Blue Eyed Grass and Blue Eyed Grass

Sisyrinchium xerophyllum (and S. solstitiale)


Today John and George enjoyed an autumn day in the Martin County Scrub.   So pretty outside now, with all the invasive exotics colorfully in flower: Pendulous Senna dangling sprays of egg yolk blossoms out of the woody shadows over the roadside snowdrifts of Mexican-Clover. No wonder they call it Florida! The native species were nice too, with the belle of the ball being Blue-Eyed-Grass, a purty lil’ “Iris” with blue flowers having a contrasting starry yellow eye.  (All photos today are S. xerophyllym, by John Bradford.)

Sisyrinchium xerophyllum 2

Sisyrinchiums may be beautiful to the eye, but they have an ugly history classification-wise, with umpteen regional variants, many of them with different chromosome numbers. A good example of a narrowly distributed species is Sisyrinchium funereum from a postage stamp in Death Valley. The same botanist-cum-ornithologist who came up with that handle in 1904, Eugene P. Bicknell, also named the species John and I admired today, or at least gave it its original name, back in 1899.   Remember that year.

If you look at older museum specimens from our general area you find specimens of today’s species consistently labeled using Bicknell’s designation  “Sisyrinchium solstitiale.”   But then comes a mystery….all those older specimens were re-labeled abruptly in recent years as Sisyrinchium xerophyllum, the name you’ll find in current manuals. That might raise your eyebrows. Something’s happening here, and what it is ain’t exactly clear.

Sisyrinchium xerophyllum 3

Now this may sound like boring bookkeeping, and maybe it is, but bear with me a moment: it is not THAT darn boring.   The label-name hijinks revealed an intriguing example of the twists and turns in the classification game.   Call it an example of why I think it is more fun to try to understand nature than to compete in fool’s arguments unrealistically forcing messy evolution into artificially tidy categories.

Looks like the problem is we’re dealing with two names for the same thing.   Oops, did I say “the same thing?”   Not so fast.   Where did that second name, Sisyrinchium xerophyllum, come from anyhow? It too dates to 1899, conceived by Civil War Veteran, erstwhile priest, and California botany professor Edward Greene.

Was this a case of an East Coast botanist and another in California merely unaware of each other  separately naming “the same thing”?   Or is life more complex?

Fast forward to the 70s. Local botanist, the late Dr. Daniel Austin and his colleague Royce Oliver studied these two “things” in depth, concluding that we’re dealing with two distinct species. One being S. solstitiale mostly autumn-flowering, evergreen, living in scrub, and having a special fondness for Sand Pine woods. The other being spring-flowering, losing its leaves each year, and preferring high-pine and flatwoods habitats.     Oliver and Austin suspected the couple to have been separated by ancient cross-Florida oceanic inundation, and further suspected the scrub species to have switched to fall flowering to avoid the dreadfully dry scrub spring drought.  They listed eight physical characteristics distinguishing the two.

sisyrinchium xerophyllum

As so often happens, despite all that documentation, the tide of opinion drifted in the other direction, with more recent taxonomists lumping both under a broadly defined S. xerophyllum.   Sisyrinchium solstitale extincted by the stroke of a pen!   Who needs a meteor strike?

I’m not interpreting what’s right and wrong.   There is no definitive right or wrong here.   Just imperfect data and interpretation. Whatever interpretation prevails, today we stared the ghost of S. solstitiale in its blue eye: in the scrub, among the Sand Pines, evergreen, and blooming in the fall.   Get’s you wondering as you’re wandering…


Note: Oliver and Austin’s study was published in the Journal of the Arnold Arboretum 1974: 291.



Posted by on November 20, 2015 in Blue-Eyed Grass, Uncategorized



What Goes Up Must Come Down (and Up…and Down…and Up…and Down)….Leaf Movements

John and George searched Jonathan Dickinson State Park today for “Cut Throat Grass” we didn’t find.   Rough walking and a thunderstorm kept camera gear in the car.   Even if we missed the bloody grass there’s always plenty to see, and today under exceptionally dark skies it was enlightening to study the leaf positions on the various Legume weeds, such as Sensitive Brier, Pigeon Pea, Milkpeas, Amorpha, and Rabbit Bells.

