Author Archives: George Rogers

(More technically, Poikilohydric Species play possum)

This week’s fieldtrip is tomorrow, so with no crystal ball, so I’m taking a little pre- trip literally to the back yard.   Today we’re talkin’ plants with no roots and no internal plumbing, or almost none.    All water goes in through the foliage as it rains, but when dry the plants look and act like death, until resurrection when soggy returns.     Although there is a spectrum of possibilities and variation in mechanisms,  the interesting cases are not mere wilting or  closing up shop, but rather extreme deeply suspended animation, essentially lifeless.   You wonder how long a plant in that state can rise from the grave.  Answer: long.

Remember the term poikilohydric (= plant zombie).

Florida is a great place to encounter these resurrecting oddballs.   We have a lot of epiphytes, some of which behave poikilohydrically, most famously our Resurrection Fern (Pleopeltis polypodioides) as photographed by John Bradford:

Spanish-Moss and its cousin Ball-Moss (Tillandsia recurvata) may not be full-blown dry-and-diers, but they get mighty thirsty hung out to dry.

Ball-Moss on tree trunk

These species use umbrella-shaped scales (think micro-toadstools for second analogy in one sentence) to capture and distribute rainfall.

The scales on Spanish-Moss leaf.  By Robert Wise, Ph.D., Univ. WI Oshkosh, with permission. Scanning electron microscope. The scale is anchored like a toadstool at the middle.

One drop can irrigate an entire scaly leaf as we will see momentarily.  The scales have intricate structure, and flip down in the presence of water.  Water moves from scale to scale like an electric current traversing a wire.  Watch this video of one drop going a long way. CLICK TO SEE!

What is so astonishing is the depths of the Snow White slumber.  Hard-core cases go through profound internal biochemical and structural reduction and disassembly.  Internal membranes collapse or disappear.   Special protective proteins, enzymes, and electrolytes form.  Internal cellular components vanish.  Some go so far as to dismantle their photosynthetic apparatus, and reconstruct it upon remoistening.    Seen with a microscope the leaf cells look deceased.  Then it all comes back.

Below is a series of photos of a liverwort from a tree in my yard.  After the photo showing the liverwort on the hot dry bark are three microscope views of the same leaf at the same magnification, the first snapshot while dry, the 2^nd  photo 15 minutes after re-moistening, and the third shot about an hour later.   Notice that in the dry condition the cells look hollow with no apparent chloroplasts (the green disks in the later photos).  The chloroplasts are either gone  or collapsed into a thin film against the outer walls of the cell.   Yet just minutes post-moistening chloroplasts appear magically, and after an hour the previously deadish cells look perky.

Liverwort on hot dry tree trunk

A dry leaf, microscope view, from the liverwort.  Notice the hollow “empty” cells.

Fifteen minutes after wetting.  Chloroplasts (little green dots) already returning!

About an hour later

The reappearance of chloroplasts is a jiffy.  What is truly mind-boggling after wetting, however, is the instant return of respiration, intake of oxygen and expulsion of carbon dioxide, such as our breathing.    In their dry state poikilohydric plants show no detectable, biological activity.    Within seconds of rewetting carbon dioxide exhales.  (The very first carbon dioxide given off is probably not from metabolism, but merely forced out of nooks and crannies by the wetting, then metabolic carbon dioxide from life renewed follows in a few more seconds.) The graph below shows warp-speed carbon dioxide release by a remoistened moss.   So fast it’ll make your head spin.

Rapid carbon dioxide release.   Moistening was at about 650 seconds.  The fast dip happened as the water was squirted onto the dry leaf, perhaps from the gases in the pipette.  The mall first peak near 700 seconds is probably non-metabolic carbon dioxide, followed by the broader breath of metaboliic carbon dioxide as Rip Van Winkle awakens.  No time wasted!

Why that burst happens is not well understood.  I mean, why would the plant start respiring, burning sugars, before it is ready to replensih its own sugars by photosynthesis.  One line of thought is metabolism revs up like “gentlemen start your engines” before the transmission engages, that is, before all the plant’s membranes and internal structures return to working capacity.    The slumbering plant needs to work fast to exploit a fleeting cloudburst.

Poikilohydric lichens by John Bradford, followed below by moist and dry Selaginella by JB.

