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Biology of Cacti

button The Shoot Saguaro landscape
button The Spines
button The Root System
The Flower
and Pollination Ecology

(Adapted from Benson, 1982 and Cullman, 1987)

The Shoot

The stems of cacti are thick and succulent. The soft, succulent tissues of the cortex and pith provide a large volume for storing food materials and water. A thick waxy cuticle covers the epidermis and restricts loss of water.

The stems may be simple columns with no side shoots or may be branched either near ground level or far above it. The stem surface may be smooth, but is most commonly covered with tubercles, or rows of them, more or less conne cted, called ribs. which represent the sublimation of all branches. Each tubercle bears an areole, a minute hump of tissue in which spines are propagated. The buds of areoles can eventually differentiate into flowers and then fruit depending on the presence or absence of appropriate environmental stimuli and particular precursors. Areoles are found only in cacti. Removal of spines causes only superficial damage.

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The Spines

Spines develop from the axillary buds and can be considered to be modified leaves. The spines are always located on the areole. In most cases two forms of spines can be distinguished: central spines that are located close to t he apex of the areole, and radial spines which grow around the circumference of the areole apex.

In addition, specialized spine forms are often characteristic of individual species. Spines that are curved into a hook are found in many species of the Mammillarias. Cacti with such spines are commonly shallow rooted. The combination of a shallow root system and hooked spines facilitates dispersal. A plant that is inadvertently ripped from the ground by large animals passing by attaches to fur via the spines; the plant is eventually discarded and can readily take root.

Members of the sub-family Opuntioideae possess distinctive hair-like spines called glochids. Glochids are only a few millimeters in length. They are thin and brittle and have fine barbs, which can only be seen with the aid of a microscope. Glochids easily pierce the skin and then break off, The barbs help to push them deeper into the wound, resulting in a great annoyance for several days. The infamous itching powders are derived from glochids.

Spines have important benefits for cacti. The central spines are commonly long, strong, rigid and brightly colored and they deter grazers. (The grazers soon learn to stay away from brightly colored spines). The radial spines are thin , flexible and are often white. White spines reflect sunlight away from the plant, (and all spines provide shade) reducing the potential for damage from excess solar radiation. Some species, e.g. Espostoa lanataon the very arid western slopes of the Andes, capture and absorb daily ocean mists on the ample woolly spines. The down-pointed central spines drip any excess droplets to the shallow roots in the soil below. During a light rain or mist, water accumulates and condenses in droplets on the tip of each spine. Downward directed spines cause the water to be deposited on the soil surface, just above the shallow roots.

Some cacti are spineless or nearly so. These tend to produce toxic substances that compensate for the lack of protection from grazers offered by spines. Lophophora williamsii (Peyote) is one such example; it produces the alkalo id, mescaline, which is hallucinogenic and is used as a narcotic legally only by one "Native American" religious group. All other users do so illegally and some states prohibit even hobby cultivation of Peyote. Epiphytic cacti, the "Jungle Cacti" (Holiday, Orchid and Mistletoe cacti) are also nearly all spineless. These grow on trees and on cliff faces, habitats mostly inaccessible to predators who would be repelled by spines. These are treated more fully in later sections.

central spines
Radial Spines     Hooked
Spineless stems

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The Root System

Commonly, the root system of cacti is very shallow and widely spread, which enables them to exploit water deposited in surface horizons by short periods of rain. For example, the root system of a young saguaro (Carnegiea gigantea), which was only 12 cm high filled an area of about 2 meters in diameter but penetrated only 10 cm into the soil (Benson, 1982).

In addition to their shallow, branching roots, many columnar cacti, including the saguaro have one or more taproots that penetrate the deeper layers of the soil. These taproots help to anchor the plant and to obtain deeper lying water and nutrients. In some cacti, e.g. the genera Ariocarpus and Lophophora, the roots actively shrink during periods of drought. In the process, the shoot is drawn into soil and the entire plant may become covered by soil. This conserves valuable water by reducing the surface area of the plant exposed to the air, and protects them from grazing predators. Such plants have been called "living roots".