Bladderpod, a Legume. The pulvinus is inside the red circle. (By John Bradford)

Bladderpod, a Legume. The pulvinus is inside the red circle. (By John Bradford)

Many plants move their leaves upward or downward at different times for different reasons, mostly up and perky in the bright of day, and collapsed and droopy at night, sometimes in-between during shade or “down” under stressful conditions. Most notably, many go to sleep at night and wake up in the morning.

Foliar “sleep movements” have fascinated biologists for about as long as there have been biologists. Charles Darwin himself tackled the topic. Perhaps he just liked to say, “nyctinasty,”   the fancy term for what we’re discussing.  As Bob Seeger might tell you, it means “night moves,”  a good name for a band:

Photo not by JB.

Photo not by JB.

Plants with moving leaves include Oxalis, Velvetleaf, members of the Maranta Family, and especially most Legumes.   Ever notice how Legume trees such as Royal Poinciana can sleep at night, or how veggies such as beans and peanuts, take a flop, or how backyard weeds, such as Tick-Trefoils collapse, or how native Legume species know when to fold em’?

The motion comes mostly from changes in cellular water pressure. Legumes have hydraulic leaf-lifts called pulvini (singular pulvinus) visible at the base of the compound leaf where it joins the stem.   The pulvinus is the thick little muscle immediately adjacent to the attachment point.   Sometimes leaflets within a compound leaf have their own little pulvini.

Initiating the motion is where it gets tricky. What pushes the “lift the leaf” button?  Environmental cues come mostly from light and dark, specifically from red and blue light.   Usually even more important is an internal clock. A built-in rhythm.  For a little taste of competing control by light and by a built-in clock, enjoy a little time lapse movie:


The internal clock needs dawn and dusk for setting. If you leave a plant in constant light for enough days the up-down cycle continues but degrades in timing and in intensity until it fizzles out.

Next time you are out and abut among trees at night, take a look at the Legumes: Royal Poinciana, Pride of Barbados, Albizia, Powderpuffs, Sennas, Princess Earring, Pongam, Copperpod, and others, do they sleep at night?


Extra credit:

Desmodium light response chart

This diagram shows deterioration of the leaf response cycle in continuous light. The vertical axis shows the degree of erectness of the leaves in terms of percent. The horizontal axis shows 5 pm through the night to 10 pm the next day. By placing three lines representing three nights) on the graph, three days of response by one Tick-Trefoil plant are condensed into one graph.

The blue line (Day-night cycle) shows leaf movements in natural day-night light-dark. The leaves dropped abruptly and fully 6-7 pm, then stayed fully down until rising abruptly and fully 7-8 pm.

The orange line (24 hours constant light) shows the same plant placed in constant light for a night after a night under natural conditions. It drooped full but required two hours 7-9 pm, and started waking up 2-3 AM and required about 4 hours to become almost 100% erect. Its response was mildly degraded.

Continued under constant light through the following night shown by the gray line, the same plant (48 hours constant light) drooped gradually and incompletely, hitting its lowest point around 4 AM, then rising slowly and incompletely until the experiment ended at 10 AM.   Its response was severely degraded.

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Posted by on November 13, 2015 in Baybean, Nyctinasty



Spatterdock is a Pumpin’ Posie

frogs on nuphar


Nuphar advena (N. lutea)


John and I tromped scrub and swamp yesterday, focused on spiders, not posies, so I’m going to turn back time one more day to my native plants class field trip Thursday with our toes in the Loxahatchee riverbank mud. A curious plant swimming with the fishes is Spatterdock, the common water lily with egg-yolk-yellow flowers shaped like tennis balls.

These simple species is a confounded series of confuddlements, beginning with its classification within the flowering plants. Every botany student knows the flowering plants divide into two huge groups, monocots and dicots.   But water-lilies stand apart.   They were around before the monocot-dicot divide.   Primitive as can be.

