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Excerpts from Jim Conrad's
Naturalist Newsletter
entry dated March 27, 2022, issued from near Tequisquiapan, elevation about 1,900m (6200 ft), ~N20.57°, ~ W99.89°, Querétaro state, MÉXICO
CILANTRILLO, TINY-FLOWERED WEED
Now during the late dry season, weedy, fallow fields waiting for rain are pretty desolate looking, and one looks for flowering plants other than Prickly Poppies, Argemone ochroleuca and Dyer's Rocket, Reseda luteola. Finally, a tiny, inconspicuous weed at the edge of what's left of a cornfield turns up, shown at the right.
It's flowering, with tiny aster-like, composite blossoms, but the flowering heads are so small they hardly show in the picture. The most obvious one appears between the tips of my two middle fingers. Below you see an open one and two soon-to-open buds, their small sizes to be compared with the friction ridges on my finger at the right.
The tiny glistening globules covering the buds are glands on very short stalks, or stipes. Below you see that such "stipitate glands" also densely cover the green, cup-shaped involucre below a flowering head atop my fingernail.
Such aster-like flowers clearly belong to what many regard as the largest of all families of flowering plants, the Composite or Aster Family, the Asteraceae. Knowing that to identify this plant it might be hard to see enough features to distinguish it from the 24,000 or so other species in the family, it was good to see this dense coating of stipitate glands, because most species don't have them. Also, the small size is important. Similarly important to notice in the above picture is that the numerous green bracts composing the involucre are slender, more or less of equal lengths, and do not overlap one another like shingles on a roof.
However, quite a number of other species are small and bear such glands on numerous slender involucral bracts of equal lengths, so now the challenge becomes to find more distinctive field marks, and for that a head was broken open, as seen below.
Among the main identification features revealed here is the fact that between the oblong, cream-colored ovaries stacked next to one another in the picture's center, there are no scoop-shaped, papery scales called paleae. That by itself eliminates thousands of species. Also, the platform on which the ovaries stand, the receptacle, is flat, not hemispheric or convex. Features atop the ovaries are critically important, but they're easier to see atop the cypsela-type fruit (hard, not-splitting-open, one-seeded), shown in the fruiting head that develops when the flowering head matures, shown below:
The hairlike projections atop each cypsela will catch in wind and carry the fruit to new destinations. Each collection of hairs is referred to as a pappus, and they're what became of the white fuzz seen atop the cypselae in the broken-open head. Thousands of Asteraceae species have no pappus, and thousands of others have pappuses of a different kind -- scales, spines, crowns and such -- so this is very helpful.
And here's something really surprising: Corollas of the yellow disc florets forming the flower head's central "eye" have fallen off, but, seen at the bottom of the above cluster, corollas of the white ray florets along the flowering head's perimeter have just shriveled somewhat and curled up, but they're remaining attached to their cypselae. Yet even more unusual is that atop the ray florets' cypselae there are no pappuses, despite the disc-floret cypselae having them. That's a clincher, leading us to just one genus among the Composite/Aster Family's over 1600 genera.
But just for good measure, here's what our plant's lower leaves look like:
With help from the Flora of North America, which doesn't cover Mexican plants, but which includes most arid-land, northern and central Mexico genera, and does provide identification keys to the Aster Family's genera, the keys lead to genus Psilactis. The great Malezas de México website ("Weeds of Mexico") has a page for a Psilactis species whose pictures and descriptions perfectly match ours, so now we have it: PSILACTIS BREVILINGULATAS, with no good English name, but in Spanish often called Cilantrillo, "Little Cilantro."
Cilantrillo is distributed from the arid US southwestern border with Mexico all through arid, upland Mexico, and has turned up in Colombia and Perú. Unlike most weeds here, it's native Mexican, occupying upland pastures, clearings of scrub and oak-pine forests, burnt areas, roadsides, and weedy fields.
Apparently it's too small to be considered a serious weed, or to have much use to humans. It's simply one of those millions of species who are sufficiently themselves, adding texture and stability to the fabric of Nature.
entry dated April 6, 2023, issued from near Tequisquiapan, elevation about 1,900m (6200 ft), N20.565°, W99.890°, Querétaro state, MÉXICO
(~N20.55°, ~W99.89°)
A SECOND LOOK
A year later I run into this species again, and develop some new insights. For one thing, now I see that in English it can be called a Tansyaster.
Our plant's genus, Psilactis, is a small one with only seven species currently recognized -- all found only in the New World. In our upland, central Mexican region, two or three species are documented, and of these our plant is easily separated by its ray florets bearing white ligules only 2-3mm long (~1/16 inch). The species name, brevilingulata, celebrates our plants "brief ligules."
Psilactis brevilingulata occurs in fields, along roadsides, stream and ditch banks, and open areas in oak and pine forests, from the Chihuahuan Desert along the US's southwestern border with Mexico south through upland Mexico, then again in northwestern South America. In the US it's habitually named the Trans-Pecos Tansyaster -- from the Easterner's perspective occurring on the far side of the Pecos River -- but the species is overwhelmingly a Mexican and South American plant, so that name won't do. Here it's better to just call it Tansyaster, a name applicable to all species of Psilactis.
In evolutionary biology, studies determine where Psilactis, with its curious lack of pappuses on ray florets, fits on the Aster Family's big branch of the evolutionary Tree of Life. One insight has been provided by the 2009 study of Jamil Vaezi and Luc Brouillet entitled "Phylogenetic relationships among diploid species of Symphyotrichum (Asteraceae: Astereae) based on two nuclear markers, ITS and GAPDH":
Branches on the evolutionary Tree of Life usually are forked, or Y-shaped, with two taxa emerging from one. Our Psilactis, in contrast, has arisen as part of a "polytomy," which is when three or more, not just two, taxa arise from a single ancestor. That can happen, for example, if the rainy climate supporting a vast rainforest, over a long time, gradually dries out. The single enormous rainforest may fracture into several "islands" of rainforest surviving in the rainiest parts of the previously unbroken rainforest. If these islands are genetically isolated from one another by intervening grasslands, over the centuries taxa within each island may evolve in ways different from in the other islands, possibly producing new species in each island. All the new species resulting from this event will appear on the evolutionary Tree of Life as three or more species forming a polytomy branching from a single point.
Our Psilactis, write Vaezi and Brouillet, form a polytomy with the genera Almutaster and Symphyotrichum. We've met a Symphotrichum species here in Querétaro, the Saltmarsh Aster, Symphotrichum parviflorum. If you compare the Saltmarsh Aster with our Tansyaster, you'll see the close similarity: similarly small flowering heads at the tips of long branches in diffuse inflorescences; the heads with white ray florets surrounding yellow disc florets; similar fruits with similar pappi... But unlike our Tansyaster, Saltmarsh Asters' ray florets develop pappi atop their ovaries, plus they're larger, and the involucres aren't densely covered with short, glandular hairs.
Aesthetically, here we're savoring variations on an aster theme, exactly in the spirit of a trio of violin, viola and cello, but with living beings bringing forth the music, by being themselves.