Lythrum – a Pair of Mysteries

The Back Story

Image of the ‘Courtyard’ Lythrum

In mid-June, a host of lightweight herbs, plants with small leaves that had grown quickly to become open-branched and a bit shrubby, began to flower.  There are a lot of plants from varying families that begin life with this look, and in our wild yard most such vegetation might be taken as simply more weeds, by many folk. But I let them grow because I want to know who they are. In anticipation of flowering, leaf arrangement (phyllotaxy) and characteristics provide some useful clues, but our surround is laden with mystery plants.

Upon flowering, the wirey plants with opposite, fine-textured leaves that I had been watching, materialized as a fleet of Lythrum, but seemingly of two different sorts.  Though different in habit, both forms keyed to the mid-territory between Lythrum lineare and Lythrum alatum. So what gives?  Am I seeing a spectacular range of plasticity in one species, or expressions of two distinct taxa?  What might be other choices?

Of the 12 Lythrum species listed in Flora of North America online (FNA), and 39 accepted worldwide by the digital Kew Index Plants of the World OnLine (POWO), the Florida ISB plant atlas reports 4 being native to the state.  Those 4 species, Lythrum lineare, L. alatum, L. curtissii, and L. flagellare fall into two camps.  The first two, Lineare and Alatum are known across much of the Eastern US, while the last two, Flagellata and Curtissii are close endemics – a familiar pattern.

Checking the literature, I’m reminded plants of Lythrum are heterostylous, a condition noted for many groups (Ganders, 1979).  Lythrum alatum is described as “distylous” by Graham and others, which means there are two separate configurations related to the length of the style and the position of anthers.  Some plants will have a short style, while flowers in others generate a long style with an exerted stigma – strategies interpreted to promote outcrossing.  Things proved, however, to be challenging. Searching Lythrum reveals an overwhelming glut of literature on the European native Lythrum salicaria, “the Purple Menace,” a European native that’s become a problematic wetland invasive, worldwide. 

POWO distribution map for Lythrum salicaria, the purple representing non-native occurrence

Taxonomically, both our native Lythrum lineare and Europe’s aggressive L. salicaria were known to botanists by 1753, and are thus among the 7 kinds described in Species Plantarum.  Not much has changed for these two plants; both retain “L.” authorship.  To be certain, there have been challenges; POWO lists 17 published synonyms to the genus, which means that many of those descriptions will have attempted to segregate some of the taxa.  Taxonomists have spurned those offerings, leaving the genus intact, with a modest number of accepted species.

After Linnaeus, the next of our Florida species to emerge was Lythrum alatum, described by the ill-fated Frederick Traugott Pursh.  Born in Saxony, Pursh migrated to the US in 1799, where he briefly managed Philadelphia’s historic “Woodlands” (the personal botanical garden of William Hamilton), soon enough moving to employment by Benjamin Smith Barton to assist in studying Lewis and Clark materials, with an unrealized goal of writing a North American flora.  Returning to England, Pursh succeeded, at the end of 1813 and in the early days of 1814, in publishing his studies in the important Flora Americae Septentrionalis (the final term refers to the 7 principle stars of Ursa Major, thus signifying northerly latitudes, in a colorful way).  Pursh acknowledged 7 species of Lythrum in the Northeast, including Linnaeus’s L. lineare, his newly described L. alatum, and the introduced European L. salicaria.

Curtis’ Botanical Magazine depiction of Lythrum alatum

Chapman published a third Florida plant, the Peninsular endemic Lythrum flagellare, in the 1883, 2nd edition of his Flora, the description of which is credited to Robert J. Shuttleworth (a wealthy Englishman reared in Switzerland). Shuttleworth was a Chapman correspondent who conducted his own explorations, but is best-known for having financed botanists and shell collectors in many regions – most importantly for us, underwriting collecting trips of Rugel (Ferdinand Ignatius Xavier Rugel) to Florida.  Shuttleworth’s entry in Wikipedia explains his herbarium, now at the British Museum, held 150,000 specimens.  Wunderlin, Hansen, & Beekner (‘Botanical Exploration in Florida’), tell us Shuttleworth held a second, personal herbarium of 10,000 sheets that was acquired by an American, Isaac Martindale, and is now housed at NA (the acronym for the US National Arboretum herbarium.)  POWO reports Shuttleworth published at least 118 taxa and new combinations (not all are accepted today).  Thirteen or more of those were first published in editions of Chapman’s Flora, including the Florida Peninsular endemic Lythrum flagellare, as well as several plants honoring Rugel (the genus Rugelia, and Paronychia rugelii are accepted names today).  The decumbent Lythrum flagellare, is considered an obligate wetland endemic, classified as Endangered.

The most recent addition to the Florida Lythrum assemblage, reported with a narrow range in the Panhandle and adjacent SW Georgia (Calhoon County), is Lythrum curtissii, described by M. L. Fernald in 1902, based on collections made by M. A. Curtiss in 1901, in “a miry swamp” in Leary’s, Georgia.  Fernald also cites a specimen he studied in the Biltmore herbarium, one collected by Chapman in 1897, in Aspalaga. That would have been among the first specimens collected in the few counties where the plant has been located in Florida.

