White Mangrove and Its Multifarious Roots

Laguncularia racemosa

(Laguncularia comes from Latin for a small bottle, in reference to the fruit.  A raceme is a type of branched flower cluster.)

Combretaceae

This morning John and I probed the saline mud around Peck Lake, near Pt. Salerno, Florida, attempting to discern a relationship between the flora and the increasing salinity levels approaching the shore of saltwater Peck Lake.    One thing I learned is that 66 11/12 is too old for scrambling over the handrail and down off of boardwalks to get personal with crabs in the mud below.  A good place though to think about the many adaptations Mangroves have to their smelly salt life.

White Mangrove by John Bradford.

Before we go on, let it be clarified that the word “Mangrove” refers to a tropical tidal swampy muddy briney lifestyle.  The many mangroves of the world are not related to each other, and the club has indefinite inclusion, where two honest observers may disagree as to whether a given species is a “mangrove” or not.

Today’s White Mangrove is more closely related to Tropical-Almond, Black-Olive, the vine Rangoon Creeper, and Buttonwood than it is to Red or Black Mangrove.    That mangroves are a result of convergent evolution is what makes them so interesting…the different ways disparate salt-swamp coastal species solve the same problems.    For instance, Red Mangroves block salt entry instead of dealing with it internally.  Reds reportedly have the lowest salt tolerance of the local Mangroves despite usually being the one with the wettest feet in the seawater.  Black and White Mangroves, by contrast, allow salt to enter the root and rise throughout the plant.

Both White and Black have tiny glands on the leaf surfaces able to pump out the brine. Yet there’s a difference.  Black Mangrove obviously dumps a lot of salt.  Its blades are usually white-crusty, and taste like a pretzel.   But White Mangrove foliage seldom has such crystal crust.  Excretion doesn’t seem to be as important to this species.   White sequesters salt in its thickened fleshy leaves, periodically discarding and replacing them.

Below is a quick and dirty demo. The first cup contains only tapwater, reading 44 salinity units (mS/cm), reflecting the fact that tapwater does contain salt, defined broadly.  Toss in 5 sliced White Mangrove leaves, wait a few ticks for salt to dissolve into the tapwater, and the salinity reading climbs in the second photo.

Tapwater with salinity measured (as EC in mS/cm).

Same water with White Mangrove leaves giving up their salt. Note increased reading from 44 to 58.

Salt glands are not the only curiosities on White Mangrove leaves.  Near the margin is a line of tiny black cavities.  These domatia are barracks for microscopic symbiotic mites.   The related Buttonwood has them as well.    On the leaf stalk (petiole) are two raised glands often misinterpreted as “salt glands.”  No…the real salt glands are microscopic, whereas those two petiole bumps are sugar fountains for symbiotic ants, in the right times and places.

Nectaries on WM petiole, by JB. The tiny smaller glands are visible scattered lower, on the leaf blade.

The most interesting parts are the roots.    First a little context.   Mangroves live in water or in tidal muds suffocating to unspecialized roots, so the Mangroves each have different coping specialties.   Red has famous prop root flying buttresses arching out from the trunk above the water.    Black has spooky vertical “dead man’s fingers” pointing heavenward  from buried horizontal roots.   White has an intricate root aeration system, which is hard to spot.

White Mangrove roots are shy, often submerged or nearly so in water or mud.  The system  entails at least four different root types, as displayed in the following 1970 diagram by botanist Jan Jenik (linked below):

Cable roots horizontal at base.  Vertical pegs rise from these, with small feeder roots along the pegs.  Clusters of little snorkels (pneumathodes) rise from the enlarged tips of the pegs.

Starting at the deepest, large thick White Mangrove roots run horizontally under the mud.   From these rise vertical medium-peg roots to a little below or a little above the water or mud.   These often have slightly swollen heads.  To the pegs are attached the thin feeder roots. What a perfect place to be a feeder root, since as the water and oxygen levels rise and fall the positions of the feeder roots probably rise and fall  by means of death and regeneration correspondingly on the pegs.

From Jenik, see text.

All those feeder roots need aeration.  For this purpose on the peg tips arise stubby, thick, short-lived chimneys (pneumathodes), venting air through a soft porous outer layer all the way to those needy feeder roots.

 

Pegs with pneumathodes today.

Outer layer of pneumathode is spongy.

Pneumathode has big white breathing pores (lenticels).

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[Forgive a non sequitur.  Part of the reason Bald Cypress knees do not seem to be root-zone air vents is that they are nowhere near the active feeder roots..]

Jenik article:

http://www.preslia.cz/archive/Preslia_42_1970_105-113.pdf