Eremothamnus marlothianus, a dicotyledonous plant oddly combining foliar spinescence with foliar succulence
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In a recent Post, I pointed out the oddness of Oberholtzeria etendekaensis (https://en.wikipedia.org/wiki/Oberholzeria).
This Namibian dicotyledonous plant seems to be the only candidate, worldwide, for a leguminous plant (https://en.wikipedia.org/wiki/Fabaceae) with succulent leaves of a xerophytic kind (https://en.wikipedia.org/wiki/Xerophyte).
This finding is remarkable because foliar succulence is vanishingly rare in the otherwise extremely diverse legumes. The few other spp. with some degree of fleshiness in their leaves, in this 'superfamily', tend to be somewhat halophytic (https://en.wikipedia.org/wiki/Halophyte), rather than xerophytic.
Here, I point out another species, restricted to a small part of Namibia, that is so rare in its combination of features that it may perhaps be the only candidate on Earth for a certain category of dicotyledonous plants (https://en.wikipedia.org/wiki/Dicotyledon).
The conceptual framework:
Foliar spinescence and foliar succulence tend to be mutually exclusive in plants.
This is because foliar spinescence is associated with a hard, dry, ligneous texture in leaves, called sclerophylly (https://en.wikipedia.org/wiki/Sclerophyll). By contrast, foliar succulence is associated with softness in leaves.
Worldwide, the main examples of plants combining foliar spinescence with foliar succulence are monocotyledonous rosette-plants (https://en.wikipedia.org/wiki/Rosette_(botany)):
- Asparagaceae: Agave (https://en.wikipedia.org/wiki/Agave), in which the spines occur at the tips of the leaves (https://en.wikipedia.org/wiki/Sisal), and
- Asphodelaceae: Aloe (https://en.wikipedia.org/wiki/Aloe), in which the spines are restricted to the sides of the leaves.
The daisy family, Asteraceae (https://en.wikipedia.org/wiki/Asteraceae), contains certain foliar-spinescent spp., and other spp. that have succulent leaves.
Furthermore, this holds true within the flora of southern Africa.
Here,
- examples of foliar spinescence include Berkheya (https://www.inaturalist.org/observations?place_id=any&taxon_id=147338&view=species) and Cullumia (https://www.inaturalist.org/observations?place_id=any&taxon_id=461737&view=species), and
- examples of foliar succulence include Kleinia (https://www.inaturalist.org/observations?place_id=any&taxon_id=186256&view=species) and Curio (https://www.inaturalist.org/observations?place_id=any&taxon_id=186256&view=species).
However, both types of adaptation are rather poorly developed in Asteraceae, including in southern Africa.
This is because
- sclerophylly is surprisingly rare in this family, so that the main examples of foliar spinescence in Asteraceae are thistles (https://en.wikipedia.org/wiki/Thistle), and
- the succulent leaves occurring in this family tend to be relatively thin, and better-described as 'fleshy' (e.g. in Didelta, Chrysanthemoides, Osteospermum, and Othonna).
Given the patterns above, there would seem to be scant chance that Asteraceae would produce a species combining the features in question.
And yet this combination has indeed been approached, in the form of the monospecific genus Eremothamnus (https://en.wikipedia.org/wiki/Eremothamnus and https://www.inaturalist.org/taxa/191899-Eremothamnus-marlothianus).
The habitat of E. marlothianus is remote. Nonetheless, the photos currently posted in iNaturalist are sufficient to show the leaves clearly:
https://www.inaturalist.org/observations/71415179 and https://www.inaturalist.org/observations/108873382 and https://www.inaturalist.org/observations/108860922 and https://www.inaturalist.org/observations/108857770 and https://www.inaturalist.org/observations/123853514 and https://www.inaturalist.org/observations/110076204.
As far as I know, there is no species anywhere else on Earth that comes close to combining foliar spinescence with foliar succulence in any dicotyledonous lineage, let alone in Asteraceae.
Both of the odd species, the legume O. etendekaensis and the daisy E. marlothianus, are herbaceous rather than woody, and occur on the edges of the Namib Desert (https://en.wikipedia.org/wiki/Namib).
However, the two monospecific genera differ in that
- O. etendekaensis occurs on slopes on the inland edge of the desert, whereas E. marlothianus occurs on flats on the coastal edge, and
- the former species occurs in the northern part of Namibia (https://en.wikipedia.org/wiki/Kaokoland and https://en.wikipedia.org/wiki/Damaraland), whereas the latter occurs in the southern part of the country (https://en.wikipedia.org/wiki/Tsau_%C7%81Khaeb_Sperrgebiet_National_Park).
Kommentarer
interesting - thanks - Oberlhozeria also monotypic? Eremothamnus was one of the sp I most wanted to see on my recent trip up that way - best I check out Oberholzeria, as may be going up there in Dec
Fascinating! And what a beautiful plant, too.
Very interesting, thank you for sharing this information
Most interesting, thank you!
About succulent leaves in Asteraceae, Senecio angulatus is reported as succulent (Wikipedia) (or shall we say 'fleshy'?), native to South Africa and well settled on the Mediterranean coast, here in dry conditions at the end of the summer.
@jeanphilippeb
Many thanks for your useful comment.
It is indeed true that, in its country of origin, Senecio angulatus would be described as 'fleshy' rather than succulent. This is because succulence is somewhat relative, and there are so many truly succulent non-halophytes in this country.
If it had been the case that S. angulatus were indigenous to Australia (which it is not), it would be have been among the most succulent non-halophytic indigenous plant species on that whole continent.
Because Senecio angulatus is now so anthropogenically widespread on Earth, it may be of interest to readers to know of its original, natural habitat in South Africa. However, the facts seem obscure.
My guess is that Senecio angulatus forms a liane in a coastal type of vegetation called 'strandveld' on calcareous sands and limestone, which would be better-described by the Australian term 'vine thicket' (see (https://www.inaturalist.org/observations/129937183). This is a chaparral- or maquis-like vegetation of evergreen tall shrubs, but differs from chaparral and maquis by being non-flammable ('pyrofugic').
@tonyrebelo and @jeremygilmore Tony and Jeremy, could you please fact-check what I have written above? What do South African botanical field-guides say about the natural occurrence of S. angulatus?
Similar vine thicket occurs widely in eastern Australia, but lacks any liane that has even the fleshy leaves seen in S. angulatus.
However, the 'natural' origins of S. angulatus are obscure, because it seems to be deeply commensal in the sense of a 'horticultural weed' (see https://www.aphis.usda.gov/plant_health/plant_pest_info/weeds/downloads/wra/Senecio_angulatus_WRA.pdf).
For example, I find it puzzling that this species is not listed as present in Albany Thicket (http://biodiversityadvisor.sanbi.org/wp-content/uploads/2015/12/Strelitzia_19_2006_Part_4.pdf), where it would seem to fit in, given the proliferation of succulent shrubs and various lianes.
Senecio angulatus cannot be described as xerophytic, but instead exemplifies a basic tendency in the southern African flora, towards succulence in various growth-forms in many families of plants, extending into mesic climates.
https://www.citscihub.nz/Phil_Bendle_Collection:Senecio_angulatus_(Cape_ivy)
The SANBI Red List describes Senecio angulatus as a species of the Western and Eastern Cape provinces, however as far as I have gathered, this only applies from the eastern side of the Southern Cape and then further eastwards.
While it does hug the coast, strictly speaking I wouldn't say a "Strandveld" species (speaking of the Fynbos-type referred to around Cape Town).
Interestingly, in the Garden Route, it seems to occur on that transition from coastal Fynbos to Afrotemperate Forest; possibly suggesting it likes the higher rainfall, which would explain why the majority of it's range does not extend much further West than this?
Its fleshy nature makes it a tough coloniser here in the West too (as a garden escapee), so its foothold in the Mediterranean is not at all unexplainable if so - especially when we think of a species like Cotoneaster glaucophyllus & friends from the Himalayas naturalising in Mediterranean-type Cape Town!
What I don't understand is the exclusion from Albany Thicket, as it surely does occur there... perhaps a mistake?
Keen to hear more opinions - if I haven't added too much controversy already :|
@jeremygilmore Many thanks for this useful information.
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