ISB Plant Atlas distribution of Lythrum curtissii

Checking the ISB Plant Atlas distribution map, you see this plant shows curious disjunction for many Panhandle plants, present in the Apalachicola River area, and then again in the northeast Atlantic coastal counties, yet absent in the region centered around the Suwannee Straits.

The Main Story

My current mystery is the identification of the two distinct forms of Lythrum flowering in early July, on our property.  Both plants key out as being of the upright, green sorts reported for our flora, most likely either L. lineare or L. alata.  One form, an entire colony (I’ll call this ‘Roadside’) branches loosely, has linear leaves, while the other, a single specimen (I’ll call ‘Courtyard)’ is more compact, with leaves I’d describe as narrowly elliptic.  Here are some observations:

Comparing the two Lythrum forms, ‘Roadside’ to the left, ‘Courtyard’ to the right
  1. Both forms bear opposite leaves, throughout.  Thus leaves up to the branching inflorescence (the books call these the distal leaves) are opposite, as well as leaves at the base of the plant (the floras call these proximal leaves.) –  Only in the inflorescence of the Courtyard form do I find departure from opposite leaves.  This leads to an identification as Lythrum lineare for the Roadside form, and likely for the Courtyard.
  2. The stems of the looser, Roadside form are distinctly square, and shallowly winged.  Minimally, they are more obviously winged than the Courtyard form.  That would suggest the first could be Lythrum alatum, while the second might be Lythrum lineare.
  3. Inflorescence architecture is very different, the Roadside form (of which there are a dozen plants) producing a very open, elongated and airy structure with a pair of flowers opening sequentially on each stem.  Inflorescenes of my ‘Courtyard’ are much more congested, generating several flowers open on a single stem each day.
  4. Petals on both forms are 4-6 mm long, slightly longer in Roadside.  That places both forms at the large end of L. lineare, described in Flora of Florida (v. IV) as 3-4 mm (1.5-2 mm in FNA), compared with petals of L. alata, listed in the key as 5-7 mm (2-7 in the description). 
  5. Having examined flowers from several of the Roadside plants, I see no examples of heterostyly among plants in this location.  All flowers have an exerted, capitate stigma.  The same holds for flowers of the single Courtyard specimen. Graham records all Lythrums as dimorphic (showing two differing flower configurations in the population). However, shortly after making these observations, on 8 July 23 I encountered a population of the same Lythrum in Tate’s Hell State Forest with flowers of a short-style form, confirming the dimorphy.
  6. Both forms shed flowers at the end of the day, or tardily the next day, with new flowers opening by 0900 hours.  (Don’t go out early, these flowers keep banker’s hours)
  7. Flowers of of both forms span a full centimeter, but seldom more than 11 mm, the petals being around 4 mm long.  All told, flower size appears to correlate better with descriptions of L. alata.
  8. The bases of ovaries in the two plants are similar, each pale green, elliptical ovary is surmounted by a 3-4 mm style topped with a capitate stigma. In both Roadside and Courtyard, the ovary sits atop a rich green, globular stipe, about half a millimeter long. I couldn’t say the green stipe as “encircling” the ovary, which would fit the description in Flora of Florida and FNA for L. alatum. But Graham writes regarding the L. lineare pistil: “basal stipe of ovary scarcely or not at all thickened into hypogynous ring,” In my specimens, you wouldn’t miss observing the swollen stipe using a dissection microscope, but I imagine it would be a challenge confirming the nature of this 0.5 stipe using a handlens. Based on her descriptions, I’m guessing Graham would label my plants L. alatum.
As above, Lythrum ‘Roadside’ to the left, ‘Courtyard’ to the right

A nice fallout of this attempt at identification has been my forced reacquaintance with characters of Lythraceae.  For years, I’ve used Lagestroemia to demonstrate “clawed” petals for students.  There’s hardly a better example.  And we cultivated some nice Cuphaeas in the SoCal gardens, reminding me of plants I regularly encountered along rocky stream beds in Panama.  I could always rely on the opposite to sub-opposite leaves, striated hypanthium, and clawed petals to get me to the right family.  But I hadn’t given much attention to Lythrum until this summer, when confronted with plants I couldn’t easily peg to family, much less to species, on my own property.

Lagerstroema,flower, showing the “clawed’ petal bases

Given closer scrutiny, I begin to appreciate the taxonomically useful characters.  Be advised, the floral features are small, with the floral tube (both calyx and corolla tissue) generating the familiar striated symmetrical tube (the hypanthium), which in these flowers is about 5 mm long.  The top of the hypanthium is marked with 6 fleshy, pointed tubercles, one subtending each of the six petals (which the Flora terms an epicalyx appendage, FNA terms epicalyx segments, and other treatments call ‘processes’).  Triangular sepal lobes on the rim of the hypanthium alternate with the 4-5 mm long petals, each of which forms a narrowed, but not clawed attachment to the tube.  The presence and size of the epicalyx appendage relative to the sepal lobes is utilized as a significant key character.  In flowers shown here from our property, the appendage is somewhat larger than the sepal lobes.  Each flower produces 6 stamens, filaments of which are free to the base of the ovary, with the anthers presented at the rim of the hypanthium.

