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Excerpts from Jim Conrad's
Naturalist Newsletter
entry dated October 1, 2022, issued from near Tequisquiapan, elevation about 1,900m (6200 ft), N20.565°, W99.890°, Querétaro state, MÉXICO
BUR RAGWEED
On a much-eroded, machete-chopped slope on the edge of town, several dense, woody-based bushes about shoulder high were flowering, but not in the usual manner. A branch of one of the bushes is shown above, bearing hundreds of somewhat messily cluttered, small, green flowers of two very different kinds. Even most of this bush's leaves were a mess, bug-eaten and fungus-attacked, but even the younger ones without bug and fungus damage were haphazardly incised with irregularly formed and spaced teeth.
Up closer, things became more orderly and you could see what was going on. This is a member of the vast Composite or Aster Family, in which flowering heads consist of several to many, usually colorful florets which are gathered within a usually greenish, cuplike involucre. However, in this species, nothing was colorful, and while the drooping flowering heads in the above picture's top, right corner clearly show numerous tiny florets packed into upside-down, cuplike involucres, the spiky items below just don't fit Aster Family stereotypes.
Florets in the the top flowering heads are unisexual male. On some of them you can barely see five, sharp-pointed corolla lobes. The burs at the bottom of the picture are harder to interpret.
If you're a plant anatomist familiar with basic Aster Family flower structure, it all makes sense. The burs are highly developed structures based on the usual Aster Family theme. The burs on the spiky fruits are no more than phyllaries on the involucre of the female flowers. Phyllaries are scale-like parts forming the sides of green, cuplike involucres in "normal" Aster Family flowers.
In most Aster Family flowering heads, phyllaries go fairly unnoticed beneath the colorful ray and disc florets above them, but here each phyllary has enlarged and its sharp tip has hardened, often forming sharp hooks making life more disagreeable for animals wanting to eat the fruits. Some anatomists interpret the barbs atop the burs as paleae modified in the same way as the phyllaries. Paleae occur in many Aster Family flowering heads, looking somewhat like little scales growing at the base of each floret.
In the above picture, the burs bear many glands, which may have something to do with the numerous minuscule insects seen ranging among or possibly stuck to the spines. Are the glands attracting the insects for unknown purposes, or serving to keep insects away by being sticky? I can't find a study answering that.
The above picture shows our plant's much branching, woody base.
Northerners who pay attention to neighborhood plants might recognize our eroded-slope bush as a species of ragweed, genus Ambrosia. It's AMBROSIA CORDIFOLIA.
In the US, Ambrosia cordifolia is restricted to Arizona where it's native, as it is across the border in northwestern Mexico. It's thought to have been introduced here in the highlands of central Mexico, where it seems to be expanding rapidly into disturbed sites, as a weed. In the US, our plant is known as Tucson Bur Ragweed, but that's not appropriate over most of its distribution, which mainly is here in Mexico, so we'll call it Bur Ragweed. That name at least distinguishes it from the well known, sneeze-causing Giant Ragweed and Common Ragweed, which also are species of Ambrosia.
What strikes me about ragweeds is that, despite their sloppy looks, they're highly specialized. In evolutionary terms, the Aster Family is regarded as one of the most recently arisen of flowering plant families, and within that family, the tribe to which ragweeds belong, the Heliantheae, has been shown by genetic analysis to be one of the most recently arisen, highly derived tribes. Unlike most members of the Aster Family, ragweeds have shifted their unisexual male flowers to one place, and their unisexual female ones to another, plus they rely on wind for pollination, not insects, which may be advantageous during this time of plummeting pollinator numbers. Ragweed structural parts are miniaturized, and fusion of certain parts have taken place, -- exactly as happened to computer parts as technology improved.
The end result is a scroungy looking plant using its resources in a highly efficient manner, and adaptive enough to thrive in more than a single ideal environment, from the sandy washes and benchs described as its habitat in Arizona, to our much abused slope consisting largely of bare rock exposed by erosion.
I think that Ambrosia cordifolia is teaching us something about life in the future dystopia and environmental catastrophe humans are preparing for themselves. The teaching is: To survive, whether you like the idea or not, you'll need to drastically downsize, and become profoundly more efficient in the use of your limited resources, even if you end up looking and maybe feeling crappy with all your diseases.
Beyond all that, nobody knows what was on the mind of Linnaeus when in 1753 he named the ragweed genus Ambrosia. In classical Greek, ambrosia is the "food or drink of immortality." Linnaeus doesn't seem to have known a botanist named Ambrose. Could Linnaeus have been making a joke?