Eastern Prickly Pear
Opuntia humifusa (Raf.) Raf.
This was the only species of the plant family Cactaceæ that grew wild along the Atlantic Coast from southeastern Massachussets to Florida, and westward as far as the Mississippi River.1 It may have been found on or near Meriwether Lewis's plantation, Locust Hill, in Albemarle County, Virginia. Carl Linnæus had named it Cactus opuntia in his Species Plantarum of 1753, but Lewis knew it only as "prickly pear." Before he left the east coast for his journey down the Ohio, he might well have read about a few species of cactus in the periodical press, or may even have seen a few imported specimens in one of the few plant collectors' private gardens.
The significance of the word "prickly" gained an additional measure when Lewis encountered the several western species of Opuntia that were to command the Expedition's attention (Figure 7, Figure 8) and patience. The word "pear" describes one of the most common among the various outlines of the genus's flat green pads—the rest being circular, lanceolate (lance-shaped), elliptic, or obovate (egg-like). All of them resemble leaves, so we can forgive Lewis for calling them that, but they are really stems that have evolved to function as leaves—photosynthesizing sunlight and storing moisture to sustain the plant through long dry seasons. Cactus stems are called cladodes or phylloclades.2
The dark bumps on the specimen pictured here, some of which have pale centers, are nodes called areoles. (The light-green spots in this photograph are droplets of water from the irrigation system in the USDA Agricultural Research Service's National Arboretum.) In most cacti, one or more long, stiff spines grow from the center of each areole. Those spines are vestigial leaves, but they lack the leaf-cell genes common to most true leaves. (Compare with Figure 4.) Typically, only a few of the areoles on each cladode of an O. humifusa produce spines, while the rest are spineless—glabrous, or "bald."
The areoles also hold clusters of short (up to 4 mm), pale-yellow to dark-brown, barbed spines called glochids.3 The glochids are so fine that they can hardly be seen, but when embedded in a person's skin they can produce noticeable irritations, and are extremely difficult to extract. Modern cactus gardeners have recourse to the 20th-century expedient of applying any sort of adhesive to pull them out. The areoles on O. humifusa are farther apart than those of the plains prickly pear, O. polyacantha (Figure 3), and may hold only one or two 5 cm (2 in.) spines each, although many lack spines altogether.
Using Lyon's specimen of the eastern prickly pear as the basis for his taxon, and choosing Cactus as the generic name, Linnaeus had classified it in the tenth edition of his Systema Naturæ as "Cactus opuntia." It was renamed Cactus humifusus in 1820 by Constantine Rafinesque (1783-1840), who published it with his description of a type specimen. He revised his narrative in 1830, and reclassified the species as Opuntia humifurus. The specific epithet has since been changed to the form humifusa.
Until recently it was generally held that the generic term came from the Latin "Opuntius," in turn from the Greek city of Opus, but that is now questionable. Instead, it may have come from the Greek opos, "fig juice," evoking the shape and flavor of the plant's fig-like fruits.4 A third but etymologically more remote possibility is that it was derived from a combination of the Aztec noun nopali (fruit) and the Latin pungere (to prick or sting).
Like a few other species of Opuntia, O. humifusa has evolved a strategy to withstand the cold winters of the temperate latitudes, which is implied by its specific epithet, humifusa meaning "spread out on the ground." In autumn it becomes procumbent, which means "prostrate or trailing on the ground, but not taking root."5 The stems lose their moisture, become limp and spongy, and flatten out on the ground to gain a little warmth from the earth and allow blankets of snow to insulate them from winter winds. The pale purple appendages on the upper cladode are nearly-ripe edible fruits that matured in the previous growing season.
Plains Prickly Pear
Opuntia polyacantha Haw.
Lewis would have been dumfounded by the contrast between the relatively non-threatening eastern species of prickly pear, and the heavily-armed plains prickly pear. The botanical terms arms and armed denote the spines and thorns of certain plants, which in this instance are defensive arms that nature has evolved to discourage rodents and other mammals from eating those moist, succulent pads.
"the more common species with the flat leaf"
Opuntia polyacantha Haw.
There are more than 120 genera and 1200 species in the cactus family, all native to North and South America. Among the most numerous species of the genus Opuntia are the plains prickly pears, identifiable by their spiny stems that in summer trail erect along the ground, plus its own seasonal flower, and fruits the size, shape, and almost the color of figs. Lewis referred to this species on 13 August 1805 as "the broad leafed pear of the Missouri." Of the seven species of cacti that are native to the Rockies and the High Plains, Lewis preserved specimens of two, including one evidently of O. polyacantha, but both have long since been lost.
