Excerpts from Jim Conrad's
from the December 17, 2007 Newsletter issued from Yerba Buena Clinic just outside Pueblo Nuevo Solistahuacan, Chiapas, MÉXICO
about 1740 meters in elevation, ± LAT. 17° 11' 27"N, LONG. -92° 53' 35"W
You can see one of our most common and conspicuously flowering roadside weeds here at the end of the wet season, a member of the Composite Family, is the one you see above.
That's BIDENS AUREA, known in central Mexico as "Acahual," a name applied to many similar daisy-like plants. Here in Chiapas most commonly it's called Mastis. The species name "aurea" means "golden," and I read that often the ray flowers are yellow, not white as in the picture.
In the picture, notice the dark brown, spherical cluster of bristly fruits, or achenes, at the left. Those fruits look a lot like the "stick-tights" produced by North America's fall-flowering, yellow- blossomed Spanish Needles. In fact, Spanish Needles and Mastis belong to the same genus, Bidens.
Keep in mind that these needle-topped achenes are specialized fruits typical of the Composite Family, not seeds, though most people think of them as seeds. Achenes are dry, indehiscent (don't open up), one-seeded fruits. When we eat "sunflower seeds" we're cracking open the achene's hard coat, or pericarp (the hard wall of the ripened ovary), expelling and eating the soft seed.
With a handlens you can see that each needle atop our Mastises' achenes is "retrorsely barbed." That means that each needle is covered with tiny, backward- pointing barbs, so when a needle enters animal hair or human socks, the needles tend to stay put.
As with North America's Spanish Needles, here often you see large sections of fields or roadsides gloriously hued with this species. In Chiapas, despite this being an introduced species, beekeepers are especially fond of Bidens aurea because its flowers produce abundant pollen and nectar. It grows mainly in disturbed areas in the pine and oak-pine zones.
By the way, the red thing in the picture is an unopened Ipomoea coccinea flower, a kind of morning-glory, kept in the picture just for the fun of it.