Sonicatin’ in the Meadow

30 Sep


Rhexia cubensis (and additional species)


When it comes to meadow flowers, the fairest of them all are Meadow Beauties (species of Rhexia), with their delicate pinky-lavender, or less often white or yellow flowers.  Do beauties in the meadow resemble the big Tibouchinas in the garden?  Yes, they’re all family, the Melastomataceae, along with Tetrazygia bicolor (misnomered West Indian “Lilac”) a popular shrub in native plant gardens.  Melastomataceae have elegantly curved leaf veins, and have unique oversized yellow curved anthers. (Anthers are the pollen-producing flower organs.)


About 10 species of Rhexia reside in Florida, representing the lion’s share of this 13-species genus mostly confined to the U.S. with minor extensions into Canada and the West Indies.  Rhexias rise up after disturbance and can spread vegetatively.  Some have tubers and/or creeping horizontal roots pretending to be rhizomes.

In our immediate zone of exploration, Rhexia nuttallii is the only species that has its anthers in a radially symmetrical arrangement at the flower center.  In the other species the anthers are longer and in a lopsided bilaterally symmetrical arrangement.  Rhexia cubensis has glands on the stem hair tips and particularly bristly fruits in contrast with glandless hairs in R. mariana and less-bristly fruits.  Rhexia nashii too has the stem hairs glandless, and differs from R. mariana by having non-bristly (or nearly so) fruits.

Do you find these beauties to be sometimes hard to identify? They vary; for instance, Rhexia cubensis has diploid, tetraploid, and hexaploid strains.  (This refers to chromosomes in pairs, or sets of four, or sets of six.) Some species hybridize.  Rhexia mariana is especially known for mixing it up with its kin.

The anthers are bright yellow, presumably as snazzy come-ons for bees, yellow suggesting yummy pollen.  There is a little controversy around that topic.  Yet yellow decoration of bee flowers is widespread, and the bee-draw of yellow on flowers is generally accepted.  (And suggests some fun experiments.)  Certain Melastomataceae have two types of anthers:  big yellow advertising anthers along with smaller, drab-colored “business” anthers to dust the bee visitors with pollen.  If dual-anther types exist in Rhexia I’m not aware of it.  How they do function in Rhexia is fascinating, however.  So read on:

They employ “buzz pollination,”  a phenomenon found in about 8% of flowering plants dispersed among unrelated families—thus a great example of convergent evolution.  Many members of the Potato Family, Solanaceae, feature buzz pollination, and now back to Rhexia.  Most pollination research has involved Rhexia virginiana, which is absent around here, so we’re extrapolating recklessly to our local species.

Those big, elongate, curved anthers draw bees seeking pollen as an important baby-bee protein source.   In most non-buzz flowers the anthers open, wads of pollen spill forth, bees get dusted, and off they go.  But that is a crude approach vulnerable to bee greed, wrong pollinators,  bad timing, and weather.  In Rhexia and other buzz flowers the anther has a single pore at the tip.  The pollen remains safely inside the anther until good vibrations puff some pollen out pore.  Not just any bee can get a buzz on.  Forget honeybees.  Bumblebees do the job, and so do multiple additional types of native bees.  A fancy term for buzz action is sonicating.

Enjoy  simulated sonication here: BUZZ

Now this is just pure speculation, but don’t those anthers shaped like a bent bottle with an expanded chamber at the base and a narrow neck look like guitars where the music box “amplifies” the buzz?  In some Rhexias the anthers change color on their second day.  Interestingly, the vase shape of the anthers repeats in the fruit, which likewise has a swollen base and narrow neck, dispersing the tiny “cochleate” (snail-curled) seeds.  As in so many other wetland plants, the tiny seeds have sculptured surfaces.

The buzz system rations pollen.  In Rhexia (virginica) only 10.2 % shakes forth per visit. Thus to complete its load a buzzy-bee is compelled to move along, and plenty of pollen remains behind for other bees on other days, extending the effective life of the flower.

[The photo is R. cubensis, by John Bradford.]


Posted by on September 30, 2012 in Meadowbeauty


Tags: , , ,

2 responses to “Sonicatin’ in the Meadow

  1. Steve

    October 1, 2012 at 12:46 pm

    I found a couple new Melastomes in my Florida botanical journeys.

    I collected Melastoma malabathricum back in 2000 from Danforth park (didn’t know what genus it was at first, and originally guessed it as a Tibouchina). It was a beautiful thing, big shrub with big showy flowers and likely in cultivation. Given it’s showy nature, I was confident that it wasn’t native (it being native to India). I fear exotic melastomes invading FL, as they love acid soils, and it seemed a reasonable threat. So the land managers eradicated it immediately (good for them, and us).

    Six years ago I found another Melastome. The genus is Acisanthera, and I originally determined it to be A. quadrata, B. Hansen upon receiving the specimen redetermined it A. erecta. Alas, I had limited time and resources to track down an acceptable monograph for the genus. Good thing I didn’t publish it with an incorrect name. (It should have been in the most recent Guide to Vascular Plants of FL, but herbarium politics prevented a timely exchange with USF). Anyway, I bring it up because he interestingly determined it to be not native (they tend to do that at USF for any recent arrivals). I haven’t been able to determine the full range of this species (A. quadrata is as close as the Caribbean), but A. erecta is at least native to Venezuela, where it’s type specimen is from. I found it in a fairly remote place outside of Naples (as remote of place that could possibly be in this day and age). I cannot imagine anyone bringing this puny thing in as a wildflower. Seeds are tiny (common name is Dustseed), and could have easily been brought in by birds if not storms. I haven’t been back to the site to determine if it was a waif, or whether it might still be there. Surely we all agree that biological organisms can still move around naturally, and things still pop up here and there likely having arrived through non anthropogenic processes. I’d be curious to know what your thoughts are.

    • George Rogers

      October 1, 2012 at 2:51 pm

      Thnak you Steve, lotsa info there. I have some thoughts, none profound or original:

      I totally agree that natural forces still move things around, as do semi-natural forces and unnatural forces. Drawing the “red line” between some of those is impossible, as everything is connected with everything. There are a whole lot of “pantropical” (or otherwise widespread) weeds where the exact point of origin is impossible to say, and their ranges keep expanding.

      My leading thought is that Global Warming is warming the Florida reception for otherwise tropical species arriving here in the many ways a species can come. A few years ago my biggest “find-an-unexpected-species” thrill was an endemic African grass growing wild in Barbados. Could have arrived in any of many ways, but the fun interpretation was seeds in the “Africa Dust” that rains down heavily on Barbados, sometimes even dropping insects.


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