Question for botanists: how do you describe the genus Euphorbia? This is not an identification question, just a matter of curiosity.
@nathantaylor is an expert on Euphorbias:
- Nathan is the top identifier of the genus, with over 200,000 Euphorbia IDs
- Nathan was mentioned in this Identifier Profile and this Observation of the Week
- Scroll to the end of the posts for more Euphorbia resources
Do you mean, what are the common characteristics of all Euphorbias that let botanists know that each is Euphorbia?
Not sure if this is what you are getting at, but personally I find it hard to describe the genus as currently circumscribed, because it contains such a huge diversity of forms worldwide as to have descriptive and predictive value more equivalent to most Tribes or Subfamilies. I find much more value in some of the narrower monophyletic groups that have been recognized within the genus. For example, I can always recognize a Chamaesyce when I see it in the field. But I can’t say the same for the whole genus Euphorbia.
they have highly specific inflorescences called cyathia, not found in any other plant, and unisexual flowers reduced to 1 stamen or 1 pistil,.
that said, there have been attempts to divide it into several genera (it would be easiest if it went to 4, Chamaesyce for mostly non-succulent mostly new-world plants (incl. things like poinsettia), Euphorbia and Rhizanthium for mostly succulent, cactus like old world plants, and Esula/Tithymalus for mostly old-world herbs). The problem is that Rhizanthium and Euphorbia are not eachothers closest relative , but are adapted to similar conditions and share both more and less drought adapted plants, and extreme xerophytes that evolved multiple times in both groups, so they are not morphologically easily definable. One solution would be to make the more or less well defined sections their own genera, resulting in maybe like 10-15 genera, or to keep it like it is, a genus united by its inflorescences, but not by it vegetative morphology. Most of the sections and subgenera are also quite old, I think. (while this is a solution, it is akin to putting all Apiaceae into Apium because their inflorescences are similar, and it makes a very large genus and obscures the diversity of Euphorbiaceae. But since genera are arbitrary it ultimately does not matter)
…or Asteraceae into Aster, or Poaceae into Poa…
@ljazz Thanks.
That is a very big question and one that a single comment cannot answer appropriately. However, I’ll give you a very broad sense,
- Phylogeny: there are 4 major clades to which Euphorbia belongs: subg. Esula, subg. Athymalus (new name for subg. Rhizanthium), subg. Chamaesyce, and subg. Euphorbia (listed in order of earliest divergence).
- Unifying character: All these species are united by the cyathium (they also all have latex, but that doesn’t get talked about as much).
- Distribution: With the exception of subg. Athymalus, all clades have species on every continent. All subgenera seem to have originated in the old world, but some have very prominent radiations in the new world.
- Body plan: In non-woody species, the basic body plan can be defined by a terminal pleiochasium, with dichasia branching off of it. Though, probably not ancestral, it is extremely widespread throughout the genus. Many groups have taken this body plan in some pretty weird directions. For example, the cyathophylls of E. milii are simply dichasial bracts (though this species doesn’t produce a pleiochasium).
- Succulence and woodiness: The genus has evolved succulence independently in at least 3 of it’s subgenera (I have not read about succulence in subg. Esula). All subgenera have evolved woodiness. Woodiness may be ancestra. The genus Euphorbia includes the only known C4 trees.
- Photosynthesis: The genus has evolved 4 different photosynthetic systems across it’s lineages: C3, C4, C2 (a C3-C4 intermediate), and CAM.
Actually understanding the diversity of the genus is best done by going beyond the subgenera and learning the sections. The subgenera are so diverse that I have not observed nor read about any unifying features. Many lineages within these subgenera include species that have extremely similar body plans (the body plan described above can be found in all subgenera). At the level of sections, remarkably distinct clades emerge (see sect. Anisophyllum, sect. Monadenium, and sect. Crepidaria for examples).
If you want to learn more about the evolution of the group from a very broad, phylogenetic perspective, this is a good article: https://www.sciencedirect.com/science/article/pii/S1055790311005483?via%3Dihub
If your new to Euphorbia but want to delve into the sections, I would recommend looking through a few of the journal posts I’ve written. I’ve organized them here: https://www.inaturalist.org/projects/euphorbia-species-of-the-united-states/journal/18753-recommended-recourses
For what it’s worth, the genus is overlumped in my opinion. However, if you wanted to adhere strictly to monophylly and have half-way diagnosable groups, I think it would take more than 15 genera. Even then, it would still be difficult to carve out useful genera in the grades at the base of the subgenera. If you relax monophylly to include some paraphyletic taxa, you could make things a bit more manageable, but still difficult and likely pretty controversial (not that any genus-level Euphorbia classification hasn’t been been plagued with argument and controversy).