The specific epithet polyacantha means "many thorns" in botanical Latin. Plains pricklypear was first described for science in 1819 by the English gardener Adrian Haworth (1767-1833), a devoted student of succulent plants.
With or without spines as numerous as those of O. polyacantha, the drying and pressing of suculent cactus specimens of many dimensions for botanical study is considerably more challenging than with most other plants, a factor which measurably protracted the development of cactus taxonomies. Analytical work normally carried out in the laboratory or "cabinet" using collected specimens had to be completed in the field.
Each little clump of cottony, waxy fluff on these prickly pear stems is a cocoon that shelters a single wingless female scale insect called a cochineal (kah-chin-eel). The cocoon serves to protect her eggs from hot sun and chilling rain.
Pediocactus simpsonii (Engelm.) Britton & Rose
The small size and scattered occurrences of this species, easily hidden among low grasses and shrubs on the prairie—pedio is Latin for "a plain"—must have made it especially annoying to the Corps' hunters. Its plants usually grow singly; rarely do two or three spring from the same root. Hunting on the move in cactus country demanded acute attention to many environmental details at once.
This was one of the first cacti carried to Europe by explorers of the West Indies, and for some years it was the only species known to Carl Linnæus, who assigned it the name Cactus mammillaris in his Species Plantarum of 1753. It had already been described by the Dutch botanist Jan Commelin in 1697 and the German botanist Paul Hermann in 1698. In 1819 Adrian Haworth renamed it Mammillaria conica. His description, "Tubercles large, conic,"6 resembled Lewis's. In 1913 Britton and Rose traced its taxonomy from the 1863 description by the German-American botanist George Engelmann (1809-1884), who had named the species simpsonii, for botanist Charles Torrey Simpson (1846-1932).
As described in Flora of North America, ball cactus can be up to 15 cm (5.9 in.) broad by 15cm high; the surface of the stem is covered with grooved pyramidal protuberances called tubercles, each framed radially by 15 to 20 spines from 5 to 21 cm. (1.9 to 8.2 in.) long.7
Escobaria missouriensis (Sweet) D.R. Hunt
Missouri foxtail cactus
Sometimes called pincushion cactus or nipple cactus, Escobaria missouriensis (Sweet) D. R. Hunt is another possible identity for Lewis's cactus "of a globular form." It is still found from the Mississippi to the Rockies. Robert Sweet (1783-1835), the author of the original description (1826) under the name Mammillaria missouriensis, was a prominent English botanist and gardener. D. R. Hunt (b. 1938) is the English specialist in Cactaceæ who changed the generic name from Mammillaria to Escobaria, the latter memorializing the Escobar brothers of northern Mexico. This species ranges in size from 2 to 8 cm (.078 - 3.1 in.) in height, either singly, or in clumps of more than 30 cm (11.8 in.) in diameter.8
Opuntia fragilis Haw.
Sometimes called the "Little Prickly Pear," the specific epithet in its official name Opuntia fragilis Haw. reflects its most obvious characteristic, the fragile connections between its cladodes, which enables the uppermost member to propagate itself by clinging to a passing carrier with its barbed spines (Figure 7-8), to be deposited in a new, often distant location where it can take root. Given that asexual mode of reproduction it is no wonder that O. fragilis is seldom seen in bloom.
In terms of the average size of its stems, Opuntia fragilis is the smallest species of cactus in North America, and the one most capable of surviving cold temperatures. Its distribution extends from southern Texas, New Mexico and Arizona, westward into Oregon and Washington, and northward to upper British Columbia and southern Alberta, Saskatchewan and Manitoba.9
The longer and darker spines on this cladode are entirely dead; the shorter, lighter ones are younger, having live cells only at their bases. The cladode pictured here is 24 mm (0.9 in.) long by 15 mm (0.6 in.) in diameter. The 3-8 spines per areole spines are up to 24 mm (.9 in.) in length.
Adrian Haworth related that the botanist Philip Miller (1691-1771) had declared, "it is called Pin-Pillow in the west Indies, from the appearance which the plump branches have to a pincushion stuck full of pins"—inserted upside down, of course.10
The grip of the tip
An electron microscopy image of the tip of a spine of Opuntia fragilis shows its reversed barbs at a magnitude of 200 times their actual size.11 No one, including Lewis, could have realized the magnitude of those microscopic barbs.
Adrian Haworth, the British botanist who earned a reputation as the pioneer expert on cacti, wrote the first description of the species, which had been introduced from the West Indies in 1690 into a collector's garden in the vicinity of London. Haworth named it Opuntia curassavica, deriving the specific epithet from Curaçao, the island in the Netherlands Antilles where it had first been found. "This is a very shy-flowering plant," he wrote, and has only once attempted to blossom with me." Indeed, the latest account of it notes, "in many places, it flowers infrequently, if at all."12 Its branches were so extremely brittle that they break off at the joints on the slightest touch, adhering to the flesh or clothes of any person coming in contact with them; and create much pain and trouble to disengage, as their long sharp prickles are thickly and reversely barbed with invisible points."
