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Excerpts from Jim Conrad's
Naturalist Newsletter
entry dated September 30, 2022, issued from near Tequisquiapan, elevation about 1,900m (6200 ft), N20.565°, W99.890°, Querétaro state, MÉXICO
MEXICAN BONEBRACT
These days of the mid rainy season -- even though this year the rains came two months late and so far aren't nearly the usual amount -- the countryside is resplendent with broad strokes of yellow contributed by members of the vast Composite or Aster family. Below, a small part of an impressive roadside colony of the species shown above provides a hint of what's going on:
Most species of yellow-flowered composites, at first glance, are fairly similar to the one shown above. However, when you look more closely, usually you'll find that a surprising number of species are at hand. The blossoms being considered here are a little different from most because in the center of each flowering head, or capitulum, the cylindrical disc florets rise unusually high above the radiating ray florets. A view from below one of this species' capitula highlights more distinctions:
The green parts are densely hairy with short, stiff hairs lying close to the surface. The five yellow rays are exceptionally broad, with a sharp crease at their tips, and notice how their rounded bases are held away from the capitulum's center by a slender stem. More diagnostic, however, is that the five, leafy, green phyllaries constituting the involucre are very narrow and widely separated from one another. Average phyllaries are somewhat triangular and often arranged in two or more series with the phyllaries overlapping one another. The green, oval objects between the phyllaries are better understood in the following picture:
In the lower part of the above image, and a little left of center, you see that the oval, green items positioned between the ray florets consist of the lower parts of unusually large paleae -- paleae being scale-like items growing beside each disc floret in the capitulum's center. These paleae, instead of being formed the normal way, like modest flakes or little scoops, completely wrap around the bottoms of the disc florets. Many genera of the Aster Family produce no paleae at all, so these giant paleae wrapping around the developing fruits are remarkable. As the fruits mature, the paleae covering them harden, then are shed with the fruit as one thing.
Also in the above photo, notice that the ray florets are sterile -- producing no cypselae at all. Having sterile ray florets, in itself, disqualifies the vast majority of other Aster Family species our plant possibly could be. It's a keystone field mark.
Leaves and stems also bear white hairs, and the blades' margins, at least on larger, lower leaves, may bear low, widely separated teeth. Leaves arise one at a stem node, not two or more as with many Aster Family species.
At first I was fairly sure that this was the same Bonebract species that was common at this time of year back in the Yucatan. However, this area's high-elevation climate and its volcanic geology are so different from the Yucatan's hot lowlands and limestone bedrock that not many species are adaptive enough to prosper in both places. Therefore, just to be sure I didn't have a different species, I did the botany. And, sure enough, our Querétaro species is a bonebract, genus Sclerocarpus, but a different, though very similar, species.
Here we have SCLEROCARPUS UNISERIALIS. If you click on the above link to the Yucatan species, Sclerocarpus divaricatus, you'll see that the Yucatan species produces very different phyllaries in the involucre.
Sclerocarpus uniserialis bears the English name Mexican Bonebract because it occurs in Texas, and then extends much farther south along the Eastern Sierra Madres south into southern Mexico, including the Yucatan, and into Guatemala. It's common in many kinds of disturbed habitats, from sea level to maybe 2100m (7000ft).
The 2022 work by Cointa Casanova-Pérez and others entitled Plantas medicinales usadas por los Tének in la Huasteca, México says that Mexican Bonebract is used by the Tének indigenous people in eastern Mexico's Huastec Region to treat diarrhea. An infusion of the plant is drunk. In the Yucatan we observed that dogs habitually ate the leaves of Sclerocarpus divaricatus Bonebract, presumably because the stiff, slender hairs covering the leaves was hard on the dogs' intestinal worms. Since the hairs of the two species are so similar, probably dogs around here eat our species' leaves, too.