Naked Embryos and Dead Man’s Fingers

18 Nov

Black Mangrove

Avicennia germinans


This week’s Friday field trip by John and George got nuked by a combination of work obligations, rain, and car trouble.  With reference to the last-mentioned, this post was written in the commodious Napleton Nissan/Kia Service lounge in Riviera Beach, with complimentary coffee and People Magazine.  So we’ll step back in time just a couple weeks.  Black Mangrove (Avicennia germinans) surprised us by practicing dispersal in massive quantities by bare naked embryos.  We found thousands of them in the tidal wash on the beach at the Hobe Sound Wildlife Refuge across the Intracoastal from Tiger Woods’s spread.  Everybody knows that flowering plants usually disperse as fruits or as seeds, but as exposed embryos?  In saltwater?  Yes.  And some were already rooting on shore, making it clear how you wind up with extensive stands of Black Mangrove.

Black Mangrove embryo castaways on the beach (photo by JB)

Googlization reveals this peculiarity to be old hat to mangrove cognoscenti, even being the basis for the specific epithet “germinans.”  But we’re nobody’s cognoscenti, thus our happy surprise at the little embryonic nudists.  It is also old hat to some plant propagators who soak Avicennia fruits to liberate the embryos for sowin’ and growin’.  By the way, the name Black Mangroves refers to the dark-colored wood.

Embryos from Avicennia species can remain viable over 6 months submerged in sea water and can float alive at least 50 km, although most dispersal is local.  The embryos are large, fleshy, and well provisioned before release.  They look like the innards from a giant lima bean.

Having escaped their fruits and seeds, and having enlarged, they  are in a sense prematurely germinated, and in this way vaguely resemble the precocious seedlings dropping from the unrelated Red Mangroves with a big prematurely sprouted embryonic root sticking out of the persistent fruit covering while still on the mama tree.

Inquiring minds may now ask, well, what about the third Musketeer,  White Mangrove?  Its fruits disperse in the traditional fashion—intact—but they have some “pregermination” going on too, as the seeds can sprout inside the fruits during dispersal.  This shared tendency is an interesting tidbit of convergent evolution where three different and unrelated species have all adopted premature germination, in different forms, to meet the challenge of saltwater-drift dispersal.

Dead man’s fingers (photo by JB)

Anyone with much experience hanging around docks has encountered spooky black gnarly “dead man’s fingers” rising vertically among Black Mangroves.    More convincingly than Bald Cypress “pseudo- pneumatophores,”   Black Mangrove pneumatophores have obvious adaptations as root snorkels.

The fingers are filled with spongy tissue suitable for gas exchange.  The finger lengths adjust to the need for air, and lenticels (breathing pores) on the fingers reportedly open and close in response to environmental conditions.





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Posted by on November 18, 2011 in Black Mangrove


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