicate stigmas.

rhyn lat spikelet diagram

Reported visitors include bees and hoverflies (syrphids), an interesting combo given that both feed on or gather pollen.

 

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2 responses to “Giant Whitetop, Broadleaf Painted-Sedge

  1. theshrubqueen

    July 20, 2019 at 8:23 am

    My favorite sedge.

     
    • George Rogers

      July 20, 2019 at 9:11 am

      Me too

       

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