Mystery Plant! #746
By John Nelson
It would not be exaggerating to say that grasses are the most important plants, economically, in the world. For one thing, it turns out that every major civilization has depended upon one (or more) grass species as a major food source. (Consider the far Eastern civilizations, based upon rice as a staple crop, the American civilizations employing corn, and the Middle East and Roman Empire, built upon wheat and barley. In a very traditionally human sense, we think of the grasses as the source of bread, or “food”, in a metaphoric sense, the staff of life. So, I sometimes I would tell my students that the grasses are the “stuff of loaf”.) Members of this family have additionally supplied humans with animal feed, turf, housing materials, and starches, used for production of sugar and alcohol, as well as ornamental species for garden cultivation. And in a negative sense, many grasses are important as weeds.
The grass family is very complex, and is broken down into a number of different divisions, or “tribes”, based on vegetative as well as floral characteristics. The stems, which are called “culms”, are generally hollow, are usually circular in cross-section. The leaves are always alternate, arising one at each node: the base of the leaf wraps around the stem, and is the “sheath”. The free part is termed the “blade”, as you might expect. The flowers and grains are of primary importance in identifying grasses, and studying these small parts takes practice (and patience).The flowers of grasses are quite small, and are clustered into small units called spikelets. Each spikelet may contain from one to many flowers, depending on the species. The spikelets are maintained in very characteristic arrangement, or inflorescences, which may my simple and spike-like, or highly branched. Small, dry bracts surround the flowers, and are sometimes ornamented with needle-like spines, or awns.
The fruit of a grass is what we call a “grain”. (Of course there’s a more botanical term for it, but never mind that right now.) A grain, no matter the grass species, will contain a single seed, and the tiny seed itself is absolutely fused to the grain’s inner wall, and when the seed sprouts, it sends its first tiny root right through the grain wall. There is no other fruit type quite like it. When you are munching on an ear of corn, you are eating grains, to be precise. Not “seeds”.
This week’s Mystery Plant is indeed a grass, and each of its flowers produces three slender awns. These awns are effective in tangling themselves into an animal’s fur, or someone’s’ socks, as an effective means of dispersing the grain. It is an annual species, native and widespread east of the Mississippi River, quite happily growing in dry, sandy fields or other kinds of open, vacant places, and is usually thought of as a good indicator of previous soil disturbance. This fall, take a walk in your area and try to get a sense for the different kinds of grasses that grow around you. Maybe you’ll see this one.
[Answer: “Three-awn grass,” Aristida tuberculosa]
John Nelson is the retired curator of the Herbarium at the University of South Carolina, in the Department of Biological Sciences. As a public service, the Herbarium offers free plant identifications. For more information, visit www.herbarium.org or email firstname.lastname@example.org.
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