Spatterdock rhizome floating

Spatterdock rhizome floating

Within the genus Nuphar, species concepts remain unsettled, so if you look up our local species you’ll find different names in different books.   Folks who lose sleep (or send condescending corrective e-mails) over such cognitive dissonance should embrace the currents and eddies of evolution. You just can’t shoehorn an enormous dynamic evolving system into division, class, order, family, genus, species. More fun to watch the sky than to name the clouds.

So let’s try to get to something interesting.   The flowers start out female (pollen-receptive), and in a day or so become male (pollen-releasing).   Their fundamental main pollination syndrome, at least in the U.S., may be more or less like this: The flowers start out a little bit open during the female phase, so that visiting beetles brush across the stigma, thus pollinating the flower if they carry pollen. The beetles may become trapped temporarily. Subsequently, as pollen release ensues, the blossoms open. Any trapped beetles would then become dusted, and newcomers could stop by, get dusted, and fly off to entrapment in other flowers still in the female phase.

Presumably in the female phase. Come on in! The stigma is just inside the door. (By John Bradford)

Presumably in the female phase. Come on in! The stigma is just inside the door. (By John Bradford)

To whatever extent all that is so, it is not the whole truth and nothing but the truth, because numerous other critters visit, including flies and bees, and in the Old World the spatterdocks clearly don’t have beetle mania.

Male phase, wide open, stamens showing. (By JB)

Male phase, wide open, stamens showing. (By JB)

The fruits ripen above the waves, and contain seeds cooked historically in mush and gruel.   Spatterdock patches in places speculatively owe their existence to pre-European aquaculture, and fossil remains (pollen) is known from ancient human coprolites here in Florida.

The rhizomes are several feet long and many inches in diameter, and starchy. They too have been on the ancient menu, and have history in medicines, often mashed into poultices.   Laundry lists of ancient medicinal uses tend to be boring, unless there are patterns or repetitions.   A recurrent use in old records now on-line perhaps useful to some readers is, “hung up inside to keep witches away.”

The rhizomes live down in stinky oxygen-deprived pond-bottom mud.   How plants manage breathe where the sun don’t shine is always a curious matter, and spatterdock is a rock star in this area. It is an example of something there ought to be (and probably is undetected) more of: active ventilation…pumping….as opposed to the passive diffusion, with wind assistance, so universally attributed to plant gas exchanges.   Spatterdocks have a genius method of “forcing” air through the plant, starting with young leaves, on down through their petioles (leaf stalks) to and through the rhizome, then up and out through older leaves.   The internal air pipes run continuously the whole nine yards.

Young leaves floating (some are red). Old leaves flapping in the breeze above the surface. (JB)

Young leaves floating (some are red). Old leaves flapping in the breeze above the surface. (JB)

Air enters the system through the floating young leaves, slowly by diffusion, and more “air” (oxygen) accumulates as a byproduct from the photosynthetic activity in the leaf. Unlike most plants, that “waste” oxygen is not wasted. Instead, the oxygenated air collects in hollow ducts toward the bottom of the leaf. The trapped air cannot escape upward to the outside because a tight barrier (palisade mesophyll) separates the air ducts from the exposed leaf surface above. (The lower leaf surface is in the water, not in contact with the air.)   With the vapors accumulating and unable to escape, pressure builds up in the young leaves.   The botanist who documented all this several decades ago, John Dacey, noted that the young leaves are often reddish.   Botanists generally interpret red in young growth to be sun screen, but in this case Dr. Dacey suggested that the red pigment absorb solar energy, heating the leaf and raising the internal pressure.

The older leaves are different. Instead of building pressure, they release it. They are the vents. The older blades are held above the water in the breeze, presumably able to release water vapor and gases from both surfaces, and the barrier that prevented vapor escape in the young leaves is stretched out with gaps, it has become porous in the old leaves.

Young red leaf at left. Old leaf elevated on right. Rhizome buried in pond mud. Blue lines are air ducts. Arrows show predominant vapor flow.

Young red leaf at left, with tight restrictive vapor barrier.  Old leaf elevated on right. The vapor barrier has big openings.  Rhizome buried in pond mud. Blue lines are air ducts. Arrows show predominant vapor flow.

