Ultimate Hitchhiker
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WAYNE'S WORD Noteworthy Plant For May 1998

Ultimate and Painful Hitchhikers

Major Topics To Be Discussed:

   Wayne's Word Top 17 Hitchhiking Plants
   Ultimate Hitchhiker From Madagascar
   Ultimate Hitchhiker Of S.W. United States
   Most Tenacious Hitchhikers Of America
   The Most Painful Hitchhiker
   Horehound: A Hitchhiking Herb
   Deadly Pisonia Tree Of Polynesia
   Additional Species Of Hitchhikers
   Conclusions About Hitchhiking Plants
   References About Hitchhiking Plants

WAYNE'S WORD Top 17 Hitchhiking Plants

[Based On The Difficulty In Removing Them From Your Socks]

Disclaimer: Ranking & SRDUs are purely hypothetical without any quantitative data or peer review to support them.

Thirteen Hitchhiking Plants.

A. Burdock (Arctium lappa, Asteraceae); B. Grappling-hook (Harpagonella palmeri, Boraginaceae); C. Horehound (Marrubium vulgare, Lamiaceae); D. Bur-grass (Cenchrus echinatus, Poaceae); E. Bur-clover (Medicago polymorpha syn. M. hispida, Fabaceae); F. Beggar-ticks (Bidens pilosa, Asteraceae); G. Sand-bur (Ambrosia acanthicarpa, Asteraceae); H. Cocklebur (Xanthium strumarium, Asteraceae); I. Krameria (Krameria grayi, Krameriaceae); J. Stick-tight (Desmodium cuspidatum, Fabaceae); K. Devil's-claw (Martynia annua, Martyniaceae); L. Uncarina (Uncarina grandidieri, Pedaliaceae); M. Grappling hook (Harpagophyton procumbens).

This Mexican species of beggar-ticks (Bidens aequisquama) has showy flowers.

Foxtail barley (Hordeum murinum ssp. leporinum)

A casual walk through a grassy field in southern California will very likely result in one or more spikelets of foxtail barley (Hordeum murinum) embedded in your socks. In fact, foxtail barley should probably be placed in the preceeding table. There are three subspecies of this annual European grass naturalized in California, and they are all excellent hitchhikers. They pose a serious problem for dogs and cats because the spikelets can penetrate the ears and eye sockets. If an entire foxtail spike gets up your nose it is difficult to remove. The spike gets pushed farther up the nasal cavity and then fragments into spikelets when you attempt to pull it out.

The Ultimate Hitchhiker From Madagascar

Fruit of Uncarina grandidieri--the ultimate hitchhiker from Madagascar.

When the Wayne's Word staff began compiling a list of hitchhikers that are difficult to remove from socks and underwear, we felt confident that our list included the most tenacious species. That is, until horticulturist John Trager of the marvelous Huntington Botanical Garden in San Marino, California told us about a fascinating tree from Madagascar (Uncarina grandidieri). This amazing species definitely goes to the top of our hitchhiker list with 12 SRDUs (Sock Removal Difficulty Units). The hitchhiker seed capsule superficially resembles a grappling hook or a space satellite from a sci-fi movie. It has long radiating spines, each tipped with sharp hooks. In fact, Mr. Wolffia impaled his left fingers on the spines, and while trying to free himself, he also impaled his right fingers. His hopeless predicament was reminiscent of a Chinese puzzle where fingers of both hands are caught. How animals in Madagascar rain forests (such as Lemurs) free themselves of these incredible hitchhikers is a mystery.

Several amazing hitchhikers of Uncarina grandidieri from Madagascar.

Uncarina belongs to the pedalium family (Pedaliaceae), an interesting and little-known family that includes sesame (Sesamum indicum), source of the delicious seeds on your favorite hamburger buns. The flowers of Uncarina are remarkably similar to those of Devil's Claws (Proboscidea) and are undoubtedly closely related to the martynia family (Martyniaceae). John Trager of Huntington Botanical Garden was able to hand pollinate the flowers to produce the bizarre hitchhiker fruits. Unlike most flowers, the anthers of Uncarina do not split open to release powdery pollen. Instead, the pollen is paste-like and must be squeezed out of the anthers, sort of like squeezing toothpaste out of a tube; or like squeezing a blackhead to release the mass of fatty material in a skin pore. According to C. Hanson (Cactus and Succulent Journal Vol. 69, 1997), this act may be accomplished in nature by pollen-eating beetles. As the beetle chews on the anthers, the pasty pollen oozes out and gets on the beetle's thorax and elytra (outer wing coverings). When the beetle enters another flower and pushes past the lower stigma lobe, the upper stigma lobe is forced down and wipes pollen from the beetle's body.