The distribution of plants with poikilohydric superpowers is broad and spotty.  The “lower” plants with no roots or veins need it, having no ability to take lift water from the soil and no pipes to move it internally:  some algae, lichens, mosses, and liverworts.     Rootless rain dependence may seem like a drag,   but it is a plus for living rootless on tombstones, telephone wires, and tree trunks.  Some plants with roots and veins, “vascular plants,” have poikilohydric representatives, including fern allies (our local Selaginella),   ferns, and assorted Dicots and Monocots, including a few grasses and sedges.

You might ask, how could so many different unrelated  plants come up with the same bag of tricks?  Well, convergent evolution is not rare, but more tantalizing, the ability of mature plants to go into quasi-death-dormancy is mirrored in something all seed plants have—seeds.  The genetic mechanisms that allow seeds to lie dormant and dry for centuries might extend sometimes in some species into adult life (and death, and life).

Hanging around the liverwort tree, by Donna Rogers

Well,  what do you do when the house is as ready as possible and you’re sitting waiting for Matthew?   Hunker in the bunker and write a blog.   Putting up the hurricane shutters revealed a little native Florida,  bringing forth lizards, a tree frog, a nervous zebra longwing, and a friendly wasp solo in its papery nest.  Hope they all creep, buzz, and flutter to safety.  Butterfly in a hurricane!?   Sounds like my autobiography.

Standing tall in the face of the big blow are Bald Cypresses.

Bald Cypress seed cones.  Photos today by John Bradford.

The relationship between Bald Cypress and Pond Cypress gets a word.  Modern molecular studies show the two as so close genetically that most botanists treat them as varieties of the same species, as for example Flora North America.

Our blog has a pre-existing condition on the tree’s knees:  CLICK FOR KNEES

Today’s question is,  why is Bald Cypress deciduous?    After all, we don’t have many fully leaf-dropping trees in South Florida.     So why such a prominent exception?

Why do deciduous trees exist to begin with?  In many warm climates having a dry season trees shed foliage when water is too limiting for photosynthetic efficiency.  I used to work in Venezuela where people raking and burning leaves at the curb reminded me of  October in Michigan.    Where’s the cider?

Maybe the Florida dry season could have something to do with the baldness.    But then again, these are trees of the wettest habitats, and only a few local trees shed in our not-that-dry winter/spring anyhow.   So dodging the Florida dry season is not a gratifying explanation.

As with all evolutionary questions,   it pays to look broadly in space and time.   Bald Cypress once spread across much of cold North America, to be shoved into the Southeastern U.S. by glaciation.    A northern origin helps explain deciduousness, given that most cold-climate trees dare to bare.  Perhaps Bald Cypress brought the condition from cold beginnings as I brought my ice skates.  Some of our other local deciduous trees, such as red maple and pop ash, are snowbirds as well, and interestingly swamp-dwellers likewise.

Yet writing off BC’s seasonal twig-shedding as northern baggage may not say it all.  This is the alpha tree of our swamplands,  and seasonal baldness seems to agree oddly with its Florida lifestyle no-matter the point of origin.

In the large world of conifers, deciduous species are unusual.    They are Taxodium, its close relatives Metasequoia and Glyptostrobus (which also makes knees),  and the more distantly related Larches, including Tamaracks.   Maybe this leaf-droppy crew tends to have something in common to help explain such rare deciduousness in conifers which otherwise cling to their needles even in the far north.  One thing the deciduous conifers have in common is prior life in ancient near-arctic wet forests.

No place for nice roots

The deciduous conifers all went through long formative periods where root function is especially challenged.  And yes, I’m out on a thin speculative limb.   Metasequoia is almost extinct so ignore it.    The other deciduous conifers flourish far from the ancient arctic, yet all with proclivities for habitats with impaired root function.   True of Bald Cypress?  Absolutely: no matter how you interpret the knees, they probably have to do with horrid soils, which not only are  oxygen-deprived under water but also probably not generous with nutrients, toxic,  and not hospitable to symbiotic microbes.

Not documented so far as I know for Bald Cypresses, but research shows deciduous larches to have special adeptness at transforming sugars from the foliage to  permanent stem wood before ditching  their temporary cheap leaves.

If Taxodium arrived here from far north, has it adjusted visibly to the Southeast after arrival?  Maybe that adjustment is the variety we call Pond Cypress.     Pond Cypress seems to have leaves adapted especially to diminish the water loss root-challenged trees can’t afford…less flat, arranged all around the twig, and pressed more or less together around it.

