Eurybia – Following Cassini’s Voyages

The Cover Story

Many people will know of the Cassini orbiter, a 20-year mission of planetary discovery, launched on 15 October 1997, exploring features of Saturn first discovered by Giovanni (Jean Dominique) Cassini (1625-1712), a brilliant Italian-French astronomer. Over three additional generations, the Cassini family would come to hold prominent positions in French astronomical and geographical study, a line ending at the 5th generation with Henri, who devoted his genius to plants rather than planets.

It strikes me a nice coincidence that our botanist, Henri Cassini (1781-1832), spent his career studying sunflowers and asters, a group of plants marked by stellar allusions.  But he was in it for the task itself.  In over 60 works  published in the first third of the 19th century, Cassini described 130 genera still recognized today, created classification schemes, and basically laid the foundations of modern synantherology  (the study of Composites).  In the Florida flora, of 140 recognized genera in the Asteraceae, 19 are attributed wholly or in part to Cassini, including the well-known Carphephorus, Heterotheca, Pluchea, and of course Eurybia.

The Main Story

Cassini was unconcerned with our local Thistleleaf Aster when he erected Eurybia.  He never knew the plant, which was described by Torrey and Gray as part of their work on The Flora of North America (see more on this topic at the end of this article).

As with so many other Florida plants, Torrey and Gray described Eurybia eryngifolia (their Aster eryngifolius) based on material supplied by Dr. Chapman, getting their story straight after learning the ray flowers are white.

This is an easily-spotted, fairly abundant plant in pine flatwoods of Apalachicola National Forest, where I encounter them in northern Franklin and southern Liberty Counties. Clusters of a few flowering heads (3-4+) with a breadth of 6-8 cm (2-3″) terminate single scapes about a half meter (1.5-2.5′) tall. Each scape arises from a rosette of linear leaves, for all the world looking like the leaves an Eryngium, or some kind of monocot, and is covered with smaller cauline leafy bracts (or bract-like leaves). Margins of the tough leaves are mostly entire, marked by conspicuous zones of spininess, often with a shredded edge.

Immature flower heads remind me of sea urchins, clustered balls of spiny bright green involucral bracts, from the apex of which white ray petals emerge, first vertically, then spreading to reveal the white rays and yellow discs.

Once the flower heads open, the plants are difficult to miss, even when driving along roadways.

What perplexes me, however, is the identity of a sister species, Eurybia spinulosa, which has been recorded in just a few counties at the heart of the Eurybia eryngifolia range. The ISB Atlas shows a single specimen, from Gulf County, and several photos contributed by Guy Anglin. Floristic treatments tell me E. spinulosa bears more heads than is typical for E. eryngifolia, but each head is smaller, sporting many fewer ray flowers, & held captive by tighter, non-spreading involucral bracts. This is a plant I’ve not encountered, so the differences remain somewhat mysterious for me.

The Inside Story and the Back Story

Taxonomic details wrapped up in the history of a plant name can become highly involved, in that following the histories of names, their connection to specimens, and the rules of nomenclature can be complex (and pocked by missing information).  Taxonomic Nomenclature, you might discover, is the definition of the word “arcane.” 

Let’s discuss history of the generic name Eurybia first. I would never have been able to appreciate that history without access to an incredible article, published by Christina Flann, Werner Greuter, and D. J. Nicholas Hind in volume 59 of TAXON:  “Cassini’s Composite genera: A nomenclatural and taxonomic assessment.”   And those authors would not have been able to determine the facts about Eurybia without access to publications by Guy Nesom, who studied the plants and traced the full taxonomic history in order to bring order to the circumstances.  

But for Henri Cassini, two centuries ago, things were different.  Working only a few decades after Linnaeus, and before rules of plant nomenclature were codified, Cassini’s approach was much looser than what would constitute valid publication today, and his train of thought can be challenging to follow.  Publishing in French, he formulated many of today’s genera as sub-genera, for example, names that only later would be made valid.   Eurybia is of that sort.  His verbiage waffled somewhat.   Having first clearly described Eurybia as a sub-genus in 1818, in 1820 Cassini wrote: “This new genus of plants, or rather sub-genus”, in the context of discussing the taxon as a subgenus.”  (translation by Flann, et al)

Eurybia was upgraded to generic status in 1821 by Samuel F. Gray, in his book, The Natural Arrangement of British Plants. Gray applied Eurybia to entry on the native Sea Hog’s Beans, Eurybia maritima .  But even that is riddled with arcane taxonomic issues.  The plant Gray named Eurybia is now considered Tripolium pannonicum, thus even though the plant he included is now known by a different name, his publication was valid enough to create the genusWhen Nesom resurrected Eurybia (having been subsumed in Aster), he selected a species Cassini had studied as the type, a plant Aiton had described as Aster corymbosa.  That plant is now considered synonymous to Linnaeus’s Aster divaricata.  The upshot is that Eurybia divaricata is the type oforthe genus. It’s all very legal. 

