Ludwigia –What a Tangled Web We Perceive When at First We Practice to Use Our Keys

13 Jun



Last Friday Billy, John and George survived a thunder squall on the Haney Creek Trail in (or near) Jensen Beach.  Beautiful site, but too much lightning and too many Ludwigias, aka Primrose-Willows.

Have you ever hurt your head attempting to key out a plant species only to find the glove don’t fit so you must acquit?  Did you pause and wonder:  is the problem the dumb key writer?  Or is it me overlooking something?  Or is it the plant?  The problem may be you or a goofy key writer, but sometimes the plant really truly does not fit.  Plants do not know we will them to fit into a stilted 18th Century system of classification.

Ludwigia maritima (by JB) (It has two chromosome sets, so does John.)

Look at it this way.  Einstein figured out there’s no such thing as space and time. They’re human concepts.  I never really understood that, but I do understand sometimes there’s no such thing as species.  They’re human concepts.  What is amazing is how often plants do sort themselves out into species, but not always.

That all begs a big question: what is a species?  Well (somewhat animal-o-centrically) we’d like it to be a lineage of organisms that all look very much alike and breed readily with each other but with nobody else.  We’d like a species to be fairly widespread and ancient.  Nobody told all the plants about this, however.

There are groups of plants whose classification has never been resolved for the simple reason that their patterns of breeding, histories, and visible differences do not resolve into traditional species.   Ludwigia is a prime example, because its pattern of variation is nutty.  Hold this thought a moment as I set the stage with a little context.

Ludwigia consists of about 82 species around the world with North and South America points of concentration. Florida has about 28 species, some of them invasive exotics and others of questionable provenance.   Our immediate blog area has about 16 species.  That’s plenty.

Ludwigia peruviana (by JB)

Florida Ludwigias range in size from little floating or mud-creeping weeds to shrubs taller than you are.  Some have big yellow buttercup flowers; several have no petals at all.   The fruits shapes are all over the map from cigarettes to dice,  the leaves can be opposite or alternate (and extremely variable even on single  individuals),  and the plants range from shaggy to bare.  In short, they are diverse, although you usually know one when you see one, like dogs.

Ludwigias are master weeds globally, including American species behaving badly overseas.   The distributions are so expansive that it is often impossible to pinpoint the exact regions of origin.  As an example,  L. octovalvis is a Florida wildflower and an Australian wildflower.  Some have moved around and have hybridized as aquarium plants, escaping when the aquarium gets dumped.  Peruvian Primrose-Willow (L. peruviana)  can reach 12 feet tall, and you could scarcely design a better wetland invader.  This South American exotic roots from floating stem fragments, and it makes seeds in massive numbers carried by water currents, birds, and anything that moves.  Needless to say, it can form monospecific stands along shores.  Yet others have restricted distributions.  Ludwigia stricta is Cuban and nada mas.

Now back to the species screwiness.  We need a quick lesson on chromosomes and hybrids.  Chromosomes usually come in pairs.  You and I have 23 pairs.  In any given pair, one member arrived from Dad in the sperm, the other from Mom in the egg.  The two chromosomes sets came together (in Rochester in 1952) and paired up in the fertilized egg that grew into me.   Many hybrids are sterile because chromosomes from the sperm of one species and the egg of a different species don’t pair up properly in the fertilized egg.  It would be like sending two kids to run through Beall’s Outlet and each toss 5 shoes into a bag.  Mismatched “pairs” will result The mismatch may prevent  the hybrid from developing or functioning fully, or perhaps from making is own viable sperms and eggs.  A mule.

But plants often do something weird — many double their chromosomes, the equivalent of a person having 92 chromosomes instead of 46.   Or the equivalent of taking that bag full of Beall’s shoes back through the store and adding in the matched shoe for each one the kids tossed in, doubling the shoe total in the bag.

Chromosomal doubling is rare but happens, creating a bizarre but true circumstance.  If a hybrid suffering from mismatched chromosomes has its chromosome number doubled,  then each chromosome has a matched  partner.  This allows plants to form stable fertile hybrids with novel chromosome numbers, having four, six, eight or more chromosome sets.   Bread wheat has six sets. Strawberries have eight.  Multiple chromosomes sets or the plants having them are called polyploids. All of this happens in Ludwigia, and many of the “species” are really stabilized hybrids with differing levels of polyploidy.  Ludwigia maritima has two sets of chromosomes (8 chromosomes in each set for a total of 16).  By contrast, L. peruviana reportedly has 12 sets of chromosomes.  A dozen!

