How can a scrawny plant, growing in disturbed soils, with painfully sharp spines all over its fruits come to be named for the mythical “food of the Greek gods”—Ambrosia—a name also related, apparently, to the Greek word for immortality, αθανασία (athanasia)? Carl Linnaeus himself, the “father of modern taxonomy,” bestowed the name in 1754. But why Ambrosia? I haven’t found an explanation. The authoritative Flora North America says “allusion unclear.” One might say it’s a crusty old botanist’s joke on posterity, but I won’t impugn the intentions of the great Linnaeus. The genus isn’t native to Sweden and he may have examined only a few specimens from North America (collected by others). Maybe they smelled nice, but he probably didn’t get to know the genus well enough.
Ambrosia acanthicarpa, annual bur-weed or annual ragweed, grows on disturbed, sandy soils, often along roadsides, throughout much of western—especially southwestern—North America. The plants seen here were on a dirt road near the north edge of the Bodie Hills, in Lyon County, Nevada. I’ve also seen it beside Hwy 270 at Mormon Meadow and I’ve probably overlooked it at other locations. (Though you’re not likely to overlook it if you encounter it while wearing open-toed sandals.)
It’s not immediately obvious, but Ambrosia is a composite—in the sunflower family (Asteraceae). There are more than 40 species of Ambrosia in the New World, mostly in western North America. Ambrosia now includes plants formerly placed in Hymenoclea and Franseria.
Despite its vicious demeanor, Ambrosia has an intriguing anatomy. The male flowers, bulging with stamens, are tightly clustered into numerous small heads, dangling along the axis of a tall raceme. The pollen shed from those anthers causes agonizing irritation of eyes and sinuses in anyone getting a face-full of the stuff. Magnified, the pollen grains look like lethal medieval weapons.
While the male flowers will insult your eyes and upper respiratory system, it is the female flowers that will draw blood from your toes and fingers. Pistillate flowers are in the axils of leaves below the staminate inflorescence—the better to catch those heavily armed pollen grains. They lack corollas and are encased, usually one at a time, in a long-spined “bur.” (These spines are derived from the paleas—in Asteraceae, the usually very thin, papery, scale-like or bristle-like “chaffy bracts” at the base of each flower.) As the fruit matures, the bur becomes very hard. The spines stiffen and become very sharp.
Did you notice the tire tracks in the first photo? Above you see evidence for one of this plant’s long-range dispersal mechanisms. The mature burs attach themselves freely to automobile tires. No doubt this is one reason Ambrosia acanthicarpa is fairly common along disturbed road shoulders and many lesser-used unpaved tracks throughout the American west.
Copyright © Tim Messick 2017. All rights reserved.
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