    Saguaros     Shallow

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The Flower

The cactus flower is similar to those of other flowering plants. It is a perfect flower, which means that it bears sepals, petals and functional pistils and stamens. Most cacti have numerous stamens. All cacti, except for some specie s of the primitive Pereskiahave epigynous flowers. The flowers of some primitive Pereskiaspecies are perigynous, which differ from the epigynous flowers of all other cacti in that the ovary is superior rather than inferior.

Cactus flowers vary in size and color. The flowers of Selenicerus grandiflorus (Queen of the Night) can be more than 20 cm in diameter (Benson, 1982). In contrast, the flowers of Mammillaria are commonly very small but cluster together to form a conspicuous garland just back from the crown of the plant. Colorful blooms and scents attract pollinators. Many desert cacti produce colorful unscented flowers that bloom during the day and attract various flying insects. Many jungle ca cti have richly perfumed large blooms, usually white or pale yellow, that open at dusk and attract moths. Night-blooming cacti tend to have large, foul smelling, fleshy blooms that attract bats.

Perfect flower
with numerous stamens
Bat-attracting blooms Garland of flowers

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To survive in very dry and hot places, cacti must be capable of

  1. surviving long periods without water;
  2. being very economical with any available water;
  3. tolerating intense solar radiation and high temperatures.

The following features contribute to these capabilities:

...Cacti are very slow growing.

Consequently, their water requirements per unit time are low.

...Cacti have special water-storing tissue.

The majority of epidermal and inner stem tissue consists of water-storing parenchymatous cells.

Spongy tissue

...Cacti exhibit Crassulacean Acid Metabolism (CAM).

This results in high WUE (water use efficiency). WUE refers to the amount of water lost by transpiration relative to the amount of carbon dioxide taken up and fixed into organic compounds by photosynthesis.

Most plants open their stomata during the day in order to allow carbon dioxide to diffuse into the leaf and, with water, to function immediately as a substrate for photosynthesis. Inevitably, this also allows water saturated air surrounding the photosynthetic tissue to diffuse outwards; at night, when photosynthesis ceases, the stomata close in order to stop the loss of water.

CAM plants have a special mechanism that allows them to take in carbon dioxide at night and hold it in organic acids; this causes the pH to fall at night (hence the name acid metabolism).

During the daytime, the carbon dioxide is released from the organic acids and feeds into photosynthesis; stomata remain closed. Opening the stomata at night, rather than during the day results in much less loss of water because of lower air and tissue temperatures. It has been calculated that cacti lose 1/6000th as much water as a mesophyte of the same surface area, by such strategies.

...Cacti are able to survive after considerable water loss.

Cacti commonly lose 60% of their water before significant physiological stress sets in. Most mesophytes, in contrast, will wilt and die with a 20% loss of their water (Benson, 1982).

...Cacti have mechanisms to rapidly seal off wounded tissue.

When a typical mesophyte is cut or wounded its exposed tissue will dry out rapidly. A leaf or branch of a wounded plant may wilt or die unless enough water can be supplied by the root system to compensate for the rapid water loss.

In arid environments, cacti cannot rely on rainfall or moisture in the air or soil to compensate for a loss of water through a wound. However, cacti produce a special, slimy polysaccharide which will cover a wound and dry into a tou gh, impermeable layer preventing further water loss. Another mechanism of sealing wounds is the production of a layer of water-resistant cork around the wounded tissue. This mechanism is common in the Saguaro.

...Cacti have mechanisms to reduce exposure to sunlight.

Reduced exposure may be needed to reduce overheating, or excess photoxidation. Light colors reflect more of the sunlight away from the plant. The waxy coatings of many species are often light or even white in color. Many columnar ca cti such as Cephalocereus senilis (Old Man cactus) have snow-white hairs covering the entire body. A major function of radial spines is to provide shade for the plant.

Water-losses and over-heating is also reduced in low profile cacti that have stems that are retractable into the soil, growing half-buried in the ground and/or being pulled underground by shrinking, napiform roots in dry times.