Subsequent to the work of Guy Nesom, Luc Brouillet submitted a treatment of Eurybia, accepting 23 species for the Flora of North America (FNA).  His conclusion was that Cassini’s 1820 Dictionary treatment does validate the genus, thus he contradicts Nesom as well as Flann, et al in accepting Cassini as the authority:

193. Eurybia (Cassini) Cassini in F. Cuvier, Dict. Sci. Nat. ed. 2. 16: 46. 1820. [Greek eurys, wide, and baios, few, perhaps alluding to the few, wide-spreading ray florets]

Brouillet includes 23 species of Eurybia native to North America and northern Eurasia.  In his treatment, Brouillet doesn’t recognize Nesom’s section Eryngiifoliae, rather he groups Thistleleaf in section Heleastrum.:  

Eurybia traditionally has been treated within Aster in a broad sense. G. L. Nesom (1994b), in his review of North American asters, showed that Aster in a broad sense does not form a natural group and proposed splitting it into several genera, among which Eurybia is one. In his treatment, Nesom included Herrickia within Eurybia, as sect. Herrickia in subg. Eurybia. Such views were generally supported in molecular phylogenetic studies (J. C. Semple et al. 2002). L. Brouillet et al. (2004) showed, however, that Oreostemma, Herrickia, Eurybia, and Triniteurybia form a grade at the base of the Machaerantherinae and that Herrickia and Eurybia are distinct.

The subgenera and sections proposed by G. L. Nesom (1994b), based on anterior taxonomy, could not be confirmed in the molecular studies cited above. I chose not to use subgeneric limits as proposed by Nesom because they may not reflect actual relationships. For instance, there is a clear gradation between members of sect. Calliastrum (Torrey & A. Gray) G. L. Nesom (subg. Eurybia) and sect. Heleastrum (de Candolle) G. L. Nesom. Also, I do not recognize sect. Eryngiifoliae (Alexander) G. L. Nesom distinct from sect. Heleastrum, as there is no clear demarcation between the two as currently defined. Finally, sect. Radulini (Rydberg) G. L. Nesom appears artificial to me, but currently there is no good way to reassign its species. The Eurybia radulina complex of western North America clearly constitute a group, but it is unclear whether the western E. conspicua or the eastern E. radula and E. saxicastelli are close to them. Members of other sections may have played a role in the reticulate evolution of sect. Eurybia, even though it is well marked by its cordate leaves and disc florets with long tubes and short, campanulate corollas. Therefore, species are described below in a rough taxonomic order, with diploids listed before polyploids of the same group.

Having recorded the saga of our genus Eurybia, I might as well relate the tale of the specific epithet. As mentioned in my Main Story above, Torrey and Gray first described the plant we call Eurybia eryngifolia, about a decade after Cassini’s death. But they did not describe it once; it was published twice! It seems Torrey and Gray described the Thistleleaf Aster as Prionopsis? chapmanii (their question mark) on page 245 in their Flora, based on material from Chapman. On nearly the final page of Volume 2 , their Flora (page 502), in the Appendix, we see a description for Aster eryngifolius, including the conclusion this was a better decision for Chapman’s plant: “As the rays of this plant prove to be white instead of yellow…” Somewhere along the line, the flower color became clear,. The rays are white, which was one reason the authors determined it must be an Aster. Of course they could not now make this new Aster eponymous for Chapman because back on page 161 they had already described a beautiful blue-flowered plant as Aster chapmanii (see below, currently called Symphyotrichum chapmanii). So there it is. Chapman was sending such a rich trove of material to Asa Gray that his botanical friends in the big cities were stumbling over themselves attempting to recognize his contributions, much less interpreting the meaning of all this material. 

I’d be surprised if this is the end of the story. Examining photos of the type species, Eurybia divaricata, this doesn’t look like a comfortably permanent home for our native Thistleleaf Aster.  Some of us won’t be around in 50 years for final resolution, but given enough protected forest, our plant will persist – perhaps under yet another name.

We end with quotations Semple and Brouillet used to introduce their 1980 publication (in AJB): “A Synopsis of North American Asters: The Subgenera, Sections and Subsections of Aster and Lasallea”:

“Notwithstanding the very favourable opportunities we have enjoyed, our arrangement of this, probably the most difficult genus in North American botany, although the result of much labor, is by no means so satisfactory as could be desired” Torrey and Gray (1842)

“I am not very confident of the success of my prolonged endeavors to put these genera into proper order and to fix the nomenclature of the older species; and in certain groups absolute or practical definition of the species by written characters or descriptions is beyond my powers.” on Aster and Solidago, Gray (1882)

%d bloggers like this: