Euphorbia albomarginata (Whitemargin Sandmat) and E. polycarpa (Smallseed Sandmat)

At most times of the year, there is a rather constant influx of E. albomarginata and E. polycarpa observations. Because of their similar appearance to each other at various parts of their respective ranges, many end up misidentified. Hopefully, this will help provide a reference for people.

The main way to separate the two is by looking at the stipules:

Euphorbia albomarginata (left); Euphorbia polycarpa (right). Photo credits: (left) dlbowls; (right) Jay Keller (used with permission).

Varieties of E. polycarpa

In the United States, essentially two varieties exist (not currently formally recognized due to complications in the phylogeny; for more details read Maya-Lastra's phylogeny paper and the Flora of North America treatment). The two varieties are separated by the peninsular ranges (including the Santa Rosa Mnts. and San Jacinto Mnts.) and transverse ranges (the San Gabriel and San Bernadino Mnts.) in California. The two varieties differ primarily in the size of the glandular appendages but may include other more subtle differences in stipules and habit. When comparing to E. albomarginata, the plants are rarely confused and easily distinguished east of the previously mentioned mountain ranges (E. polycarpa produces somewhat mounding plants with very small appendages). However, the plants west of these mountain ranges are more difficult to separate from E. albomarginata. The difference is that the apical stipules of E. polycarpa are divided (rarely united into a lanceolate segment) and relatively insignificant, the stems don't root at the nodes, the plants may be hairy, and the leaves lack a small, white membranous margin around the leaf blades. Another complication is that the stipules of E. polycarpa are often fused on the undersides of stems. A close look at the nodes may alleviate this complication. The nodes of E. polycarpa lack the prominent bumps of root initials which are usually present in E. albomarginata.

It is also worth noting that E. polycarpa may be completely hairless or densely hairy on both sides of the mountain ranges. This variation has been given varietal status in the past but shows no distributional trends and both forms may pop up in the same population.


Euphorbia polycarpa west of previously mentioned California mountains (left); Euphorbia polycarpa east of previously mentioned California mountains (right). Photo credits: (left) katgrom; (right) Jay Keller (used with permission).


Euphorbia polycarpa stipules (west of mountains). Photo credits: (left and right) Jay Keller (used with permission).


Euphorbia polycarpa, hairy form (east of mountains). Photo credits: Jay Keller (used with permission).


Euphorbia albomarginata (left); Euphorbia polycarpa (right). Photo credits: (left) dlbowls; (right) katgrom.


Euphorbia albomarginata leaf membranes. Photo credits: (left) lonnyholmes; (right) Alex Abair.


Left: iNaturalist observations of E. polycarpa. Right: iNaturalist observations of E. albomarginata.

Posted by nathantaylor nathantaylor, August 16, 2018 03:45

Comments

Any advice on distinguishing hairy E. polycarpa and E. melanadenia?
Looking at the two descriptions differences include:

E polycarpa has slightly shorter stipules
E polycarpa has slightly shorter petioles
E polycarpa is more prostrate on average
E polycarpa blades can get a tiny bit larger
E polycarpa glands are smaller
E melanadenia always has showy appendages (?) whereas E polycarpa can have very short appendages in the eastern part of its range
E polycarpa styles significantly shorter (up to 0.4 mm vs above 0.5)
E polycarpa capsules smaller
seeds quite different but we hardly ever get to look at those
range largely overlaps

That's all very nice, but without the ability to take measurements or compare specimens at scale I'm not sure how I'm supposed to make much use of this information. I think that either the ratio of leaf to gland size, or the length of the styles, may be most useful.
I do notice that the language used to describe the hairs is different, but I also find that the language used by different people regarding hairs tends to vary from person to person enough that I can't always figure out a consistent meaning.

Posted by trh_blue 6 months ago (Flag)

Maybe I can add something to this someday. The main character I use for photos is as follows:
E. polycarpa: hairs straight, generally not densely obscuring surfaces (epidermis of young fruits generally always visible). Good comparative observation (especially since I got it wrong a year ago): https://www.inaturalist.org/observations/41267070
E. melanadenia: hairs almost always appressed or at least curved, often densely obscuring surfaces leading to a gray appearance, especially young fruits (epidermis of young fruits rarely visible through the hairs). Good comparative observation (magnificent view of the the behavior of young hairs and beautiful shot of a cyathium): https://www.inaturalist.org/observations/70479554

Identification is relatively simple once you get the hang of it. And, of course, it is only a problem west of the California mountain ranges mentioned above. Everywhere else, appendage length will easily separate the two.

Posted by nathantaylor 6 months ago (Flag)

another request, Nathan. Could you add a set of photos clearly showing how to tell whether these plants are rooting at the nodes / what it looks like in photographs, where you can't go poking at the plant physically. The nodes themselves look approximately the same to my untrained eyes (apart from the stipules of course), and when the plant is not in direct contact with the soil it looks as though neither is rooting.

Posted by trh_blue 5 months ago (Flag)

Bit too busy to do that right now. Would you remind me after the 30th?

Posted by nathantaylor 5 months ago (Flag)

Hey there! I'm reminding you.

Posted by trh_blue 5 months ago (Flag)

Hmm. I could have been sure I had a better root primordia photo uploaded. Anyway, maybe you can get what you need from this image:
But see also here: https://www.inaturalist.org/observations/2104117
It is impossible to tell if the plant is rooting at the nodes or has root primordia unless the stems are flipped over and the undersides are visible so this usually isn't helpful for photo IDs.

Posted by nathantaylor 4 months ago (Flag)

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