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Excerpts from Jim Conrad's
Naturalist Newsletter
entry dated February 23, 2022, issued from near Tequisquiapan, elevation about 1,900m (6200 ft), ~N20.57°, ~ W99.89°, Querétaro state, MÉXICO During the heart of the dry season, in certain parached-gray-brown, fallow agricultural fields, the little wildflower shown above does weed service with a handful of other species. In general form it's similar to numerous yellow-flowered members of the Composite or Aster Family, the Asteraceae, so to identify it certain important field marks had to be noted. First of all, it was noted whether the disc florets in the blossom's center were gathered on a flat or elevated receptacle. The receptacle was raised, or "convex," as shown above. Next, the leafy scales, or bracts below the receptacle can vary tremendously from species to species, so the bottom of the flower head, or "capitulum," was noted: This receptacle's individual leafy parts, the phyllaries, are unusually rounded at their tips, they're arranged in two overlapping series, and they're short hairy -- all good features to note. However, some of the most important field marks to notice can be seen only by breaking a captulum apart, as seen below: The disc flowers' ovaries, the future one-seeded, cypsela-type fruits, below each cylindrical, yellow disk-flower corolla bear no hairs or spines atop them, and this absence is a critical feature. However, each ovary is partly enshrouded with a scoop-shaped, chaffy, scale-like thing called a palea, and that's important, too. At this point, already the possible genera have been whittled down to just a handful. The next picture shows another critical identification feature: That's the bottom of a ray flower, and what's important is that the ovary is tiny and flat, as if empty, and the Y-shaped style above the ovary is much less developed than those atop the disc flowers with their plump ovaries. Therefore, in this species, unlike most Asteraceae species, the disc flowers are fertile, but the ray flowers are sterile. The flat ovary and undeveloped style-arms are vestigial. These features were enough to key us to the genus Heliopsis, which embraces about 18 species occurring only in the Americas, with the center of species diversity here in Mexico. To figure out the species, vegetative characters were needed, so here's how the leaves and stems looked: Vegetatively, with its opposite leaves and fuzzy, purple, four-cornered stems, it looks like a member of the Mint Family, but with flowers like that, that's impossible. Comparing pictures of Mexican Heliopsis on the internet, we're led conclusively to a common weed of arid, upland central and northern Mexico, sometimes called Cabezona, meaning "Big-Head." It's HELIOPSIS ANNUA, native and endemic throughout its distribution area, which means that it's one of the few native plants able to occupy such disturbed habitats as the fallow field in which we find it. Most weeds are invasive species. But Cabezona is described as common along roadsides, in cornfields, and fallow agricultural fields in genera;. The last crop in the field where our photos were taken was oats, but plenty of remains of corn, or maize, littered the ground, too. Interestingly, some fallow fields have Cabezona plants, often lots of them, while in other fallow fields there are none. It's clear that Cabezona needs the soil to be tilled at a certain time during the season. Too little time lying fallow, or too long, and there's no Cabezona.
CABEZONA