Several of the Ludwigia species in the Southeastern U.S. belong to an odd complex mix of four chromosome sets, or six.  Those with four sets cross freely, a process aided by human disturbance which creates unnatural intermixing.

Making matters worse, the plants reproduce clonally by fragments, stolons, and by self-fertilization, so  any given hybrid can spread.  When you try to identify a Ludwigia, are you holding a “species,” or some sort of hybrid resulting from free mixing and clonal propagation ?

Here are some selected examples to underscore the screwiness:

Ludwigia linearis and L. linifolia are similar with linear leaves and a single chromosome pair.  They differ because a fragment of one chromosome has flipped within the chromosome. They cross to make fertile offspring.

Ludwigia alata is apparently a stabilized hybrid with six sets of chromosomes between L. lanceolata (four sets of chromosomes) and L. microcarpa (with two sets)

Ludwigia curtissii has eight sets of chromosomes, suggesting that is a stable hybrid of perhaps four other species.

Are you confused?  If so, good, my work here is done.  The point being that sometimes plants mix and match in ways that don’t represent slowly evolving, evenly branching evolutionary trees.  The take-home lesson is that:  if the identification key and nature don’t match, maybe the problem is nature and not you (but it is probably you).  On a serious note, nature does as nature does, and there are limitations to our taxonomic system.   You can’t capture all variation with families, genera, and species any more than you can capture all people with a chart of personality types.


Here is a mini-guide to selected local Ludwigias from a booklet John and I threw together back in 2007. Don’t bet the farm on it.

Leaves opposite: creeping (or floating) weed of wet shores…Ludwigia repens

Leaves alternate:

Petals absent:

Flowers in a tight spike:  Ludwigia suffruticosa

Flowers not in a spike:

Fruit < 2 mm long: L. microcarpa

Fruit 2-3 mm long: L. curtisii

Fruit 3-4 mm long, plant hairless, stem winged: L. alata

Fruit 3-4 mm long; plant hairy: L. pilosa

Petals 4 in number:

Stamens 4:

Capsule round, longer than wide:  L. linifolia

Capsule square, as wide as long: L. maritima

Stamens 8:

Petals 4-5 mm long: L. erecta

Petals 10-30 mm, plant not very hairy:  L. octovalvis

Petals 25 mm, plant fuzzy: L. peruviana

Petals 5 in number:

Stem shaggy with long hairs: L. leptocarpa

Stem hairless or with short hairs, usually floating: L. peploides


Posted by on June 13, 2012 in Ludwigia


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11 responses to “Ludwigia –What a Tangled Web We Perceive When at First We Practice to Use Our Keys

  1. Mary Hart

    June 14, 2012 at 3:43 am

    Thank goodness I don’t think there are any Ludwigia round me – I’m too easily confused!!

    • George Rogers

      June 14, 2012 at 7:42 am

      That’s the beauty of it — you just say, “these are all crazy, so I’m off the hook.” I’ve never looked into it, but I’ll bet there are U.S. species growing as unwelcome invasives in the UK. That is true with a vengeance in France, so why should France have all the fun?

  2. mudfish

    June 14, 2012 at 6:51 am

    Oh, that was really sweet! Well said! I’ve thought about this many times before – that maybe the same sort of thinking could apply to math, perhaps. The different “maths”, as we know them, are merely our best representation, just our latest and most current attempts at quantifying and writing down our observations of the universe. It’s like using a map – the map isn’t the actual truth about the land around you, it’s only someone’s best understanding and attempt at explaining it. A map can never actually be perfectly accurate.

    #2 – does this same chromosomal hybridization apply to the scrub oaks? I’ve always been told that they (scrub live oak, Chapman’s oak, myrtle oak) hybridize easily and can be difficult to pin down. Yes? No?

    • George Rogers

      June 14, 2012 at 7:54 am

      Now that’s thinking cosmically! Every now and then I pick up an outdated Discover Magazine waiting in the doctor’s office and try to understand the space time continuum, string theory, and trying to exceed the speed of light and you gotta be right—it’s all in the math. (By the time the doctor finally sees you you could explain an entire cosmic theory.)