Ultimate Hitchhiker Of The S.W. United States

Fruits of stick-tight (Harpagonella palmeri)--an ultimate hitchhiker.

One of the "super hitchhikers" of southern California, Baja California Norte, and the offshore California islands is a low-growing annual called "stick-tight" (Harpagonella palmeri). In the official Wayne"s Word Top 17 Hitchhikers, this species receives 10 SRDUs, although it is almost as difficult to remove from your socks as the previous species. This hitchhiking plant is occasionally encountered on dry slopes and mesas of coastal San Diego County. The fruit of this rare wildflower is composed of two small nutlets enclosed in a burlike calyx. The persistent calyx is armed with 5-9 spines, each with minute, hooked barbs. The generic name Harpagonella is derived from the Latin word "harpago" or grappling hook. Like a grappling hook, the burs become deeply embedded in fibrous material and are practically impossible to pull out.

Stick-tight belongs to the borage family (Boraginaceae), a large family of attractive wildflowers and several very effective hitchhikers. Hikers in the Sierra Nevada of California have undoubtedly noticed the little seed-bearing nutlets of a blue wildflower called forget-me-not or stickseed (Hackelia species) firmly attached to their shoelaces or socks. But when it comes to a hitchhiking bur that could render a pair of socks or fine-mesh cloth net worthless, Harpagonella gets the highest award. And a cold night in a sleeping bag, with "scratchy" stick-tight burs embedded in your long underwear, is an unforgettable experience.

Highly magnified view of the seed-bearing nutlets of comb-bur (Pectocarya linearis ssp. ferocula). The borage family (Boraginaceae) contains many species with minute, hitchhiking, seed-bearing nutlets, often produced in clusters of 4. In this species the nutlets separate into an X-shaped structure, each nutlet fringed with glistening, curved hairs. In the center of the X is the remains of the old stigma.

A Tenacious Hitchhiker Of The Colorado Desert

Fruits of (Krameria grayi) resemble miniature grappling hooks.

Two more hitchhikers with Wayne's Word ratings of 10 SRDUs are krameria (Krameria grayi) and stick-tight (Desmodium cuspidatum). Krameria is a small, purple-flowered shrub of the Colorado Desert region of southeastern California. The amazing hitchhiker fruits are like miniature versions of the legendary Uncarina of Madagascar. The fruit is covered with radiating spines, each spine tipped with several minute hooks or barbs resembling a tiny harpoon. In a related species (K. parvifolia) the barbs are scattered along the upper portion of each spine. Although the flowers resemble a lovely orchid, krameria actually belongs to its own family, the Krameriaceae. Kramerias are also quite fascinating because they are partially parasitic on the roots of nearby shrubs.

Stick-tights or beggar's-ticks (Desmodium cuspidatum) produces slender legume fruits that break into small, one-seeded joints covered with tiny barbed hairs. Technically, this special kind of legume fruit is called a loment. The individual joints are so flat that they are exceedingly difficult to remove from your socks. Like little flat ticks, you must individually pull off each one. This can be exasperating when your socks are covered with them. Several species of this remarkable hitchhiking herb are native to the midwestern and eastern United States.

Stick-tights (Desmodium cuspidatum) attach to socks like flat ticks.

Naturalized hitchhiking plants often grow very well in disturbed sites, and this is how some of California's most noxious weeds were introduced. The earliest reported collection of puncture vine in California was made at Port Los Angeles in 1903, presumably the result of a ballast dump. Within a few decades it had spread throughout the state, reaching epidemic proportions in some cultivated valleys. The burs were carried in the wool of sheep, in hay, straw, other feed, manure, melons, alfalfa, cotton, potatoes, picking sacks and boxes, tents, sand, gravel, farm and industrial machinery, and in rubber tires of automobiles and airplanes. Puncture vine is the scourge of every bicyclist and is a major factor in the popularity of inner tube repair kits. Although it only has a SRDF of 3 in the above table, this may be a gross underrating for this naturalized hitchhiker.

Puncture vine (Tribulus terrestris), an Old World annual weed that has colonized interior valleys and roadsides throughout California. Each plant forms a prostrate mat composed of trailing stems that spread in all directions. Spiny fruits develop in the leaf axils, each fruit splitting into five seed-bearing sections (burs) called carpels. Since the fruit splits into indehiscent, seed-bearing sections, it is technically referred to as a schizocarp. The spines of each section are arranged so that one is always facing upward, like the medieval weapon called a caltrop. The spiny, seed-bearing burs readily penetrate shoes, clothing and skin, where they hitchhike to new locations.

Puncture vine belongs to the caltrop family (Zygophyllaceae), so named because of the shape of the wicked fruits. At matury, the fruit dries and breaks apart into five seed-bearing sections called carpels. Each section is armed with several sharp spines that readily penetrate bicycle tires or your shoes.