So then a mix of oxygen and other vapors flows from the pressurized young leaves, down through the air-hungry sunken rhizome, and then out through the leaky older leaves.   You may ask, “if vapors escape easily through big holes on old leaves but enter reluctantly through a tight barrier in young leaves, how can pressure continue to build and flow?”   The answer is gas-generation inside the plant; it self-pressurizes with its own metabolic activity: oxygen from photosynthesis, carbon dioxide from respiration, and internally released water vapor.

I just love it when plants “do stuff.”


Posted by on November 7, 2015 in Uncategorized


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Salt, Sun, and Succulence

Here is an experiment conducted on a hot sunny day Nov. 3 2015. he vertical axis hsows degrees (centigrade). The horizontal axis shows time, spanning roughly 4 hours. he orange line shows he temperatures at the surface of a regular flat leaf. he blue lines shows the temperatures at the same time and place on the inside of a succulent (Aloe) leaf. It stayed mores stable and mostly cooler

Here is an experiment conducted in full sun on a hot sunny day Nov. 3 2015.  The vertical axis shows degrees (centigrade).  The leaf temperature become high, often exceeding the air.  Forty degrees C = approx. 104 degrees F.   The horizontal axis shows time, spanning roughly 4 hours. The orange line shows the temperatures at the surface of a regular flat leaf. The blue line shows the temperatures at the same time and place on the inside of a succulent (Aloe) leaf. It stayed more stable and mostly cooler.

Succulent plants are odd and beautiful , and I can’t resist them.   Today in a big box store attending to home plumbing repairs, the errand became more fun distracted by the purchase of two non-native succulent charmers, Lithops and Fenestraria presently sitting on the kitchen counter.

By “succulent” I’m referring to plants with thick jello-filled stems or leaves.   Succulence has evolved many times in many places in many unrelated families. Today’s focus is on leaf-succulents.

Most succulents occupy sunny dry habitats. That tempts us into the obvious interpretation of succulence as an adaptation for water storage, like the hump(s) on the camel.  Makes sense—if you go to the desert better have a canteen.   But, as with many simplistic assumptions, a truthful reply is, “well sorta, but not exactly.”  Not that I claim to have total answers! This discussion is established plant physiology mixed with an effort to weave it into a local narrative, sort of like a historian connecting the dots with fragmentary data. You are warned.  These are musings, not hard fact.

“Kiss Me Quick,” a locally native succulent often encountered in hot dry weedy places. (By John Bradford).

So let’s get busy. First of all, why does a plant need to take in water? 1. Photosynthesis requires some. 2. Cells need some for basic maintenance, for nutrient transport, and to remain pressured so they don’t wilt. 3. Evaporative cooling.  Evaporation (or to speak botanically, transpiration) is the big user, reportedly accounting for as much as 97 percent of a plant’s water uptake.  Some trees transpire over 100 gallons per day bringing nutrients upward and cooling the excessive sun impinging on the foliage.

In a hot sunny dry habitat, a plant can’t afford to lavish non-existent water in massive quantities on the evaporative cooling that hot sun demands. And to make the water crisis worse, many arid plants have C4 photosynthesis,  something not to explore today beyond saying it  shuts down transpiration even further.   (As a local example, the somewhat succulent tree Rose Apple can have C4 photosynthesis.)  So you see, plants in dry hot circumstances are in a living hell! With curtailed evaporative cooling, they need a “plan B.”

Plan B is two-pronged: 1) Heat tolerance, and 2) Buffering from spikes in temperature, especially lulls in the cooling wind. Thick watery leaves resist temperature change. Think of it this way, if you pour a gallon of water out into a thin layer (representing a normal thin flat leaf) in the hot sun the water warms instantly.   But if you keep that water in a jug, or in a succulent leaf, the temperature resists change. Think how long it takes to bring a gallon pot of water to boil.

Succulent leaves are not simple water bags, as anyone who has ever sliced one can attest. They don’t generally drip when cut, and their volumes don’t seem to change much relative to wet and dry weather.   They are made of cells, and the cell contents are more of a gel than a liquid.   Think of the gel in a disposable diaper, wet and not prone to give it up.

