What is a cactus?

Cacti - the "ugly ducklings" of the plant world, are unique plants. With hundreds of elaborate shapes, sizes and colours they have become a popular object for collecting.

A cactus is a plant that is different from other plants in the way that it grows, lives and survives. Cacti are widespread throughout the New World, the deserts of Mexico and USA are home to a large number of cactus species while in the grasslands and mountains of South America they are the predominant vegetation. Most cacti prefer an arid environment but there are species that inhabit the rainforests of Central and South America as well as Baja California. First of all, all cacti are succulents (succus - juicy, lentos - leaf, grow, Lat.), that is they store water in their trunks to survive almost waterless conditions.

Most cacti have spines, this is a major adaptation, the spines are actually reduced leaves which grow from structures called areoles. Cacti are the only plants that posess areoles. Cacti often have a candle - like or barrel - like shape. This is often what appeals to the collector, along with colourful spines and flowers and growing habits. These plants add life to landscapes that would otherwise have been empty. Cacti liven up a room or garden, adding an air of exoticity. The most beautiful arrays are group placings where many different plants re placed together. Balance is of course essential. A single well cared for cactus placed somewhere visible under good illumination livens up a living room or entertaining space. Species with long, brightly coloured spines work better for this purpose. The Christmas cacti have the feature of the owner being able to make the plant flower when it is desired. This is handy because the plant can be made to flower for a special occasion or a public holiday. Of course, collecting cacti provides a hobby and a pastime, giving years of enjoyment and good time.

Anatomy and Structure of Cactus
Areoles
The distinction between the Cactaceae and the order succulents is that the Cactaceae contain areoles. An areole is a structure that bears spines, the buds that eventually turn into flowers and then fruit and sometimes the areoles bear glochids. On most cacti, the areole is located on the tubericle or elsewhere on the rib region. Only several species in the Opuntia genus contain glochids (located on the areole). The rest of the order succulents (not including the Cactaceae) contain spines that grow not form the areoles, but directly from the succulent tissue. The adaptation of areoles in the cactus family is essential, thus if the spine of a succulent is removed, the tissue surrounding the spine is damaged. Rather when a spine is removed from an areole, the tissue is not damaged because the areole bears the spine, the tissue does not bear the spine on a cactus.

Spines
A spine is termed as a structure developed from a leaf. All spines in the Cactaceae are located on an areole. Spines are identified as two groups: the central spines and the radial spines. The central spines are usually located on the center of the areole; the radial spines are located around the margin of the areole. The adaptation of spines are used for several purposes. The main purpose of spines is to protect the plant from predators seeking food or water from the cactus. Another purpose of the spine is when atmospheric water (mists) falls onto the spine, the structural shape of the spine concentrates the atmospheric water into water droplets, thus falling onto the soil and the water is absorbed by the shallow root system of cactus.

Flowers
A cactus flower is very similar to other flowering plants. Its colorful blooms and scents attract pollinators. The flowers that are located in arid regions that will receive prolong periods of drought are design to open as soon as it rains, thus leaving its dormant stage to flower. When they are pollinated, some can produce seeds in less than one month. Most desert flowers bloom during the day and produce colorful, unscented blooms that attract mainly flying insects. Many jungle species have large blooms that open at dusk; these are pale and richly perfumed so that pollinating moths can easily locate them. By contrast, some flowers are nighttime-flowering that have foul smelling, fleshy blooms that attract bats.

Root Systems
Cacti have an extremely shallow and wide root system, enabling them to absorb water quickly, especially during dry periods when water will evaporate quickly. For example, Cannon (1911) found the roots of a large barrel cactus (Ferocactus wislizenii) to occur at an average depth of 3 cm. Even during long periods of drought, distal portions of the roots remain receptive to some water and rapid growth of the root hairs make much more available. The fallen joints of chollas, prickly pears, fragments or uprooted stems of cacti can root rapidly. In certain genera this may result in asexual reproduction, thus during periods of unfavorable conditions, branches or even fruits of some species may root and grow.

Growth
The growth of cacti is extremely similar to that of other plants. An apical meristem continues to produce new cells, which slowly mature into stem tissues. Beyond the maturity of the plant, no further cell divisions occur in the soft parenchyma and the fleshy parts of the cactus disintegrate after the limited lifespan. Generally, each cell type has a definite lifespan: leaf cells - 6 months to 1 year, stem cells - 1 to several years and parenchyma cells in wood - 5 to 10 years. In most Cerae, stem growth is monpodial, i.e. the apical growing point continues cell divisions. If this apical meristem is injured, a lateral bud takes over its function, producing, for example, the candelabrumlike growth form of the saguaro. In Rhipsalis and Opuntia, typically growth is sympodial, one joint being developed from another, the axis not necessarily continuing in a straight line. This pattern is followed even in the fruiting of Opuntia fulgida, in which a new flower may be produced from an areole of a fruit of the preceding year. Continuation of this process year after year results in zigzag branched chains of fruits of as many as 23 age groups hanging down from the older joints. Since this species never or rarely reproduces through seeds, ordinarily it makes little or no difference whether or not the seeds are dispersed.

Because of the special metabolism of cacti, carbohydrates are synthesized to the fullest extent after cool nights (when evaporation is low), therefore cacti grow the most when cool nights are followed by warm days. This adaptation clearly displays that cacti adapted in growth response, as well as succulent metabolism.

Water Relations
Almost the entire stem of a cactus is parenchymatous water storage tissue, thus 80-90 percent of a cactus is water. A cactus plant will lose less then one-thousandth as much water as a mesophytic plant of the same weight. Cacti of arid regions can withstand considerable water loss. For example, a mesophytic plant would wilt or possibly even die when it loses 10-20 percent of its water. Cacti can withstand a water loss of 60 percent. Cacti such as the saguaro have an adaptation in which plant tissues around any wound are sealed off by a thick layer of cork.