During medieval times, a vicious weapon called a caltrop was used in European warfare. This was an iron device with four points so designed that one was always facing upward, whichever way it landed, to impale the hooves of enemy cavalry horses. A similar device was also used in World War II to destroy truck tires on enemy supply convoys. The widespread water caltrop (Trapa natans) also has a four-pronged fruit that resembles a caltrop.

The Undisputed Most Painful Hitchhiker

A spiny stem segment of jumping cholla (Opuntia bigelovii).

Certainly one of the most painful hitchhiking plants in the southwestern United States is jumping cholla (Opuntia bigelovii). Although it only received 5 SRDUs for removal from interwoven socks, the spines are exceedingly difficult to pull out of rubber soles and elastic human skin. Like other kinds of cholla cactus, the cylindrical stem segments are densely covered with slender, barbed spines. In fact, this species is sometimes called "teddy-bear cholla" because of its dense covering of spines. What makes this cholla so unique is that the stem segments or joints break off with the slightest touch and become firmly attached to various body extremities. Unlike the unrelated but truly amazing Mexican jumping beans and California jumping galls, this cholla doesn't really jump. If you barely touch or brush against the spines and then suddenly jerk away, the fuzzy stem fragment will be instantaneously upon you. Trying to pull out the barbed spines is not only frustrating and excruciating, but usually results in the joint or fragment becoming attached to another part of your anatomy. On a field trip to the Anza-Borrego Desert of San Diego County, several students attempted to remove a segment from a lady's shoe, only to have it transferred to the shoes of each chivalrous male. It finally ended up on the hand of a screaming (bleeding) student who promptly flipped it full force into Mr. Wolffia's groin region. From that day forward the Wayne's Word staff always carries a pair of needle-nose pliers when walking through jumping cholla country.

Closeup microphotography with a scanning electron microscope (SEM) reveals why the spines of jumping cholla are so tenacious and difficult to pull out of skin. The spine is covered with sharp, overlapping scales or barbs that lie flat and allow the spine to penetrate skin readily like a very sharp needle. When you try to remove a spine, you are pulling against hundreds of tiny scales. In the process, other spines penetrate the skin from all directions, making the extraction very painful and seemingly hopeless.

Close-up view of the spine shaft from jumping cholla (Opuntia bigelovii) as seen through a scanning electron microscope (350 x). The numerous, sharp scales or barbs show why the spine is so difficult (and painful) to pull out.

The easily fragmented stem segments of jumping cholla are one of nature's most effective methods of hitchhiking and vegetative reproduction. Thickets of jumping cholla covering entire hillsides or alluvial fans may have developed from fragmented stem segments that became rooted in the desert soil. Although jumping cholla produces flowers, the seeds of most populations are typically sterile and reproduction is accomplished without sexual reproduction (technically referred to as apomixis). You could say that jumping cholla is a master in the art of hitchhiking and cloning itself.

The Deadly Pisonia Tree Of Tropical Pacific Islands

One of the most interesting cases of hitchhiking involves seabirds and a tropical pisonia trees of the four o'clock family (Nyctaginaceae), including Pisonia umbellifera and P. grandis. On some atolls of the South Pacific, pisonia is the only dominant tree on otherwise barren islets. Marine birds use the large, edible leaves to construct nests in the trees. Pisonia trees produce tiny white flowers in terminal, many-flowered clusters. Individual apetalous flowers have a tubular, petaloid calyx that resembles a sympetalous corolla. The lower portion of the calyx tightly enwraps the ovary and is persistent around the fruit as an anthocarp. The calyx base plus the enclosed seed-bearing fruit is the unit of dispersal. In some members of the Nyctaginaceae, the persistent calyx base bears sticky glandular projections that aid in dispersal by adhering to the bodies of animals. This is especially true in pisonia trees in which the numerous glutinous anthocarps stick to the feathers of seabirds. This is an effective method of dispersal to distant atolls and islands of the South Pacific region. Sometimes a hapless seabird is completely covered by clusters of the sticky anthocarps, to the point where flight is difficult or impossible. Unable to remove the water-resistant, glue-like anthocarps from its feathers, the seabird drowns in the surf and is consumed by ravenous beach crabs.

Inflorescence of a pisonia tree (Pisonia umbellifera), a common tree on Polynesian islands. A similar species (P. grandis) grows on islands and atolls of Polynesia and Micronesia. The ovary at the base of each flower in enclosed by a persistant calyx bearing sticky (glue-like) projections. These seed-bearing anthocarps readily attach to the feathers of seabirds and are dispersed throughout the South Pacific islands.