In Palm Beach County Florida most native succulent plants live by the sea: Saltwort (Batis maritima), Sea-Purslane (Sesuvium portulacastrum), Marsh-Elder (Iva imbricata) , Sea-Rocket (Cakile lanceolata), and more.    Is it tough to take up water in maritime circumstances? Sure, rocky and sandy soils, and more importantly, salty habitats are “physiologically dry.”    And to pour salt on the wound, if a plant takes up large quantities of salty water for transpiration, remember that 97%,  its plumbing might choke on crud.   So no surprise seaside plants are often succulent.

Sea-Purslane, a maritime succulent (by JB).

Sea-Purslane, a maritime succulent (by JB).

Often, but not always: those able to shed salt don’t all need succulence. For example, Crested Saltbush (Atriplex pentandra) disposes of the salt into hairs shaped like lightbulbs. When the hair fills with salt it bursts, and goodbye salt.   This species is not particularly succulent.

Those unable to dispose of salt externally, by contrast, hold it in, and those are the seashore succulents. They store salt compartmentalized in each cell,  within a large water balloon filled with salt water (technically the vacuole), like the waste collection system in an RV.   A saltwater bag in each cell, drawing in ever-more water by osmosis,  swells and forces bloated water-retention, suggesting why the beach is rich in thick-leaved species. The plant sheds the excess salt when it sheds the loaded leaves, like that bloated disposable diaper.   Beach-succulents cultivated under salt-free conditions experimentally have shown diminished succulence.

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Posted by on November 1, 2015 in Succulents



Lazy Jack Hanging in the Bush

Jack in the Bush, Siam Weed

Chromolaena odorata


John is away for a couple weeks so I must represent alone, although that’s easy after a week of class field trips.   A curious species often encountered in class is Jack in the Bush, a native with a big personality in disturbed places, usually sunny. It is a native in Florida but a mixed minor blessing and major horror escaped invasively around the tropical world.

Jack with violet flower heads (by John Bradford)

Jack with violet flower heads (by John Bradford)

A problem with deliberately introducing plants and beasts—and in some marriages—surfaces when minor blessings later turn into major horrors.   Jack in the Bush has spread far and for different reasons, including during WWII on equipment and personnel, as a living mulch, as a cover crop, and in coffee plantations.   Why is it deliberately spread?   The robust growth crowds out other weeds, is reportedly allelopathic (naturally herbicidal), insect and nematode repellant, and easily generated green manure. It is beneficial to some fallow fields. And being so utterly willing to grow anywhere, our species has attracted attention as a potential cover for mine tailings. Hooray. Bring it!

Jack with white heads (JB)

Jack with white heads (JB)

Too bad it grows too well, and in Africa, India, Asia, and beyond Jack has gone crazy, dominating farm fields, disrupting cropping cycles, invading tree crop plantations, and becoming a general smother as well as hosting pest insects and probably pest fungi. The leaves are petri dishes hosting powdery mildews and many other fungi. Researchers have found the soil near Chromolaena to be oddly high in the spores of certain molds, as if somehow promoting them.

Derisive common names are revealing, such as “King Kong,” “Cholera,” and Rey de Todo (King of Everything) reflecting an imperialistic tendency to form vast single-species stands.

Rey de Todo (See how it looks wilty?)

Rey de Todo

This plant knows how to grow. Is it an annual or perennial? Both. Whatever top growth survives drought, fires, grazers, and mowers from year to year resprouts from buds.   It can regrow directly from the roots, which can enlarge into immortal storage organs, and from tiny seeds (achenes) blowing forth on parachutes to new colonization opportunities as crowded as 2000 seedlings per square meter. Once the growth starts, stand back, as observers have noted, “it grows like a crop.” If the main stem finds a support it can shoot up to 30 feet. If no support is encountered, side branches take over, often growing out at right angles. Growth rates can exceed an inch a day.

The flower heads have an odd trait: variable coloration, whiteish, bluish, violetish, and pinkish. That might tie in with the dozens of butterfly and moth species recorded to visit and pollinate. With so much help the plant can establish anywhere. And who needs butterflies anyhow? It reportedly can set seeds clonally without benefit of bugs.

