Knotted Spikerush All Over The Place (Yet We Scarcely Know It)

29 Dec

Eleocharis interstincta

(Eleocharis translates loosely as beautiful marsh-dweller.   Interstincta means speckled.  The basal leaf sheath is dotted with dark purple.)

Cyperaceae (Sedges)


Today’s botany time with John was in Savannas State Park near Jensen Beach, Florida preparing a friendly grass, sedge, and rush public workshop for the morning of January 13.  Join us!   CLICK for a link. 

Most of the park is beautiful, low, and marshy….and in this morning’s cool fog, mysterious.   The perfect encounter for emergent aquatics, which have their own peculiar ecology, such as by forming massive rhizomatous monoculture populations spreading until they engage in a slow-motion collision with opposing spreading species.  Knotted Spikerush  is one of the most abundant and conspicuous emergent species of open shallow water in South Florida, lining shores and blanketing acres giant marshland lawns.     But the species remains oddly under-studied.

E. interstincta 2

Knotted Spikerush, monoculture.  Today’s non-microscopic photos a mix by John Bradford and by me.  I’ve forgotten who took which.

Has it expanded like Cattails as a result of nutrient pollution?  Unclear.    Limited research fails to show much effect from varied nutritional levels, although there is room for far more data.

Eleocharis interstincta short stem

Short stalk with flower cluster, surrounded by tall stalks.  Not much wind down here!

A Spikerush oddity has bugged me for decades:  the flowering stems exist  in two distinct heights.  Tipped with a flower cluster, most are around a yard tall.    Others are mere inches tall, mixed with and “at the knees” of the taller stems.  Why stunted stalks?  To take advantage of two different pollinators?  This is almost certainly a predominantly wind-pollinated species, with those tall stems waving in the breeze.    But the shorties are tucked down far below the windy heights, maybe supplementing wind pollination by attracting insects  down in the shadows.  There are hints of plausibility:  the flower heads are showy white, and those on the similar Eleocharis elegans are fragrant.  (Fragrance in E. interstincta is subtle, or at a time I’ve not sniffed, or absent.)

E. interstincta inflorescence 2

Flower cluster at stem tip.

A likely alternative explanation for the height disparity might relate to fluctuating water levels, where the short spikes hold forth during the drier months, and the tall ones in the high-water season. This week during late December short stems are in full bloom, as in the photo above taken this week,  whereas the tall ones are instead forming or dispersing fruits.   Seasonality is consistent with insect visitors, but I’ll place my money on fluctuating water levels until I catch bugs in the act.

Now another mystery to ponder.  The stem tops cook under the tropical sun, but the lower regions, rhizomes, and roots lie smothering under water and mud.  How do emergent marsh plants ventilate the basement?    Many have pressurized air exchange.  The exposed tops pump fresh air down where the sun don’t shine, like mechanical pumps aerating a mineshaft.

Most marsh plants have open channels and/or porous ventilation tissue called aerenchyma leading from their airy tops to their suffocating roots.     In Eleocharis interstincta and some related species the stems are hollow except for thin partitions making the stem resemble a miniature bamboo.

Eleocharis interstincta partitions

The stem partitions have a bamboo look.

The disks are made of loosely packed cells permeable to pressurized gases, as I’ve tested by blowing through to make bubbles.    Additionally, as visible in the electron microscope image below, channels around the stem periphery may facilitate air movement.

Eleocharis interstincta 3_preview

Porous stem partition surface, scanning electron microscope image, courtesy of Dr. Robert Wise.

Having open tissue helps, but you need to force fresh air down deep, and to let exhaust escape.   Studying an array of emergents, plant physiologists have identified three main systems to force airflow.

  1. Wind blowing across broken stems lowers their internal pressure, this drawing in air from other stems less exposed to wind. Probably unimportant with today’s species.
  2. Engineers move gases with a “Knudsen pump*” that works by having a hot chamber connected by tubes to a cold chamber. The hot chamber expands the gases within, generating pressure, just like car tires have higher pressures when hot.   The cold chamber, by contrast, has diminished pressure, allowing gases to flow from the hot high pressure end to the chilly low pressure end, and then escape. In a spike rush, the warm chamber is the hollow stem portion up in the sun.   The cold chamber is the portion down in the cold water.    Knudsen pressure pushes warmed air down to the cool lower regions where it vents out.
  3. The inside of the hollow stem is humid due to submersion in water and because all stems contain water. Because the water vapor accounts for much of the gas inside the hollow stem, other gases are more concentrated outside.  As those outside gases move inward to even out their concentrations they add to the internal stem pressure.

Our local Knotted Spikerush has never been studied pressure-wise.  But it bubbles from the submerged base.

Eleocharis bubbles2

Bubbling underwater root on a clump with the top up in hot sun.

Bubbling is not definitive proof of pressure from the top,  but there is an even better indicator:  detailed quantitative study of the pressures and gases in the related E. sphacelataCLICK HERE.

Emissions from marsh plant roots appeared in the blog last week where manganese oxide stained the root paths at the hands of oxygen-using soil bacteria.  Oxygen released in the root zone supports favorable microbes and diminishes toxicity.

A tight-fitting sleeve rising as high as a foot wraps around the base of the stem and around the region where the stem base, rhizomes, and roots come together.   Perhaps the sleve prevents gases from escaping prematurely, although I’m just guessing.  Interestingly, in the much-studied marsh grass  Phragmites the basal sheath helps generate the gas pressure.

Eleocharis interstincta sheath

Knotted Spikerush purple leaf sheaths. Do they prevent gases from escaping prematurely?

After pollination, the small flowers disappear behind little scales where the fruits mature.  Like many marsh plants, the fruits are oddly attractive, having a beak on top, bristles rising up around the fruit, and a decoratively sculptured surface.  Such sculpturing is common, maybe even characteristic, of the fruits or seeds of unrelated marshland plants, so there must be something “good” about it.    Botanists speculate the waffle sculpturing may help the fruits (or seeds) cling to mud on the legs and bodies of animals, wading birds for instance, thus helping with dispersal. That might help explain the spotty geographic distribution of the species.

E. interstincta achene

Fruit, with sculptured surface,  surrounded by barbed bristles.   Built to cling!

*For the sake of technical correctness, I feel duty-bound to note that in an actual engineered Knudsen pump,   the gas molecules are more concentrated in the cold end, so there is a movement of molecules back toward the warm end.  How it works overall will depend on the inlets, outlets, and venting.


Posted by on December 29, 2017 in Knotted Spikerush, Uncategorized


2 responses to “Knotted Spikerush All Over The Place (Yet We Scarcely Know It)

  1. theshrubqueen

    December 29, 2017 at 7:20 pm


  2. Uncle Tree

    December 30, 2017 at 11:12 am

    Love the info and the micro in every post, George and John!
    Hope you both have a happy and fruitful year! 🙂 Peace, UT


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