Sharper than a needle
An electron microscope image here compares one of the spines (bottom) that is attached to the cladode shown in Figure 11, with the tip of an ordinary sewing needle. The tip of the spine has been damaged—a few barbs are missing—but is still considerably sharper than the sewing needle. The barbs to the left of the damaged tip cannot easily be felt with fingers calloused by the kinds of work the Corps and its captains carried out daiy. It is only upon pulling them out of one's flesh that the tenacity of the barbs is sensed.
Glochid from an Opuntia fragilis
This is the irritating end of a glochid (with a broken tip) that is one of a cluster of short (3- to 4-mm), yellowish, flexible and retrorsely barbed spinules that radiate from the upper margin of each areole on the stem of an Opuntia fragilis. The glochid in this EM image, shown at 800 times its actual size, is from one of the 20 areoles on the cladode pictured in Figure 11, each holding from 5 to 7 glochids that are 37.5 um (micrometers) in diameter—about half the thickness of a fine human hair.
Glochids are easily detachable from areoles by the slightest contact with human skin, but they are extremely difficult to extract. Being firmly embedded in the epidermis, they become a sub-visible, burning irritant fully as irritating as a mosquito bite, which is refreshed with each contact with some other surface.13 Modern cactus-growers keep a roll of adhesive-coated tape handy for relief.
Areoles and the glochids that grow from them are the vestiges of branches and leaves that evolved on desert plants to reduce water loss and enable them to survive in extremely dry habitats.
"The greatest pests" in perspective
Clay Jenkinson, a former director and editor of Discovering Lewis & Clark, kneels (carefully!) beside four mats of Opuntia fragilis Haw. Typically, mats are more thoroughly camouflaged than these are by grasses and other low plants, which accounts for Lewis's remark—on June 4, 1805, during his first exploration of Maria's River—that these cacti "are so numerous that it require[s] one half of the traveler's attention to avoid them." Imagine the frustrations of the hunters, whose primary job was to keep watching for game. Moreover, it is pointless to try to walk a straight line through areas where such plants grow.
"when the thorn touches your mockerson . . ."
With several spines of this cladode firmly embeded in the tough, thick leather of a modern boot, one can appreciate Clark's plaint of September 8, 1805, the day prior to their arrival at the creek the captains named "Travellers-rest":
Homesteaders in the west, who were required to improve their claims within a given period after filing, could do so in part by clearing their land of cactus. The most efficient way to do that was to burn it early in the spring, before the rains came and grasses greened-up. It could require several seasons of burning before all his land could be plowed.
2. Cladode or cladodium is Greek for "many shoots"; phylloclade, also a Greek word, denotes "leaf-like." OED Online. (Accessed 18 March 2010.)
3. Britton and Rose, (1912-18), 1:127.
4. Urs Eggli and Leonard E. Newton, Etymological Dictionary of Succulent Plant Names (New York: Springer-Verlag, 2004), 172.
5. Linnæ-Turton, General System, vol. 7, "Explanation of Terms."
6. N. L. Britton and J. M. Rose, The Cactaceæ, 4 vols. in 2 (reprint of second edition, 1937; New York: Dover, 1963), 4:70.
7. Kenneth D. Hell & J. Mark Porter. 2004. Pediocactus simpsonii. In: Flora of North America Editorial Committee, eds. 1993–. Flora of North America North of Mexico, 15 vols. (New York: Oxford UniversityPress, 1993–), 4:214.
8. Flora of North America, 4:222-228.
9. Flora of North America, 4:93, 124, 146. Eric Ribbens, "Opuntia fragilis: Taxonomy, Distribution, and Ecology," Haseltonia, Yearbook of the Cactus and Succulent Society of America, Number 14 (December 2008), 94-110.
10. Haworth, Synopsis Plantarum, p. 187.
11. The following EM images have been provided by Jim Driver of the Electron Microscopy Facility, Division of Biological Sciences, University of Montana, Missoula. The EM Facility is supported in part by grant #RR-16455-04 from the National Center for Research Resources (Biomedical Research Infrastructure Network program) of the National Institutes of Health. The EM Facility's website is at http://emtrix.dbs.umt.edu/
13. Stanley L. Welsh, "Utah flora: Cactaceæ," The Great Basin Naturalist, Vol. 44, No. 1 (January 31, 1984), 69.