The plant is wilted when nobody else is.

The plant is wilted when nobody else is.

Reluctant to stand on its own two feet, Jack is sort of a shrub, sort of an herb, and prone to sprawl across and climb more substantial shrubs.   Why form wood when others do it for you? Cut open the stem, even a big one, and you find it to be made disproportionately of pith, soft, cheap, air-filled “styrofoam,” easy, lightweight, and fast to make, and of no substance.

The stem has more soft but useless pith (white) than supportive water-conducting wood (light green).

The stem has more soft but useless pith (white) than supportive water-conducting wood (light green). “Quick & Dirty” No wonder it flops.

That probably explains why Jack in the Bush so often looks wilted.   Water travels in wood (xylem), not in pith (parenchyma). So if your stem is super pithy and deficiently woody, you probably can’t move much water. If you can’t move much water you could spend a lot of time wilted. Ecologist K. Naidoo discussed “severe wilting” of Jack in the Bush as a probable adaptation to help with water use efficiency and avoid leaf damage at times of water stress.

There’s an apparent trade-off in play, just speculating.   Lightning fast growth at the cost of building a soft flimsy stem with minimal water-conducting wood. Let the host plant supply the support, grow like mad when there’s plenty of water, and take it easy wilted in the meantime.


Posted by on October 24, 2015 in Jack in the Bush


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Confounded Compounded Leaves

John and I visited the woods yesterday and watched an ant crew complete with supervisors, guards, overachievers, and slackers hauling bits of fungus and winged pine seeds along a 10 foot trail (= 5 ant-miles).   They extract the core from the pine seeds and ditch the wing. The size of the pine seed relative to an ant is about like an airplane wing relative to a poodle. And now on to native plants.

The forms and functions of leaves may be boring, but boring is my middle name. So today’s topic is boring compound leaves. To talk about a topic we better define it. My Webster’s Collegiate Dictionary defines compound leaf as…..Dontcha just hate presentations that start like that!?

Here is a single compound leaf on Gumbo Limbo, with 5 leaflets. The leaf ends in a leaflet. (All photos today by John Bradford)

Here is a single compound leaf on Gumbo Limbo, with 5 leaflets. The leaf ends in a leaflet. (All photos today by John Bradford)

A compound leaf is a single leaf that looks misleadingly like a stem with multiple small leaves.   The small fake leaves are called leaflets. Now ask, what’s the difference? Most leaves, compound or not, drop free as single units. A stem-leaf junction is always immediately below a bud, but a leaflet revealingly has no bud.  A stem ends in a bud, but the compound leaf stalk ends in a leaflet or it just peters out budless. Examples of compound leaves are plenty. A vegetable gardener says green beans.   A suburban landscaper might cite Schefflera. A fruit fancier could think of starfruit. Stonewall Jackson may mention hickory. Teenagers behind my fence know of marijuana. Those who loath invasive exotics lie awake over Brazilian Pepper. Native plant enthusiasts have umpteen examples. Enjoy a dozen:

  • Ash
  • Paradise Tree
  • Hercules Club
  • Wild Lime
  • Torchwood
  • Elderberry
  • Most ferns
  • Coontie
  • Poison Ivy and Poisonwood and Poison Sumac
  • Milk Pea, Cow Pea, Butterfly Pea, Rosary Pea, Partridge Pea, Sensitive Pea
  • Bay Beans, Coral Beans, Velvet Beans, Cool Beans
  • Things that stick in my socks (Spanish Needles, Tictrefoils)
This Sesbania has about 9 compound leaflets visible, each leaf with numerous little elliptic leaflets.

This Sesbania has about 14 compound leaves visible, each leaf with numerous little elliptic leaflets.

And there exist twice-compound leaves. Huh? Ones where the leaflets themselves are “compound leaves.”   Fact is, a leaf can be twice, thrice, or more compound.   Think of some frilly ferns, or native Nickerbean. This all leads into nomenclature we shall ignore.

