Excerpts from Jim Conrad's
from the February 21, 2016 Newsletter issued
from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán MÉXICO
In Mexico the name ÁRNICA is applied to several large, yellow-flowered members of the Sunflower or Composite Family. Locally the name is used mostly for what English speakers sometimes call the Giant Mexican Sunflower, Tithonia diversifolia, shown at http://www.backyardnature.net/yucatan/tithonia.htm.
I'm thinking about the name Árnica now because this week a yellow-flowered member of the Sunflower Family turned up still blossoming despite the lack of rain for several weeks, and the Maya often call it Árnica-che, the word "che" usually meaning bush. So, this week's plant was the "Árnica Bush," a name possibly appearing in print here for the first time. But, what was this Árnica-che in terms the outside world can understand?
It was a branching, head-high, woody bush whose spineless stems arched like those of blackberry canes, and forming a blackberry-bramble-like thicket in a small, partly shaded clearing in the woods that had been abandoned for several years. Below, you can see the bush's rough, veiny leaves, two per stem node, and some branch-end flower-clusters:
Two peculiarities about the plant visible in that photo are that each flower cluster consists of several heads atop long stems, or peduncles, of more or less equal lengths, plus in older flowering heads the ray flowers have dried up and shriveled, but remained on the head. Normally ray flower corollas fall off after pollination. When something withers but remains in place, it's said to be "marcescent," and these marcescent corollas are a good field mark for this species. Eventually this plant's old corollas do fall off, but not before giving the flower cluster a messy look -- which doesn't seem to stop pollinators from visiting the remaining fresh heads.
Above we see some heads from which the ray flower corollas have fallen, showing that disc flowers in the head's "eye" also have marcescent parts. Also, the greenish part holding the florets -- the involucre -- is composed of broad, scoop-shaped, overlapping scales.
These are all field marks used when "keying out" the genus in the online Flora of North America. Fortunately, one species of the genus occurs in the southwestern US, so the genus is included in the Flora. Other features used in the key, absolutely necessary to notice when differentiating this genus from closely related ones, are displayed below in the broken-apart fruiting head on the tip of my finger:
Notice the marcescent disc flower corollas and anther tubes. More important for identification purposes is the white pappus atop each grain-like, cypsela-type fruit. The pappus segments are not slender hairs but rather long, slender, sharp-pointed scales, purple at their bases, and seeming to unite at their bases where they attach to the cypselae. Each individual cypsela is separated by its neighbor by a pale, papery, scoop-shaped scale, a palea. And the cypselae themselves are a little angular in shape but don't display wings or sharp edges.
All these features lead to LASIANTHAEA FRUTICOSA, with no good English name, occurring throughout central and southern Mexico southward throughout Central America. The genus Lasianthaea is one I'd never heard of, embracing eleven species. Mexico appears to be its center of evolution.
I was glad that my photos of the tiny technical features turned out OK because on the Internet most photos of this species show the petal-like ray flowers as relatively longer than on our plants. However, with those technical details, I'm fairly sure of the ID. Maybe on plants past their period of peak flowering and/or growing in somewhat shady spots ray flowers are just shorter. Or maybe we have a different variety here.