from the August 14, 2016 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins,
central Yucatán MÉXICO
Along a weedy trail in the Hacienda's woods an herb about a foot tall currently bears one or a few clusters of white flowers above relatively large, heart-shaped leaves, as shown below:
Wildflower admirers might recognize the leaves as typical of certain members of the Composite or Daisy Family, the Compositae, such as the sunflowers and cockleburs, plus the flowers are a little like those of the eupatoriums, also in the Composite Family. Below, up close, we see flowers that are definitely Composite Family:
One Composite Family field mark obvious here is that each flower's five blackish anthers are united at their sides into cylinders surrounding the ovary's wishbone-shaped style. Also, the many flowers are clustered tightly together into a head, or capitulum, subtended by leafy bracts forming the capitulum's "involucre." A look at the involucre is shown below:
Among Composite Family involucres, you seldom see such a loose arrangement of overlapping, leaf-like bracts, so this is an important field mark. If you look much closer at the individual flowers in the head, the "florets," other good features appear, as shown below:
Among the composites, a very important field mark always is the floret's pappus -- the structure that may or may not exist where the corolla's base connects with the cypsela-type fruit. In the above picture, the floret at the center-left seems set into a crown-like structure atop the cypsela, and that's exactly the kind of pappus this one is described as having, a "coroniform," one very unlike the white hairs and stiff bristles more commonly seen in other species, so this is a great field mark.
With such distinctive features, it was easy to figure out that this was LAGASCEA MOLLIS, a common weed throughout nearly the entire tropical world. It's thought to be native of Mexico and Central America. Among its English names, Silkleaf may be the most commonly encountered
Silkleaf is fairly common here in weedy areas, and often you see several individual plants forming a small community. A research paper by Elizabeth Murillo and others, available on the Internet, explains why that might be: Silkleafs are "allelopathic." That means that they produce chemicals that retard the growth of other plants around them. Murillo's study found that Silkleaf extracts have a "significant effect on cellular division and radicular growth" (early root growth) on oat seedlings. Since oat plants are grasses, this raises the suspicion that Silkleaf might not be the best weed to have in fields of rice, corn, sugarcane, wheat and rye, which are all grasses.