Topical Summary: Eastern prickly pear, Opuntia humifusa – Winter repose – Plains prickly pear, Opuntia polyacantha – The more common species – Mountain cactus, Pediocactus simpsonii – Escobaria missouriensis, Missouri foxtail cactus – Opuntia fragilis – "Pin-pillow cactus" – The grip of the tip: electron microscopy – Sharper than a needle – Glochid from Opuntia fragilis – "The greatest pests" in perspective – "when the thorn touches your mockerson."
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 (Fig. 7, Fig. 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 Fig. 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 (Fig. 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.Plains pricklypear
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 succulent 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
|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 their eyes peeled 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":
On this part of the river on the head of Clarks River I observe great quantities of a peculiar Sort of Prickly peare grow in Clusters ovel & about the Size of a Pigions egge with Strong Thorns which is So birded [bearded] as to draw the Pear from the Cluster after penetrateing our feet."
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.