07/12/2014 It’s Good to be a Cactus

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Prickly Pear has been one of the most valuable plants in Texas. Historically, all parts of the plant have been used by humans. In addition to its use as a food source, its sap has served as a medicine; its spines as medical instruments and tattoo needles; and its pads were made into containers. Robert Benson Photo.

Anyone who has had a run-in with a cactus, won’t willingly cross it again. Cacti are armed and dangerous. It is a wonder that cacti aren’t completely shunned by humans and animals alike.

Ironically, some cacti are the most valuable plants in the state of Texas, especially in arid areas. As a food source for wildlife, livestock and people, cacti have been indispensible. Matt Warnock Turner in his Remarkable Plants of Texas states that cactus, especially “prickly pear, of all our native plants, has been most responsible for keeping humans and beasts from starving in times of deprivation.”

Cacti have many adaptations to life in a harsh, dry climate. Leaves lose moisture too quickly for plants to survive in a desert. So, leaves have been eliminated in most cacti. Instead, the leaves have evolved into spines. These protect the plant from being eaten too readily.

Without leaves, what we are left with are their stems, which are enlarged, green, and covered with a waxy, waterproof coating. In some species the sections of stem are flattened into pads. In others, the stems are column-like with expandable ribs or ridges. Quite a few species are ball or barrel shaped. A globe shape minimizes the surface to volume ratio. It is the most efficient design for reducing evaporation from the body of the plant.

Photosynthesis, the production of glucose sugar, is carried on in the layer of green cells just under the surface of the stems. Sunlight, falling on the green pigment chlorophyll, causes water and carbon dioxide molecules to chemically combine. Water is taken up by the plant’s roots. Carbon dioxide, a gas in the air, enters through pores in the leaves. Oops, cacti have no leaves! Well, they still have the pores, called stomata, but they are on the stems.

In another fascinating adaptation, the stomata only open at night, significantly reducing water vapor loss along with the intake of carbon dioxide.  However, the plant cannot photosynthesize at night. So what does it do with the carbon dioxide?

The cactus stores it. But it cannot store very much of it as a gas. In a metabolic adaptation present in 98% of cacti, the carbon dioxide is chemically converted to malic acid. In this form, the carbon dioxide can be stockpiled until photosynthesis starts up again in the morning. This mechanism is known as Crassulacean Acid Metabolism and is a special photosynthetic pathway that has evolved in plants that grow in arid areas.

With this biochemical advantage, cacti are among the most successful plants in the Americas. They can make food for themselves (and those that eat them) even in an arid environment. They store that food as sugars and carbohydrates in their fruits, and as protein in their seeds. Early humans quickly learned how to use the food resources of cacti. The fruits could be eaten fresh, or dried and stored.

The seeds, which often passed through human digestive tracts undigested, could be collected, ground into a kind of flour, and re-eaten. This “second harvest” was probably practiced by several Native American tribes in the southwest. Certainly, it is documented among the native peoples of the Sonoran Desert who reclaimed the seeds of Organ-pipe Cactus from their own dried feces. I wonder how they organized this. It is thought that certain tribes “practiced the custom of defecating in the same area,” thereby facilitating the collection of the dried seeds.

Mexican Fencepost Fruit 300x400

Mexican Fencepost is a columnar cactus that can be planted side-by-side in a line to make a sturdy, living fence. In summer, the plant produces lovely flowers and the resulting fruits are edible. But watch out for spines! Linda Vegh Photo.

A relative of Organ-pipe Cactus is known as Mexican Fencepost Cactus. This columnar cactus grows to 20 feet tall. These trunks, or stems, are up to eight inches in diameter and are ridged with five or so ribs. The ribs are armed with small spines that, with caution, can be avoided. Planted close together in a line, these cacti grow rapidly into a living fence that is sturdy, tall, and not too spiny.

I have found Mexican Fencepost Cactus growing in some yards in Bee County. These are specimen plants that the gardeners have decided to grow for their unusual shapes and exotic flowers. Recently many of these cacti have bloomed and now are on their way to producing the edible red fruits. I am watching them ripen and hope to sample one soon. I hear they taste like watermelon!

ESSAY BY KAREN L. P. BENSON
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