As mentioned in Observation #8 above, the superior pistil is small.  In these flowers the narrow, cylindrical ovary, about 1-1.5 mm long, sits atop a globular green stipe, about 0.5 mm tall.  Flora of Florida indicates that flowers of L. alatum produce a nectariferous ring surrounding the base of the ovary,, though it isn’t clear this thickened stipe corresponds to that structure, which in flowers I examined is clearly basal to the ovary, not surrounding.  Atop the ovary rises a style ranging from 3-4 mm long, which positions the capitate stigma exerts visibly beyond the rim of the hypanthium.  If there were alternative floral types in this population, I’d expect so see a short style, setting the stigma almost immediately atop the ovary (see photograph below). I’ve memorialized the structure of the long-style flowers here in a crude sketch, which I include so as to clarify the description above.

Determination remains a mystery. Shirley Graham, who gave greater attention to the study of Lythrum alatum has some cautionary words:

“Most widespread of the United States species of Lythrum, L. alatum has been divided by various authors into as many as four species. Shinners (11953) recognized as present in the Southeast L. alatum, L lanceolatum, L. dacotanum, and provisionally L. cordifolium. He considered L. alatum and possibly L. cordifolium as rare southeastern endemics and felt that the epithet alatum had been incorrectly applied to the common midwestern plants that he referred to L. dacotanum. Floras treating southeastern United States or adjacent areas have recognized L. alatum and L. lanceolatum as distinct species, as varieties of a single species, or have followed Shinners’ taxonomic and nomenclatural interpretations….The evidence presented in this study supports recognition of only one species in the L. alatum complex in the Southeast, with two varieties defined primarily by stature, leaf shape, and geographical range.”

Given the evidence, floral structure suggests these plants represent Lythrum alatum. Vegetative features suggest they are Lythrum lineare.  As mentioned above, with on-going examination, I discovered a population of the same plant in Tate’s Hell in which flowers show the short style flower form, as can be seen in the photograph below.  

Flowers of a short-style, long-anther form in Tate’s Hell, 8 July 23

On to the second mystery, circumstances surrounding Lythrum flagellare and L. curtissii, plants — plants I’ve not yet encountered.  Examining keys, descriptions, specimens, and photographs available in ISB, these two species pair as having thicker stems and broader leaves, with reddish coloration in stems and foliage, very different from the wispy, green look of Lineare and Alata.  There appears to be no mystery in separating the two.  Lythrum flagellare, a species described by Shuttleworth and published by Chapman in 1883, is prostrate, native to wetlands of Peninsular Florida, and bears slightly larger, magenta flowers than the Panhandle’s erect L. curtissii

The mystery here relates to Fernald’s encountering a Chapman specimen at Biltmore, collected in October, 1897.  Perhaps this was simply too late.  Chapman had issued the final edition of his Flora that year, and he died less than two years later, in April, 1899.  By this time he knew the regional flora better than anyone else, and would have certainly recognized the specimen he collected as representing an undescribed species.  It would be nice to see the Biltmore specimen, simply to learn whether or not Chapman had made annotations as to it’s identification.

The final element, and the real kinker, is that the Graham description of Lythrum curtissii differs significantly from images in ISB, and reads very similar to my Courtyard plants. Only more field encounters will help resolve my dilemma, but I’m happy to hear from others.

The Inside Story


Linné, Carl von,      1707-1778 Species plantarum : exhibentes plantas rite cognitas ad genera relatas, cum diferentiis specificis, nominibus trivialibus, synonymis selectis, locis natalibus, secundum systema sexuale digestas…1753 Sp Pl v1  Dodecandra Monogynia – 7 species treated, pages 446-447,

Pursh, William, 1814 (1813). Flora Americae Septentrionalis; or, A systematic arrangement and description of the plants of North America. Containing, besides what have been described by preceding authors, many new and rare specie… pg 334 publ of Lythrum alata Source: Missouri Botanical Garden, This is 1814, though POWO says 1 Sept 1813 as the publication date.

Fernald, M.L., 1902. Some Little Known Plants from Florida and Georgia, Botanical Gazette 33: 154-157.

Houghton-Thompson, Jamie, Harold H. Prince, James J. Smith and James F. Hancock, 2005. Evidence of Hybridization Between Lythrum salicaria (Purple Loosestrife) and L. alatum (Winged Loosestrife) in North America, Ann Bot, 96(5): 877-885

Shinners, Lloyd H., 1953. Synopsis of the United States Species of Lythrum (Lythraceae),  Field and Laboratory: Vol. 21 : No. 2.

Graham, Shirley A., 1975.  Taxonomy of the Lythraceae in the Southeastern United States, Sida, 6(2): 80-103

Flora – Main Index

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