This Royal Fern has a twice-compound leaf. Only one leaf is in this photo. It is

This Royal Fern has a twice-compound leaf. Only one leaf is in this photo. It is “doubly compound,” divided into about 12 main subunits, these subdivided into numerous smaller units.

Compound leaves have evolved separately many times in numerous unrelated plant groups. Many more plants have lobed leaves approaching compound, but not quite.

Lobed and almost compound.

Lobed and almost compound.

Some species have a portion of the leaves simple (simple = not compound), with other leaves on the same individual lobed or compound. Red Mulberry, Calloose Grape, Marsh Mermaid Weed, and the invasive exotic Arrowhead Vine come to mind. The mix reveals different pros and cons of different leaf forms depending on the leaf’s age, position, or physiological state.

Mulberry simple leaves.

Mulberry simple leaves.

Mulberry lobed leaves.

Mulberry lobed leaves.

Bald Cypress pretends to have compound leaves in a converse fashion. Its branchlets with many small leaves behave like single “compound leaves” in the sense of being seasonal, flat, and deciduous.

As Miami botanist Steve Woodmansee recently commented on this blog, there’s an evolutionary two-way street.   Although clearly simple leaves have evolved many times into compound leaves, the reverse occurs too. A locally familiar example of compound-to-simple is Coin Vine, a Legume uncharacteristically having simple leaves.

There must be something compelling to induce “big” simple leaves to subdivide over evolutionary time into “small” more or less separate leaflets and lobes.   Many botanists have wondered why, and several answers exist, none of them “the” single revealed truth.

Clammyweed. Each leaf with 3 narrow lobes.

Clammyweed. Each leaf with 3 narrow lobes.

First off all, it might seem optimal for a leaf to be as big as an umbrella and just steal all light from competitors below.   We have solar panels on our campus as big as patio surfaces. Not many plants however evolve umbrella leaves. There must be advantages toward small independent blades.

First disadvantage of big leaves: they fray and tatter in the wind, and are “expensive” to replace.

Second disadvantage of big leaves: bright sun exposes a leaf to more light than it can handle. Too much light diminishes photosynthetic ability and generates often unwelcome heat. On average, any leaf can use only about 20% of full sunlight. A big umbrella shadowing all below is wasting 80% or more of the incoming light while suffering possible damage.   A plant is usually better served by having vertically layered smaller leaves (or leaflets or lobes) capturing collectively far more than that original 20% at the top layer.  Light becomes more diffuse deep in the shadowed understory, so that the varied orientations of the leaves, leaflets, and lobes, and their flutterings allow more efficient capture of dim light, also light angling in at dawn and dusk, and sporadic bright “sunflecks” as the wind fleetingly parts the canopy above.

Frangrant Eryngo with some fancy lobes.

Frangrant Eryngo with some fancy lobes.

Third disadvantage of big leaves: Growth is not limited by light alone.   Every third grader knows, I hope, that leaves take in bad carbon dioxide and give us good oxygen. Breezes blowing across small leaves, leaflets, or lobes are more effective at gas exchange than those passing over large surfaces.   Bigger surfaces have more surface-breeze friction. Also, the margins tend to be younger fresher tissue. It is easy to show as a classroom demonstration more photosynthesis happening near the margins than “inland.”

Fourth disadvantage of big leaves:   As foliage absorbs solar energy it doesn’t merely photosynthesize, it heats up, and heat stress can be trouble. Without protective adaptations leaves would sometimes be hotter than the surrounding air, but they have evaporative cooling. Small blades shed water vapor and heat directly to the air better than big leaves. (By the way, on cold clear nights leaf blades can be colder than the surrounding air so that the ability of smaller blades to conform to air temperatures might sometimes help at both ends of temperature stress.)

One thing I like about compound leaves is the ability for individual movement by the leaflets.   Leaf movement in response to built-in rhythms and to environmental cues, including light vs. dark, is fascinating, especially in complex compound leaves.