      The Oak thing, spoken like a true resident of JD Park! You all have the messiest oaks in town. Everywhere I’ve lived oaks have misbehaved about conformity to species. Up in Michiagn I used to do a class lab where the students would bring in the entire diversity of oak leaves and try to sort them into species. Let’s see, on my shelf is a book “Oaks of North America.” The index takes us to p. 45 says, “Accidental crossing or hybridizing among oaks is common and has caused some controversy.” Don’t people have anything more important to fight about, like parallel universes? To be honest, I don’t know anything aobut the chromosomal shennanigans of Oaks.

  3. Steve

    June 18, 2012 at 9:21 am

    I never realized Ludwigia was such a mess. Thanks for elucidating its confusion! What is fun also is that it may be possible to have an endemic exotic, if the invasive species evolves into something different after its invasion…

  4. George Rogers

    June 18, 2012 at 11:10 am

    A mess, or amazing example of the hand of nature? In hte Ludwigia case thanks a series of chromosome-savvy botanists over decades, mostly at the Missouri Botancial Garden. I agree about the post-invasion genetic mischief. Just think, Ludwigia peruviana has already a jaw-dropping 12 sets of chromosomes. Maybe it’ll gobble up another set in Florida and go to 14 sets.

    I think that in this day of increasingly high-resolution DNA biotech, we’ll see “interesting” cases of Frankenstein genome combos. I have a suspected case or two, but I keep my mouth shut (at least on that) due to total lack of data. I’d also bet that ghosts botanists from eras past see post-disturbance and post-invasion intermediates they never encountered back in the day. My old teacher Roger Sanders documented the apparent sneaky mixing between Lantana depressa and L. camara, although a look at my Wunderlin shows lack of consensus, which really now is a problem for DNA study instead of measuring the angles of leaf serrations

    Untoward mixing is possible in-state too with rare endemic species of tightly limited natural distributions. When they are brought together unnaturally, there’s potential for mixing up a whole lot of subtle evolution. As unavoidable as that is in a modern world, I’ve often wondered if we should avoid doing it on purpose, for example by propagating rare & endanged tightly endemic species and then “reintroducing” those genomes outside the orginal habitats. As we get better and better at fine DNA study, this sort of thing might go from an abstract extremist concern to scientifically significant.

    Come to think of it, I’m the sorry result of mixing a whole lot of alien genomes.

  5. Steve

    June 23, 2012 at 9:56 pm

    Good points on rare plant augmentation and reintroduction problems. I am not certain there is a good answer for that right now, other than leaving it to the experts who are knowledgeable about artificially induced genetic drift. Even common things, like Coreopsis leavenworthii, have different forms throughout the state. Not knowing about the different forms, I dumped some seed that must have come from the panhandle, in my pine rockland garden in Miami, and the plants that came up looked completely different from our indigenous southern forms. I await it to “disappear” before I add the indigenous material.

    Even the common Heliotropium polyphyllum has subtle differences. It is white flowered on the west coast of FL, and yellow flowered on the east coast FL, and somewhat in between in the middle, I’ve seen white and yellow flowered forms sold by side at nurseries.

    I always recommend to customers to try and buy plants with germplasm that came from less than 50 miles away (not easy to do though).

  6. George Rogers

    June 23, 2012 at 10:04 pm

    Right—and oh so many little differences, including physiological variations, we have no idea that exist. My memory is foggy thinking back too many years, but I think I recall a well documented (at the DNA level) case of two lizard “subspecies”(?) each endemic to different glades in Missouri losing their long-evolved genetic identities by sudden access to each other, probably by habitat alteration. Wish I could remember that study better. I’m afraid we’ll see more and more of that sort of thing that as DNA works gets better, faster, and cheaper. At least it warrants thought and caution as you indicated. I remember sitting in meetings years ago where such concerns were scoffed right out the door as “reactionary.”

  7. George Rogers

    June 23, 2012 at 10:07 pm

    Hey Steve, if you see this, your message popped in while I was lookng for a photo of Calyptocarpus vialis in flower. Had one, but seem to have lost it. You happen to have one in your collection?

  8. MTV Roadies

    April 14, 2013 at 2:03 pm

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    • George Rogers

      April 14, 2013 at 3:15 pm

      Thanks a billion!


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