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Rubbervine Among the Mangroves

17 Jul

Rubbervine

Rhabdadenia biflora

Apocynaceae

Peck’s Lake, which is a part of the Intracoastal between Hobe Sound and Stuart is a popular fishing hole, and on the shore is a boardwalk providing comfy passage through swamp then mangrove jungle, past a shell midden, and onward to the salty shore with an observation dek to watch the boats go by. The species along that transect are plentiful, including magnolias, mangroves, and mastics. And porpoises, said hello as we stood on shore. All three mangrove species along with their friend Buttonwood were in flower.

Rubbervine at Peck’s Lake, Florida. (This and all photos today by John Bradford.)

The showpiece was the pretty Rubbervine, or Mangrove Vine, with its large white blossoms scattered all through the White Mangrove zone and spilling over onto the boardwalk.

Why a vine would select mangrove stands as its main domicile is a question to ponder. Perhaps that’s a select opportunity for the “right” vine where the brackish conditions suppress competing twiners and climbers.
OKAY then, why can this particular species thrive there in the brineland? As a broad speculative possibility, it is a member of the tough Dogbane Family, the Apocynaceae, which tend often to tolerate harsh living, thin nasty soils, blazing sun, and, probably most importantly, dryness. But a mangrove stand isn’t dry, is it? Well, yea, physiologically dry, with a saline soil. Osmosis you know. So maybe standing up to that sun and salt is in the vine’s family tree. The flesh drips white milk when broken. The toxic latex is generally regarded as a feeding deterrent, but I’ve always wondered if the latex, found commonly in Apocynaceae, also helps with all that dryness we’ve been fretting.

Rubbervine and other Apocynaceae are larval host to various sphinx (hawk) moths, who drink themselves nasty on the poisonous plant sap. They proclaim their toxicity with bright warning coloration.

The blossoms are showy, the fruits are mildly weird, and the seeds are even weirder. Starting with the flowers, they are presumably pollinated by sphinx moths, given that they are standard “moth” flowers, white with a long narrow tube penetrable by little other than the long mothy proboscis.

The gate at the flower throat.

Being members of the Apocynaceae, the flowers have a particularly noteworthy family characteristic. They are more or less funnel-shaped, and just at the point where the funnel narrows a “cap” made of five anthers pressed together edge-to-edge blocks the way. (A little reminiscent of a folded paper cootie-catcher if you are old enough to know what the heck I’m talking about.) You need a long fine needle to get between those anthers and poke deeper into the flower, a great gatekeeper for allowing a proboscis probe while thwarting “nectar thieves.”

It gets weirder. The stigma (pollen receptive surface) sits immediately under the anther- cap. The stigma is in the shape of a can on a stick (the stick is the style, a narrow stalk). The business part of the can-shaped stigma is the underside of the can, the bottom end. When a proboscis probes the flower, pollen is scraped off during proboscis removal, as it slides out. The Apocynaceae are the only plants I know with scrape-pollination on the bottom of the stigma.

Flower slit open to show the anther cap (the cone) at the throat. The stigma hides under the cap. The long thread is the style.

Then come the fruits resembling perky paired beans. Apocynaceae fruits generally come as twins, like bunny ears. Some readers may have observed this in Frangipani or Madagascar Periwinkle. The pods open to release seeds that look like quill pens. Each seed is two inches long counting the poofy quill. The body of the seed is a long (maybe an inch) heavy torpedo, which open landing penetrates the thick mangrove stand vegetation all the way down to the mud. How many wind-dispersed species do you find in mangrove stands? It’s a pretty good system, allowing the vine to spread through any given stand and to hop to another. It would not be hard to imagine the silky parachute clinging to the landing gear of a bird for those long hops.

Pod releasing seeds

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1 Comment

Posted by on July 17, 2012 in Rubbervine

 

Tags: , , , ,

One response to “Rubbervine Among the Mangroves

  1. Mary Hart

    July 18, 2012 at 3:45 am

    Ain’t nature fascinating!!

     

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