There are quite few ways to go about this, but here’s what I do.
First, if there’s a field guide or similar resource that covers the right geography, that’s likely to be the best, easiest place to read the distinction. In the case of Glebionis, I’d be looking for something that covers Asteraceae / Asteroideae in Europe or the Mediterranean. In general, it’s best to avoid older guidebooks, as species may have been added or renamed in the years since publication.
For some regions there may be a comprehensive online key. For example, the Jepson eFlora is a comprehensive guide and key to all wild plants in California (native and naturalized). In this case, both species of Glebionis are naturalized in the state, so the Jepson eFlora gives us a key pair that distinguishes the two species.
1. Leaves deeply 2–3-pinnately dissected; ray fruit 3-winged; disk fruit 4-angled with adaxial angle winged, faces ribbed … G. coronaria
1’ Leaves simple , toothed or ± shallowly 1(2)-pinnately lobed; ray fruit 2-winged; disk fruit ± cylindric or obovoid, not winged, 10-ribbed … G. segetum
So that gives us two ways to distinguish the species based on the fruits (also know as achenes), which are probably really difficult to see in most iNat observations, plus one way to distinguish the species based on the leaves, which seems more promising.
It’s going to be helpful to read the definitions of bipinnate, tripinnate, dissected and lobed. But, based on the Jepson key, my take is that plants with intricate, fern-like leaves would be G. coronaria and those with less extremely divided leaves are G. segetum.
But we were just fortunate to find both species in the Jepson key. What if that wasn’t available? A great starting point is the Tropicos site from the Missouri Botanical Garden. Despite the name, Tropicos covers all vascular plants globally. You’ll need to request a (free) account, because Tropicos recently limited how anonymous users can use the site due to abuse.
Once you’re on Tropicos, you can get a list of all the species described within a genus. Here’s what the search for Glebionis shows.
From this, I can see that this genus has very few taxa, which should make the task easier than the genera with hundreds or thousands of species. I can also see that the species iNat recognizes were described in 1841 and 1869. If you click the links, Tropicos has a page on each species, and many times there will also be a link to the original description, generally hosted by the Biodiversity Heritage Library.
For G. coronaria this will take you to a half-page French description of the species from 1841. For G. segetum there is only a single line in an 1869 flora of the Rhône region. (This is more sketchy than usual.) At this point I might focus on more recent articles on the genus, which might help make some sense of the distinctions. Tropicos has a “References” tab for each taxon, so I’d check that to see what I have access to.
I might also look to see which taxa in the genus were most recently described, on the basis that these articles are more likely to contain a comprehensive, up-to-date key and might be available online. I can see that Glebionis coronaria var. discolor was described in 2004, which seems promising, but once I read that the citation is “Turland, Nicholas John. Taxon 53(4): 1073. 2004” I realize that this is likely to be a closed-access article, and from more detail in the Tropicos record I can see that article was where the variety was transferred into Glebionis from Chrysanthemum, so I might assume that was just a brief nomenclatural note and I’d be less confident that the article provides a detailed distinction between the species.
Another tactic is to search Google Scholar (or another academic search engine) for text such as “Glebionis” and “key”. The first result from that search is a 2017 article that goes into some depth on how to distinguish the two varieties of Glebionis coronaria.
- Cano E, Musarella CM, Cano-Ortiz A, Piñar Fuentes JC, Spampinato G, Pinto Gomes CJ (2017) “Morphometric analysis and bioclimatic distribution of Glebionis coronaria s.l. (Asteraceae) in the Mediterranean area”. PhytoKeys 81: 103-126. https://doi.org/10.3897/phytokeys.81.11995
In fact, this 2017 article raises Glebionis coronaria var. discolor to a full species as Glebionis discolor, so maybe we need to consider three species now! But, if we check with POWO, we see that they have chosen to accept an opinion from 2018 that treats Glebionis coronaria var. discolor as a synonym of Glebionis coronaria. The easiest approach for now is to trust the iNat and POWO taxonomy until/unless we learn enough to think that it’s wrong.
Back on the hunt for clear distinctions G. coronaria and G. segetum, the Cano et al 2017 article makes clear that Turland’s 2004 article is where this distinction was laid out.
- Turland, N.J. (2004), “(1647) Proposal to conserve the name Chrysanthemum coronarium (Compositae) with a conserved type”. Taxon, 53: 1072-1074. https://doi.org/10.2307/4135582
We’re in luck that this is actually a free access article! Most of it addresses an unfortunate collision between taxonomic rules and 250 years of botanical history. Fortunately for our purposes, Turland gives a clear distinction between the species.
The two species are quite distinct: C. coronarium has leaves green, 2-pinnatisect, ray florets either yellow or white with a yellow base, and achenes of ray florets with an adaxial wing; C. segetum has leaves glaucous, irregularly incised-dentate, ray florets always deep yellow, and achenes of ray florets without an adaxial wing; in both species the disk florets are yellow.
That seems consistent with the Jepson key, and adds additional info on the color of the ray florets and leaves.
Anyhow… I hope that gives you some ideas about how to start researching species differences more generally!
