D e s e r t E x p o s u r e
June 2010
A Thorny Feast
Despite their prickly defenses, when cacti bloom and bear fruit, they play a vital role in the food chain of our Southwest deserts.
Story and photos by Jay W. Sharp
From southwestern New Mexico westward to the Sonoran Desert and eastward to the Pecos River, the cacti hold a prominent place in the plant communities of the scrublands of the arid basins. In spite of their prickly defenses, they serve up important foods for wildlife, including the human species. They provide medicines for curanderos (Mexican folk healers). They yield hallucinogens (often highly dangerous ones) for shamanistic trances. They hold a showy place in many Southwestern gardens. The cacti stir the imagination, taking a role in the folklore and legend of our area.
Cacti, with the family name of Cactaceae, first arose, in the New World, some tens of thousand of years ago. Evolutionary newcomers to the botanical kingdom, they evoke a sense of the non-traditional or the modernist. They could have sprung from the canvas of the famed 20th century Mexican artist and muralist Diego Rivera.
"They are, without doubt," said authority Clark Champie in his Cacti and Succulents of El Paso, "the oddest of the oddballs of the plant kingdom to the unending delight of the cactus fancier and utter despair of the plant classifier. In no other plant family do the members play more tricks on the taxonomists as in this one. For instance, some plant classifiers insist that the number of genera [groups of species within a family] should run in the neighborhood of 20 to 25. Other taxonomists think it might take as many as 150 genera to bring order to this unruly group of plants. And the agreement on the number of species is no closer, with the conservatives thinking that something like 800 might do the job while those of a more liberal bent would like to the number of names running closer to 1,800."
Our cacti here in the Southwest vary wildly in shape, size and bloom. They occur, for instance, in the form of buttons, balls, barrels, globules, branching pads, branching stems, branching trees and towering columns. They range in size from the humble thimble plains cactus, which measures perhaps an inch and a half in height and an inch in diameter, to the stately saguaro, which may reach more than 60 feet in height and two to three feet in diameter. A representative of the smaller species weighs no more than a few ounces. A large saguaro, by comparison, may weigh 10 tons or more.
Most of our cacti bloom from early spring into the fall, producing flowers in a rainbow of colors that bejewel the desert. The blossoms range in size from a fraction of an inch to several inches in diameter.
While we think of cacti as progeny of the desert, they "can be found in jungles, on high cold mountains, bare or grassy plains, along seashores, in sub-tropical areas, as well as in arid and semi-arid regions," as W. Hubert Earle noted in his Cacti of the Southwest. They have extended their range across much of the Caribbean and the North and South American continents.
Survival Strategies
Their variability notwithstanding, the cacti share certain defining characteristics that equip them for surviving desert drought. They have thick, hard, succulent and wax-coated green stems that harbor water.
The stems swell and shrink — a botanical camel's hump — reflecting the store of water in the tissues. Unlike any other plant family, the cacti stems bear circular- or oval-shaped organs called areoles from which spines grow, flowers blossom and new stems branch. The areoles wind in perfectly arranged spiral rows around some species. They crown phalanxes of conical-shaped tubercles in other species. They march, in orderly pattern, along the edges of the ribs of barrel and columnar cacti. They fall in regular columns along the edges and on the flat surfaces of prickly pear pads.
The spines — actually modified leaves — minimize water evaporation and discourage some animals' foraging. The spines, according to Earle, can be porrect (extended horizontally), curved, hooked, round, stiff, acicular (needle-shaped), papery, feather-like, hair-like or sheathed. The ironically named teddy bear cholla, for instance, with its bristling spines, ranks as the porcupine of the plant world. The senita, or old man, cactus, with its long wispy spines, suggests an elderly gentleman with flowing white hair and beard.
Generally, cacti root systems spread radially near the surface, first in line for any rainfall, serving as an extensive network of conduits for conducting water to the stems. Within hours after a rainfall, the roots "produce great number of tiny rootlets which quickly gather water and send it to the fleshy reservoirs of the plant," according to George Olin in House in the Sun: A Natural History of the Sonoran Desert.
Unlike most of families within the plant kingdom, cacti have perfected a biochemical method for gathering carbon dioxide — a compound integral to the daytime business of photosynthesis — during the cool of the night rather than under the heat of desert sunlight.
"Normal" plants open their stomata — or the pores in their leaves — during the day to "inhale" carbon dioxide, incorporating it immediately in the solar-energy-fueled process of photosynthesis. This makes sense, especially given the modern notion of "just-in-time" deliveries of raw materials to factories, but it proves costly in terms of the water that escapes through the plants' stomata. Cacti, by contrast, open their stomata, or pores in their stems, during the night to gather carbon dioxide, converting it into an organic acid and effectively holding it in short-term storage. When the sun rises and the heat climbs, the cacti batten down their stomata hatches. They free the carbon dioxide from the organic acid internally as needed for photosynthesis during the day. This extra step in the process of photosynthesis impedes efficiency — and in fact, as a result, cacti may grow more slowly than other plants — but it saves water, which can equal as much as 80% of the plant's total weight. A fair tradeoff in the desert.
Family Members
In our Southwestern deserts, the Cactaceae family includes an array of clans, or genera, that encompass variously defined species, often reflecting classifiers' conservative or liberal biases. The better-known genera include, for a few examples, the pincushion (Mammilaria), the hedgehog (Echinocereus), the barrel (Ferocactus and Echinocactus), the columnar (Giant Columnar), and the cholla and the prickly pear (Opuntia).
The roughly cylindrical-shaped pincushion cacti compose perhaps several dozen species in the deserts of the Southwest. A diminutive upright plant, they measure only a few inches in height. The stems come adorned with long spiraling rows of tubercles tipped with areoles that give rise to a dense cloak of spines, the central ones taking the shape of a fishhook in some species. Some pincushions grow as solitary plants, others, in dense clusters. They bloom from early summer into the fall, producing a small club-shaped reddish fruit. They take their name from the mammalian mammary gland because some of them produce a milky-looking sap.
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Hedgehog cactus blooms. The hedgehogs are among the earliest bloomers. |
The cylindrical-shaped hedgehog cacti includes several dozen species. Also a small upright plant, they may grow from a few inches up to a foot in height. Unlike their pincushion brethren, the hedgehog cacti have stems with ribs instead of spiraling rows of tubercles. Their areoles, located along the edges of the ribs, have needlelike (never hooked) spines. They typically grow in clusters of several stems. Most bloom in the spring, producing a dazzling flower and a large tasty light tan or green to reddish fruit. According to Earle, the hedgehog cacti had its named conferred by Europeans who thought that the spiny little plants resembled the bristly hedgehogs of the Old World's hedgerows.