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Excerpts from Jim Conrad's
Naturalist Newsletter
Entry from field notes dated June 30, 2023, from along narrow, very steep, cobblestone road descending on the south side of El Doctor, passing among cultivated fields and pastures; in the mountains of east-central Querétaro state, municipality of Cadereyta de Montes, 12 straight-line kms due east of Vizarrón de Montes but much farther by twisting roads; elevation ~2740m (~8900 ft), Querétaro, MÉXICO, (N20.84704°, W99.58573°)
BIDENS OSTRUTHIOIDES
With small, cylindrical florets crammed together to form the above flowering heads' "eyes", and petal-like ray florets radiating outward, the above floral structure announced yet another species of the mind-bogglingly numerous, yellow-flowered members of the vast Composite/Aster/Sunflower Family, the Asteraceae. To figure out which species it is, you really have to "do the botany." Main field marks to note above are that both disc and ray florets are present, and they're both yellow, which already disqualifies many possible species it could have been.
Above, the brown items are the disc florets' five anthers united into cylinder surrounding the style branches, which on older florets at the cluster's margins have expanded to form Y shapes. At the picture's bottom, note that the Y-shaped forked styles arising from the throats are well formed but lack brown anthers around them. Later this obscure observation proved to be critical for determining the species.
Below the flowering heads, the involucre were very different from the vast majority of members of the Aster Family. Above, note that each head, or capitulum, is subtended by five green, fleshy bracts, and between each pair of bracts arises a much larger, wider and yellowish bract. A cluster of genera produce involucral bracts separated into two, strikingly dissimilar series, the best known genera containing species of cosmos, coreopsis, dahlias and the Spanish needles or beggarticks, all members of the Aster Family tribe Coreopsideae.
Above, a broken-open capitulum exposes slender ovaries -- the future cypsela-type fruits -- separated from one another by scoop-shaped scales called paleae, which is to be expected for the Coreopsideae tribe. Often, members of that tribe bear needle-like bristles atop their cypselae, but above I see no hint of that. 
However, the picture at the right shows part of the debris resulting from crumbling a capitulum. On the image's left side, a white cypsela top is seen with very tiny, slender bristles which might grow as the fruits mature. In the picture's top, right corner, the darker green item is a less mature cypsela-top bearing what looks like even less developed bristles.
Above it's seen that leaves were trifoliate -- divided into three leaflets -- and they arose in pairs at stem nodes. The leaflets' margins are saw-toothed.
Our plant was a bush with many branches, with the lower stems semi-woody, as shown above. The stem is sprouting a new branch, and the young leaves' bottoms bear a few long, soft hairs. Here's what our bush looked like along the road, next to a roadcut into the limestone:
In our region of upland central Mexico, if you have a member of the Aster Family that is a semi-woody bush over a meter tall bearing yellow disc and ray florets, its involucral bracts are arranged in two series very different from one another, its florets are separated from one another by paleae, and its ovaries and cypselae are slender and roundish in cross-section, not flat or angled, you have the genus Bidens, commonly known as Spanish needles, beggarticks and other names.
Moreover, if the ray florets on your Bidens displaying all the above features produce well formed style branches emerging from their throats, you have BIDENS OSTRUTHIOIDES.
Bidens ostruthioides, with no English name, is distributed from uplands of northeastern and central Mexico south into Costa Rica, and in many places is considered moderately abundant, especially in oak-pine forests. The Flora del Bajío reports two or three, sometimes four, needle-like bristles atop mature cypselae, up to 3.5mm long (0.14inch), so our bush's fruits will produce those later. The bristles will bear backward-pointing barbs along their sides, helping the fruits stick to animal fur, and hitch a ride to someplace new, as Spanish needles/beggarticks are known to do.
I found it interesting that our bush's ray flowers having well developed style branches distinguished it from so many other species. The 2008 study by Mark E. Mort and others entitled "Phylogeny of Coreopsideae (Asteraceae) inferred from nuclear and plastid DNA sequences," found that the genus Bidens is "not monophyletic." That means that someone needs to reassign some Bidens species to other genera, or else create a larger genus into which Bidens will be sunk, and no longer exist. But so far no one has done that. It'll be interesting to see where our plant with its ray florets bearing well developed style arms ends up.
The 2015 study by Miguel Ángel Bello-González and others entitled "Plantas útiles de la comunidad indígena Nuevo San Juan Parangaricutiro, Michoacán, Méxco," reports that in the Mexican state of Michoacán Bidens ostruthioides is used medicinally for skin problems, and in ceremonies. As a medicine, its above-ground parts are boiled in water, which is used to wash the affected skin. Its branches are incorporated into wreathes during Day of the Dead activities. The Flora del Bajío says that livestock eat the bush, and bees gather nectar from it.