...cold-adapted Cacti apply 'anti-freeze' strategies.

Glutinous sap, dark, heat-absorbent colouration, sparse spination, are the main enhancers of internal temperature of plants adapted to high altitutes or high latitudes.

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Reproduction and Pollination Ecology

Cacti reproduce both sexually and asexually. In asexual reproduction all new plants are genetically identical to their parent plants. Asexual reproduction can occur naturally from detached joints, offsets, cuttings and cell propagati on, and including scions in grafting which produce roots after they have separated from the parent plant. Joints which do not develop roots eventually die.

Sexual reproduction is associated with the processes of flower formation, pollination, seed production, dispersal and germination. Plants can be self-pollinated if pollination occurs within the same flower or a different flowe r of the same plant. Cross-pollination occurs when pollen grains are transferred to a flower of another plant.

Most cacti ensure cross-fertilization in two important ways. First, most cacti are protandrous, meaning that the anthers shed pollen before the surface of the stigma is receptive. Secondly, most cacti are self-sterile, which p revents self-pollination.

Cacti accomplish pollination in a variety of ways. The conspicuous and variable cactus flower color, form and scent serve to attract pollinators such as birds, insects and bats to the food supplies that the flower offers. Pollinators disseminate pollen grains, and fertilization is encouraged. The different forms of the cactus flower often reflect the differences in the body structure of the pollinators that are adapted to fertilize them.

Pollinators frequent the flowers because of the food they offer, the pollen and nectar produced by the flower. Pollen is rich in both fats and protein and is eagerly sought out by bats and beetles. Many cactus species, such as the Saguaro Carnegiea giganteawill produce large quantities of pollen. A single flower of this cactus plant may contain over 3000 anthers and subsequently produce hundreds of thousands of individual pollen grains.

Nectar is a watery solution of sugar, secreted at the base of the flower, that is consumed by many insects and birds. Cacti in many genera have developed elaborate tissues for the production of nectar.

Insects, birds and bats are important cactus pollinators.


The most important cactus flower pollinators are the Hymenoptera (bees and wasps), the Lepidoptera ( butterflies and moths), and the Coleoptera, (beetles).

Most insects have a well developed sense of sight and smell due to their antennae and faceted eyes. These insects prefer the rotate, raylike flower shape that most cacti possess.

Epiphyllum oxypetalum (Dutchman's pipe) is specialized to suit certain nocturnal hawk moths. This cactus has a floral tube that is exactly 30 cm long, with nectar at its base and it flowers only at night. Attracted by the flow er's scent, the hawk moth hovers close to the flower, unrolls its proboscis that is also 30cm long and inserts it into the tube.


Birds form the largest group of cactus pollinators next to insects. The bright, variable shades of red, yellow, or color combinations of red and yellow, red and green and red and blue serve to attract specific birds.

There are approximately 2000 species of birds capable of approaching and pollinating cactus flowers. The hummingbirds are the single most important group to visit the cactus flowers. They have poor sense of smell; flowers that att ract these birds are brightly colored, generally scentless and possess stout floral tubes that are suited to their beaks and long tongues.

Schlumbergeras, the Christmas and Thanksgiving Cacti are well adapted to hummingbird pollinators. The dimensions of the flowers are apated so that their long-projecting filaments allow pollen to be easily dusted off onto the head of the bird. When the humming-bird flies to another flower, the pollen is readily brushed off onto its stigmas.


Bat pollination is common for some species of cacti that have relatively large, funnelform, white and pungent-smelling flowers that produce large amounts of nectar and pollen.

The most important nectar-feeding bats are the nocturnal, long-tongued vampires Glossophagidae. They are highly specialized to visit the flowers of many columnar cacti such as the Saguaro and the Organ Pipe cactus. The bats either cling to the large waxy blooms or remain more or less in fluttering flight, extracting large quantities of nectar and pollen by rapidly extending and retracting their long tongues from the flower.

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