Edit: I just counted them up and there are 64 sections of Euphorbia (Athymalus: 7; Chamaesyce: 15; Esula: 21; Euphorbia: 21). Adhering to strict monophylly and diagnosable groups would probably force you to accept all or nearly all of those sections as genera. I hope that gives you some sense of the scale of the problem. For extra context, there are over 2,000 species within the genus.
@nathantaylor Thank you, I really want to know more about them, so I’m going to save everything.
Adhereing to monophyly is non-negotiable, (complete) diagnosibility is.
not really all of those would need to be split. Esula and Chamaesyce are diagnosable by themselves, and I think some sections of Euphorbia and Athymalus group together in a way that could be sensible to split, even if some spp. would be the odd ones out (at least in plants, that happens all the time, I think). Anyway, I am aware of the conundrum and the 2000 spp.; I also agree it is overlumped (and at the very least, Esula should be separated). I am not a spurge expert, but I talked with one, and he seemed to think it is not so bad
Esula overlaps quite a bit with sect. Nummulariopsis. The woody Euphorbias would also likely pose a significant problem. Also, what do you mean by Chamaesyce? Subg. Chamaesyce is certainly not easily diagnosed, but sect. Anisophyllum (syn. genus Chamaesyce) is probably the most diagnosable group in the genus with both anatomical and structural characteristics that distinguish it from others.
You might try and run the Flora of North America through a few species in the US (especially species of sect. Alectoroctonum that lack appendages). The characteristics are about the best they could have come up with, but it still fails pretty badly with some species. While not quite a local scale, it’s not global either and reflects a good middle ground to think about the problem.
Just to be clear, I agree with you. I’ve spent quite a bit of time thinking about how I would carve it up and might even attempt to do so one of these days. My only point is that the problem is more complicated than it appears. Even if done successfully, many will want to work to reverse it if history is anything to go by. The history of Euphorbia is one of numerous name changes accompanying switches from treating recognizable groups at genus, subgenus, or section. This forces one to ask the question of whether its worth it or not. The situation now is not ideal, but it has very little impact on me given that I access to databases and lists that tell me which section almost all the different species belong to. If I want to mention a group within Euphorbia, it’s easy for me to do so. I recognize that it’s harder for others. That said, if you have more than 15 genera, would the people who don’t know these associations be able to identify the species to those genera?
These are the things I think about whenever I’m trying to address this problem. The easily recognizable groups pull people towards splitting and the messy, poorly recognizable groups pull people towards lumping. Euphorbia has both kinds of clades, and thus, both kinds of taxonomists. This means that the group will likely always be caught in the tension of a data-agnostic lumping/splitting problem. This makes the problem more philosophical than a scientific at this point in time. As such, I tend to spend my time concerning this problem thinking of ways to make it more scientific by describing the problem with data, but that’s a much different kind of discussion and plagued with its own challenges.
I think I can understand your question. The genus Euphorbia is a very large and extremely variable genus. It may seem very odd that the small creeping, often weedy herbs from the section Chamaesyce are in the same genus as the leafless succulent tree size species from section Euphorbia. They seem to have nothing in common. It has indeed been tried many times to split the genus but this has never been widely accepted. Modern phylogenetic evidence has even enlarged the genus further with previously accepted genera having been sunk under Euphorbia now (Monadenium, Synadenium). The only, but rather essential, thing that all the different looking groups in Euphorbia have in common is the unique morphology of the flowering/fruiting stuctures called Cyathia, which separately contain both female and male flowers that do not look at all like the “traditional” flowers with petals and sepals. The Cyathia present themselves like one flower-like structure to the outside pollinating world, albeit in a uniquely weird form. It is very differnt looking but not unlike the functional principle of the composite capitula in Asteraceae. I believe that that unique Cyatha structure is what (so far) keeps the genus Euphorbia together as one,
Hope this helps,
greetings, Bart
As a slight aside, there are at least three genera in Euphorbiaceae but outside Euphorbia that have produced pseudanthia that are very similar to the cyathium (Anthostema, Neoguillauminia, and Dalechampia). Dalechampia seems to have evolved the structure independently, while (if memory serves), Anthostema and Neoguillauminia are very closely related to Euphorbia. I highly recommend looking them up. Also, the following article has a good discussion on this: https://bsapubs.onlinelibrary.wiley.com/doi/10.3732/ajb.94.10.1612.