Excerpts from Jim Conrad's
from the November 3, 2008 Newsletter written in Yokdzonot about half an hour by bus west of Pisté, Yucatán, MÉXICO
Nowadays a different manner of prettiness is offered by an abundant roadside weed, a composite (member of the Daisy or Composite Family) with aster-like, yellow-centered, white rayed heads presented in random sprays, star-like, against its own diffusion of green leaves, as shown along a nearby road below:
A close-up of a flower head composed of yellow disk flowers and white ray flowers is below:
Lots of composite species present this yellow-eyed, white rayed configuration and it can be hard to distinguish them. However, along with the flowers there are fruiting heads, and those fruits are fairly distinctive, each topped with two spines, as shown below:
If you're up north and have a dog given to wandering the fields this time of year, surely that dog returns home covered by fruits similar to these, the "stick- tights," "tickseeds" or "Spanish needles" produced by several weedy species of the genus Bidens. I think the one illustrated here is BIDENS ALBA, earlier lumped with Bidens frondosa.
In the genus Bidens only the disk flowers produce fruits, the peripheral white ray flowers being sterile. The fruits, thought of as seeds by most people, are technically achenes, which are dry, one- seeded fruits. All members of the vast Composite Family (daisies, asters, dandelions, sunflowers, goldenrods and many more) produce achene-type fruits (so sunflower "seeds" are actually fruits). When you see achenes topped with two to four stiff, slender bristles, often with the bristles themselves bearing tiny, sharp, backward-pointing spines, a good bet is that your composite is a Bidens. Cosmos produces similar achenes but Cosmos bristles arise from a "beak," or slender stem, atop the main fruit body, while you can see in the last picture that Bidens bristles arise directly from the main body.