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Wayne's Word Noteworthy Plant For April 1998

The Remarkable Cocklebur

Worldwide Hitchhiker & Nature's Velcro®

See Article About The Ultimate And Most Painful Hitchhikers

Probably everyone has gotten cockleburs in their socks or clothing, especially if you enjoy walking in riverbed areas or along cultivated fields and moist pastures. Cocklebur plants (Xanthium strumarium) produce hundreds of little football-shaped burs, about one inch (2.5 cm) long and covered with stiff, hooked spines. [Another species that is less common but widespread across North America is called spiny cocklebur (X. spinosum). Spiny cocklebur is unmistakable with its stout, forked spines at the base of each leaf.] Each cocklebur fruit contains two seeds that may remain viable for many years. The prickly burs hook into your clothing and become tightly attached, like the Velcro® fasteners on shoes and day packs. Often the vicious burs form tangled clots in the fur of animals, and must be cut out of the hair. In fact, these remarkable burs have enabled the cocklebur plant to hitchhike all over the world.

A cocklebur plant (Xanthium strumarium) bearing prickly, hitchhiker burs.

Some Good Information For General Biology Students

There is some disagreement among botanists as to exactly how many varieties of common cocklebur (Xanthium strumarium) exist, and precisely where is their native (indigenous) habitat. There are several named varieties listed in botanical literature, including var. canadense and var. glabratum; however, some authorities believe Xanthium strumarium is one cosmopolitan species with many highly variable populations around the world. Since cockleburs can colonize new areas quite easily (particularly disturbed areas), they are good examples of the "founder effect." The founder effect is genetic drift that occurs when a small number of individuals, representing a fraction of the gene pool, establish (found) a new colony and only certain alleles (genes) of the original population are passed on to the next generation. The founding colony does not have the genetic variability of the main population, and the frequency of certain traits may increase greatly by genetic drift compared with the much larger ancestral population. A classic example of the founder effect is the "Dunkers," a politically incorrect name for a German Baptist religious sect that settled in Franklin County, Pennsylvania between 1719 and 1729. Since the original families (who did not marry outside their religious sect) settled in Pennsylvania, there has been a dramatic change in some of their gene frequencies. For example, the frequency of type A blood in the "Dunkers" is now 60 percent, compared to 42 percent for the United States and 45 percent for West Germany. The founder effect also explains the high frequency of dwarfism and polydactylism (extra fingers) in the Amish of Lancaster Pennsylvania, a colony begun by a few individuals (at least one of whom carried these traits). There is some evidence that the first humans to reach North America (across the Bering Straits land bridge) brought with them gene frequencies not representative of the Asiatic population they left. The unusual variation in bark, foliage and growth characteristics in isolated groves of cypress (Cupressus species) throughout coastal and mountainous regions of California may also be due (in part) to the founder effect; however, some traits, such as glandular (resinous) foliage, are more drought resistant and probably evolved by natural selection in the hot, dry interior regions of the state.

An assortment of prickly cockleburs (Xanthium strumarium).

See Selection & Genetic Drift In California Cypress

The cocklebur is a classic example of a short-day plant (i.e. it only flowers when the nights are long). At least one leaf of a cocklebur plant needs 15 hours of darkness to undergo various complex biochemical reactions leading to the release of a hypothetical flower stimulant called "florigen." A protein leaf pigment called "phytochrome" controls the release of florigen from the leaves. One form of this pigment (P-660) is formed during the hours of darkness and is essential for the release of florigen. The phytochrome P-660 pigment is very sensitive to specific wavelengths of light, and a flash of light during the 15 hours of darkness can instantaneously convert it into another form called P-730 which inhibits the release of florigen, thus blocking the flowering process. Cocklebur plants can bloom in the tropics where the days are short and the nights are long, thus greatly increasing its range and potential for seed production. In North America cockleburs typically bloom during the fall months when the days are shorter and the nights longer. They will not bloom during the long days of summer or near a street light.

Shoes and socks after walking through a field of cockleburs.

Cockleburs And The Related Hitchhiker Burdock

Cockleburs belong to the enormous sunflower family (Asteraceae), the largest plant family with approximately 24,000 species. The seeds are typically enclosed in a one-seeded fruit, called an achene, which is often airborne with a plume of silky hairs resembling a miniature parachute. In the cocklebur, the achenes are enclosed in a bur or involucre that is covered with hooked prickles. Another related hitchhiker in the sunflower family, called burdock (Arctium lappa), is equally adapted for clinging to animals and inanimate objects. Like the cocklebur, its hooked prickles are very difficult to remove from a shag carpet or sweater. Other hitchhikers, such as bur clover (Medicago hispida) and teasel (Dipsacus sativa) belong to the legume family (Fabaceae) and teasel family (Dipsacaceae). Teasel bristles are stiff enough to raise and straighten the nap on woolen cloth. The large, spiny heads of teasel were used in carding wool in early days. The heads were split and mounted on belts or rollers that moved over the cloth.

A. Cocklebur (Xanthium strumarium); B. Burdock (Arctium lappa); C. Bur Clover (Medicago hispida). Note the curved prickles that hook into the fiber of clothing like a Velcro® fastener.