So to sum up my daydream, maybe Bald Cypress came southward pre-armed to deal with root-nasty conditions, and then went a little farther by spawning even-more-self-protective Pond Cypress?  Something to  think about here in the cone of increasing certainty.

Aristida sp.

Poaceae

Today John shot a new panoramic photo (gigapan) in Jonathan Dickinson State Park to continue documenting recovery after a prescribed burn.  That was before we left the IPad in the woods in the thundershower.  (No worries—it had a happy ending, and compelled a second trip to a lovely park.)

Monoculture.  See notes on photos below.

Prescribed burning keeps much Florida acreage in a fire-dependent  “early” successional stage.  A recurring thought overlooking such fire savannas is the prevalence of grasses, as opposed to understory plants from other families.  Several reasons, not of much interest today, help explain this, including fire-resistant rhizomes and the ability of grasses to rise from basal growth points after the tops succumb to  flames, to deer, or to John Deere.  Many grasses, including three-awns,  have what’s called C4 photosynthis, a special advantage in hot climates.

Let’s zoom  like a gigapan on one component of grassish domination… monocultures of just one grass genus, Aristida.     Aristidas are a big bunch, over 300 species around the warm world.   Why so successful?

To start with a fun if not important answer, their name gives a clue about one advantage, they are three-awn grasses.  An awn is a bristle, and each spikelet (seed-head) has three of them.   What’s so great about that?

So many great things!   Herbivores probably don’t much cotton to needle salad.     Moreover, associated with the “seeds” (achenes) the awns catch on fur, wind, and water to disperse.  Also they may help lodge against other vegetation or take hold on the soil.   That awns serve diverse functions is hinted by their diversity in abundance, lengths, orientations,  postures, shapes,  barbs, stiffness, and probably responses to moisture.

Awns are three, easy ID

In some species all the awns are coequals, while in others the central awn dwarfs the other two.   Many additional grasses have awns, if not in triplets, and looking beyond Aristida, awns contribute to photosynthesis and help position the seed for optimal water uptake.   To drift  into left field, I wonder if they absorb water or nutrients.

Speaking of nutrients,   how does a monospecific understory of grass secure the nitrogen it needs?    In contrast with grasses,  legumes and many  other plants have nitrogen-fixing nodules.  And frequently free-living soil microbes contribute, as does decaying organic matter.

Doing the twist

Now we could do the obvious and fret over how meadows of grass get the nitrogen they need to survive as the fittest.   But turn the beat around… and take a page from the presedential politics playbook.   There are two ways to get ahead,  rise up on your merits (boring), or tear down the opponents.

So, in honor of 2016, how do Aristidas manage to become President of the Park?   They are negative campaigners.  The explanation requires some shameless speculation and extrapolation of limited research.   Just read on:

Aristidas are early-succession plants, pioneers.  Any ecology book tells us that pioneer species soon give way to different replacement species, these soon booted out by bigger better species, and so forth.    Another name for pioneer species is weeds, and some very talented weeds are legumes with their nitrogen-fixing nodules. Legumes are all over Jonathan Dickinson Park, but not among the Aristidas.  Why don’t legumes and their power-nodules move in there?

Aristidas filibuster.  They expand and overstay their fair turn in the succession parade.   Some of their advantages are obvious, such as those awns, and tough rhizomes, but there’s a more interesting trick up their sleeves.   Three-awns repulse their would-be replacements by destroying the nitrogen-fixing bacteria critical to the interlopers.   Aristidas loathe legumes, turning the pretty pink nodules to black.

How’d you like a mouthful of that?

Of course if you deprive the foe of nitrogen you do need to get your own.   Three-awns  can probably acquire it rapidly after a fire, but so much more intriguing is the increasing research on nitrogen-fixation by grasses with the assistance of bacteria different from the ones the grasses suppress.    Purple three-awn  has associated with its root sheath nitrogen-fixing bacilli belonging to, or similar to, Bacillus (Paenibacillus) polymixa, perhaps cryptically contributing to the grasses persistence in the face of competitors.

Time to wind down,  but let this then be a lesson to all political hopefuls.  You too can grow up to be President.  Be stubborn.  Be bristly.  Be toxic.  And most importantly,  have grass-roots support.

Note on today’s photos.  These are from a project John and I worked on ca. 2007.   I do not recall who shot which.  We know what the species are but that info is suppressed as a distraction.  The essay concerns ecology, not taxonomy.