In Pigeon Pea the leaflets can all move in unison, resembling the blades in venetian blinds. You see it in the time lapse below experiencing the end of a day, the night, and waking up the next morning:  CLICK to see the pea

In the following time lapse through a semidark night and the morning after, the Trefoil (left) and Oxalis (right) have individual leaflets all doing their own dance. (The classroom lights come on a couple times early in the evening— cleaning crew/security guard.)  CLICK

Inkwood compound leaf with 4 leaflets

Inkwood compound leaf with 4 leaflets


Posted by on October 18, 2015 in Compound Leaves



Coastalplain Golden Aster and the Florida Marcescent Lifestyle

Chrysopsis scabrella


Golden Asters in the sun (John Bradford)

Golden Asters in the sun (John Bradford)

Every time John and I botanize through an open scrubby area, such as today, we enjoy an odd-looking species, Coastalplain Golden Asters rising awkwardly from the white sand at varied angles to attain irregular heights. Very martian! To add drama, the funny stems retain a covering of dead withered foliage, more properly called “marcescent” leaves.

The dead leaves stay put.

The dead leaves stay put.

Dead leaves usually fall away and decay in most plants, but not always. The Golden Aster appearance always evokes the same old memory for me. Back in the Reagan Administration I had the good fortune to work at high elevations in South America where several unrelated plants resemble Golden Aster by having marcescent leaves covering an otherwise bare stem.   Around the world, this life form has evolved repeatedly, usually in exposed habitats where a drying risk is coupled with fluctuating temperature extremes, often intermittent frost alternating short-term with warm temperatures. Of a few examples here in South Florida,  Golden Aster is the most striking.   (We’ll look at Rabbit Tobacco another day.)

Coastalplain Golden Aster is generally described as a “biennial,” hunkered down the first year as a rosette on the ground, with the stem then rising the second year to flower and fruit.   New rosettes form at the stem base.   I’m not 100% sure the plant always obeys its biennial characterization.

Here is Espeletia in Ecuador:  CLICK

Here is Golden Aster in Stuart, Espeletia Jr.:

Every stem with a skirt of marcescent leaves.

Every stem with a skirt of marcescent leaves.

We might say blithely, “well, the dead leaves protect the stem.”   OK, but exactly how, from precisely what?   If anyone has looked into it at a physiological level in Chrysopsis, I can’t find it. But botanist Alan Smith back in the 70s took a hard look at Espeletia in Venezuela, and provides inspiration for a better look at our similar local case. Dr. Smith found the Espeletia habitat to feature strong seasonal differences in rainfall, like us. And there were wide strong short-term temperature fluctuations, like us. The greatest temperature stress and moisture stress occurred during the dry season, likewise the case in Florida if the main temperature stress is frost. (We live near the southern limit of the all-Florida geographic range for Chrysopsis scabrella.)

Smith and other botanists have interpreted marcescent leaf blankets as a buffer against fluctuating temperature extremes. Removal of the dead leaves cost a lot of Espeletias their lives.   The main apparent reason was that during times of frosty nights alternating with warm days stems with their dead leaves intact never dipped below freezing, whereas the ones with marcescent leaves removed dipped and died. Those dead old leaves don’t radiate heat at night.

It may seem odd to speculate that frost protection might be the “main” benefit of marcescent leaves, especially in a plant like Golden Aster so obviously exposed to extreme drying.   Don’t those dead leaves merely protect the stem from hot dry winds? Maybe, but two reasons suggest otherwise:

A.  In general, water loss from stems is not severe.   The stem probably does not need much protection from direct drying.  (Cacti are all-stem.)

B. Frost stress is drying stress. One of the worst aspects of frost for a plant (in a not-very-frosty borderline setting) is that freezing in the stem diminishes water passage from the roots to the leaves. An plant in a super-dry setting with temperatures hitting 80 degrees by day and dipping below freezing by night has much to fear from Jack Frost.   The warm day, especially at dawn, creates high demand for water to the living leaves, but if frost-impaired stem tissue can’t deliver, well that’s tragic.  Walking through the scrub in 90 degree weather and 90 percent relative humidity, it takes some faith to see those stem-blankets of dead leaves as possible winter coats.

Fruiting heads (John Bradford)

Fruiting heads (John Bradford)


Posted by on October 9, 2015 in Uncategorized




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