The tenacity of common burdock (Arctium minus) was described in an article by J. Raloff in Science News Volume 154, 1998. Burdocks are well-known for their ability to stick to bare fingers, but actually trapping an animal was unheard of. While migrating through Rock Creek Park in Washington, D.C., four ruby-throated hummingbirds became entangled in clusters of burdock fruits near the tops of this prolific weed that can grow to 6 feet (2 m). One of the birds was rescued, but the other three were so securely snared by the hooked prickles that they died right on the plants. The birds were unable to fly away, and their struggling caused them to touch other burs, ensnaring them even more. A thicket of burdock plants poses a potential death trap to small birds that might land on a cluster of the spiny burs.

Burdock (Arctium lappa) in full bloom. Like the cocklebur, the one-seeded achenes of burdock are enclosed in a bur or involucre that is covered with long spines which are hooked at the tip. The dried burs readily cling to the fur of animals or clothing, thus making this plant one of nature's most successful hitchhikers. The minute hooks at the tips of the spines can even penetrate the dead layer of skin on your fingers. Note the katydid (order Orthoptera) that was feeding on this plant.

The Invention Of Velcro®

Oone of the most amazing stories about hitchhiking plants is the origin of the fastener known as Velcro®. We are not sure whether this remarkable invention came from the cocklebur (Xanthium), burdock (Arctium), bur clover (Medicago hispida), teasel (Dipsacus sativus) or another species of "bur," but this amazing discovery is certainly pertinent to any discussion about hitchhiking plants.

Teasel burs (Dipsacus sativus). In real life they do not hatch from eggs.

The Remarkable Invention of Velcro®

One day in 1948, an amateur Swiss mountaineer and naturalist, George de Mestral, went on a nature walk with his dog through a field of hitchhiking bur plants. He and his dog returned home covered with burs. With an intense curiosity, Mestral went to his microscope and inspected one of the many burs stuck to his pants. He saw numerous small hooks that enabled the seed-bearing bur to cling so tenaciously to the tiny loops in the fabric of his pants. George de Mestral raised his head from the microscope and smiled thinking, "I will design a unique, two-sided fastener, one side with stiff hooks like the burs and the other side with soft loops like the fabric of my pants. I will call my invention Velcro® a combination of the words velour and crochet. It will rival the zipper in it's ability to fasten."

From: The Mining Company (Feature 09/12/97)

Microscopic view of the two sides of a Velcro® fastener. The numerous hooks (left) become embedded in the meshwork of loops (right). This is essentially how many plant hitchhikers (such as cockleburs) adhere to your clothing. The hooks on the cockleburs attach to the meshwork of threads in your socks. In fact, the original inventor of velcro came up with his marvelous invention after an encounter with a plant hitchhiker. Copyright (c) W. P. Armstrong.

Mestral's idea was met with resistance and even laughter, but the inventor "stuck" by his invention and together with a weaver at a textile plant in France, Mestral perfected his "hook and loop fastener." By trial and error he realized that nylon when sewn under infrared light, formed tough hooks for the bur side of the fastener. This design was finally patented in 1955. The inventor formed Velcro Industries to manufacture his invention and soon started selling over 60 million yards of the product each year. Today Velcro Industries is a multi-million dollar company.

Athough the word "velcro" was originally use by George de Mestral for his novel invention, it has become a common household word synonymous for "hook and loop" fasteners for clothing, shoes, straps, packs and numerous other products. Today Velcro® is the registered trademark for Velcro Industries' products, but there are other brands of similar fasteners on the market. Many people in the United States refer to facial tissue as "kleenex" because this was one of the original and popular brand names. But Kleenex® is the registered trademark of one brand of facial tissues, and there are other registered brand names, each manufactured by different companies. This illustrates a problem Velcro Industries and other inventors face when the names of their products become generic terms. Many words used frequently in everyday language were once trademarks, for example "escalator," "thermos," "cellophane," and "nylon." When names become generic terms, the U.S. Courts can deny exclusive rights to the trademark, so that other companies can produce the same type of product under a different registered brand name.

In order to protect a name it should be officially registered with the U.S. Patent and Trademark Office. When officially registered, the name is followed by ® (an R with a circle around it). Otherwise someone else can use your name for their product. This is precisely why the name Wayne's Word® is an official registered trademark; however, it is doubtful that it will ever become a household name like cellophane or Velcro®.

Cockleburs have literally hitchhiked across Europe and North America. In some lowland areas it is considered to be a troublesome weed by farmers and ranchers. But there may be at least two interesting uses for cockleburs. Because they readily attach to cloth material, they can be used as "darts" in a cocklebur dart game, and sixteen of the spiny burs can be glued together to form a perfect little poodle dog.

A cocklebur dart game.

A poodle dog made from sixteen cockleburs.

For More Information About Hitchhiking Plants:

See Devil's Claws: A Hitchhiker On Big Animals
Go To Ultimate And Most Painful Hitchhikers

References About Hitchhiking Plants

  1. Abrams, L. and R.S.Ferris. 1965. Illustrated Flora of the Pacific States Vol. 4. Stanford University Press, Stanford, California.

  2. Armstrong, W.P. 1979. "Nature's Hitchhikers." Environment Southwest Number 486: 20-23.

  3. Armstrong, W.P. 1988. Biology Laboratory Manual and Workbook. Burgess International Group, Inc., Edina, Minnesota.

  4. Hickman, J.C. (Editor). 1993. The Jepson Manual: Higher Plants of California. University of California Press, Berkeley, California.

  5. Raloff, J. 19998. "Botanical Velcro Entraps Hummingbirds." Science News 154 (16): 244.

  6. Robbins, W.W., M.K. Bellue and W.S. Ball. 1941. Weeds of California. State Department of Agriculture, Sacramento, California.

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