& OTHER SUCCULENTS
(Adapted from Benson, 1982 and Cullman, 1987)
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.
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.
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".
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.
To survive in very dry and hot places, cacti must be capable of
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.
...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.
...Cacti are able to survive after considerable water loss.
...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.
...Cacti have mechanisms to reduce exposure to sunlight.
...cold-adapted Cacti apply 'anti-freeze' strategies.
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).
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.
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.
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