Flowers of a pisonia tree (Pisonia umbellifera), a common tree on Polynesian islands. The ovary at the base of each flower in enclosed by a persistant calyx bearing sticky (glue-like) projections. These seed-bearing anthocarps readily attach to the feathers of seabirds and are dispersed throughout the South Pacific islands.

The seeds of some parasitic mistletoes are also coated with a very sticky substance that aids in dispersal by birds. Desert mistletoe (Phoradendron californicum), a common parasite on cat-claw acacia (Acacia greggii), produces juicy, bright red berries that provide numerous birds with food and water during the winter months. If you walk along a desert wash in the Colorado Desert region of the southwestern United States, you can often spot a black bird called the phainopepla (Phainopepla nitens) near clumps of mistletoe. This bird is easy to identify because of the conspicuous crest on its head. Mistletoe seeds are covered with a glue-like substance that sticks to the bills of birds. When birds try to clean their bills, the seeds adhere to the limbs of other trees and shrubs. The seeds also pass through the bird's digestive tract and are transported from one bush to another in the bird's droppings. In fact, this probably explains the derivation of the word mistletoe: from two Germanic words: mista (dung) and tan (twig); referring to bird droppings on a branch or stem. Apparently when the word mistletoe was first used in Europe, people were already aware of the dispersal of mistletoe seeds by birds.

Bright red berries of desert mistletoe (Phoradendron californicum), a favorite food of the phainopepla.

Additional Hitchhiking Plants: Photographed Since Original Publication of This Article

A sock infested with the seed-bearing fruits (schizocarps) of hedge-parsley (Torilis arvensis). Each minute fruit contain numerous barbs and readily clings to cotton socks. This troublesome European weed is spreading rapidy in San Diego County. It is a good candidate for the list of top hitchhiking plants.

Hedge-Parsley (Torilis arvensis). During the fall months, the minute, seed-bearing fruits readily attach to clothing and are spread to new locations. The close-up view (inset) shows a spiny fruit (schizocarp) compared with the head of an ordinary straight pin. It is composed of two seed-bearing sections called mericarps.

Conclusions About Hitchhiking Plants

There are literally hundreds of additional plant species with remarkable hitchhiking seeds or fruits that use animals for their dispersal agent. Some of the Large Hitchhikers On Big Animals, such as the North and South American devil's claws (Proboscidea and Ibicella) may have ridden on the legs and fetlocks of large extinct mammals that once roamed vast plains more than 500 thousand years ago. A large African hitchhiker by the name of grappling hook or devil's claw (Harpagophyton (syn. Harpagophytum) procumbens ssp. transvaalensis) rides on many present-day hoofed mammals of the vast South African veld. A member of the Pedaliaceae (like the number one rated Uncarina grandidieri), the octopus-like fruits have long, hooked tentacles that are a menace to the feet and jaws of wild animals and grazing cattle. Although they are large and very effective hitchhikers on large animals, they only received 7 Wayne's Word SRDUs compared with 12 SRDUs for their Madagascar counterpart Uncarina grandidieri. They can be removed from socks rather easily, except your socks may be irreparably damaged with tears and small lesions. As an agent for plant dispersal, you have probably carried some small hitchhikers on your body while walking through a grassy field or meadow. So the next time you find a little seed or bur clinging tenaciously to your socks, think about how far its ancestors must have hitchhiked and the many perils of its long journey.

Large hitchhiker fruit of Harpagophytum procumbens clinging to a shirt.

In an article by S. Chrubasik, C. Zimpfer, U. Schutt and R. Ziegler (Phytomedicine Vol. 3: 1-10, 1996), the South African devil's claw (Harpagophytum procumbens) may be useful in relieving acute lower back pain. We are not suggesting here that you place one of these hooker pods down the rear of your pants as a counterirritant. The underground tubers of this remarkable hitchhiking plant contain harpagoside, an anti-inflammatory, analgesic glucoside (glucose + nonsugar molecule). In the study, some patients taking two 400-mg tablets of devil's claw extract three times a day reported relief from their intense pain. Although the study is not conclusive, this plant may offer a safe alternative for other more conventional and potent drugs, such as muscular relaxants and opiates.

References About Hitchhiking Plants

  1. Armstrong, W.P. 1992. "Devil's Claws." Pacific Horticulture 53: 19-23.

  2. Armstrong, W.P. 1982. "Beware of the Jumping Cholla!" Environment Southwest Number 498: 3-5.     

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

  4. Armstrong, W.P. 1979. "Beware of the Jumping Cholla" Desert Magazine Number 42: 8-11.

  5. Hanson, Chuck. 1997. "Pollination of Uncarina." Cactus and Succulent Journal 69: 305.

  6. Letty, Cythna. 1962. Wild Flowers of the Transvaal. Division of Botany, Department of Agriculture,
    Pretoria, Republic of South Africa.

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