& OTHER SUCCULENTS
Mammillarias occur naturally in the southwestern U.S. and in Mexico, with a few representations further south and in the West Indies. It is one of the largest of the genera of the Cactaceae, having the greatest number of species next to Opuntia. Species number now exceeds 200 and is increasing as new explorations reveal new species. The genus Mammillaria was established in 1812 (Cullman, 1987). The name was derived from the Latin word mamilla, meaning teat or nipple, an allusion to the form of the warts or tubercles on the plant members.
Many Mammillarias are fairly small cacti, mostly clustering or branching, low, globular or cylindrical, sometimes much elongated. Not many species grow to more than 12 cm wide or more than 20 cm tall (Cullman, 1987). Mammillarias have no ribs and the body is completely covered by tubercles. The tubercles form pairs of spiraling rows which cross over each other in an alternate, mathematical, intersecting spiral relationship called a "Fibonacci series", each series having the number of spirals which is the sum of the two preceding Fibonacci number s e.g. 1, 2, 3, 5, 8, 13, 21 etc. In a proportion of species, the central spine is curved into a very conspicuous hook at the end. These hooks play a part in the vegetative dissemination of the species. For example, a passing animal's pelt is caught by the hooks, the shallow-rooted plat is uprooted, transported elsewhere and dropped to take root and continue the spec ies there. They are also useful as fishhooks.
An important feature of the Mammillarias is the position of the flowers. The flowers do not grow from the areoles at the tip of the tubercles as they do in other other cacti, but rather from the axils between the tubercles. The flowers, though often small, are very attractive and are usually arranged in a ring below the crown. The fruits are smooth, juicy club-shaped berries which protrude beyond the tubercles and are often more distinctive than the flowers. They are the very popular, tasty "chilitos".
Members of the genus Opuntia are generally regarded as being amongst the more primitive genera of the Cactaceae. With more than 300 species, Opuntias have a distribution that extends from the south of Canada through both of the American subcontinents. Opuntias successfully inhabit many other parts of the world, including Australia and South Africa where, through introduction for use as garden ornamentals and as forage in the early twentieth-century, certain Opuntia species flourished, naturalized and began to take over the land.
Plants can vary in form from shrubs or trees to creeping plants 5 cm to 8 m high that spread over as much as 40 m on the ground (Benson, 1982). Characteristically, opuntias possess jointed stems; cylindrical or conic leaves on young stems; spines that are smooth, variable in color, straight or curved, sometimes none; rotate flowers with stamens sensitive to touch; fruits with thick rinds and seeds comparatively large, rounded in one plane and flattened in the other. One of the most obvious distinguishing characteristics however, is the presence of glochids. The glochids are very slender, firm, tiny barbed bristles able to spear whatever soft tissue they may pierce with their sharp points. In contrast to other spines, when the plant is mature the glochids become so loosely held that they come off the areole at the slightest touch.
Economically, Opuntias are the most significant of all cacti. The flat-stemmed Opuntias, known almost universally as the Prickly Pears are the main food-producing cacti. The stems are fed to cattle and the young pads and fruits are widely cultivated as crops in Latin America, Mediterranea, the Middle East and elsewhere and are sold for human consumption. The use and success of certain species of Opuntia as a cash crop was one reason for these plants being introduced into some other parts of the world, where they have since become noxious weeds. In Australia, in the early 19th century, a single plant of Opuntia stricta was introduced as an ornamental and as vineyard hedges. It soon became naturalized. With attempts to remove the cacti by plowing under,the plant's ability to propagate vegetatively was stimulated and the number of cacti only increased as the chopped up pieces readily produced more roots.
Famed Horticulturalist Luther Burbank (1849-1926) bred a new strain of spineless Opuntia among hundreds of other new species. In 1914 the hybridized spineless prickly pear cacti were shipped to Australia for forage. The plants flourished and produced viable seed. However, the seed produced both the spiny and the spineless plants. Cattle and sheep would not eat the spiny plants, and by 1925 the prickly pear cacti in eastern Australia were infesting new range land at the rate of 100 hectares per hour.
The Australian government took action by searching for biological control agents. The larvae of the moth Cactoblastis cactorum, became the major control agent, and by 1933 moth predation eradicated almost 90% of the prickly pear plants. In addition to the use of biological control agents, numerous hunting parties were sent out to shoot crows, emus and scrub magpies so as to prevent the dissemination of seeds through the movement of such birds. Overall, 270,000 emus were killed (Nobel, 1994).
Platyopuntia's competitive ability and high capacity for biomass production have allowed the cacti to outcompete the local flora. The presence of certain Opuntias is a problem that still persists in Australia today.
The Lobivia species are native to the South American Andean uplands of Argentina and Peru. They inhabit the high mountain systems of the Andes and Cordilleras, and therefore are mostly adapted to withstand the unbroken rays of the sun, drought, heat and extremes of temperature. They are reasonably tolerant to variations in cultural conditions because of the arduous conditions they are often subject to in their native habitat. As they are endemic to mountainous regions, they need bright light and appreciate marked temperature differences between day and night and summer and winter.
Britton and Rose introduced the generic name Lobivia as an anagram of the name Bolivia, to indicate the main center of distribution of this genus. Geographically, the species of Lobiviaall inhabit a well-defined niche, occurring in places with roughly identical ecological conditions, stony, grass and scrub covered eastern and central valleys and slopes, in places with irregular and low rainfall. The individual species can be termed cline species as they often merge with each other (Riha, 1981).
There are about 80 species of Lobivia occurring at higher altitudes. Species of Lobivia differ widely in their appearance, spine formation, and in their flowers. However most species are often small spherical cacti, often forming clumps, with bodies that are tough and spines that are often strongly developed. The flowers are red, yellow or white, and usually remain open for just a single day. The colored flowers mostly open by day, often before sunrise, while the white flowers usually open in the evening. The flowers are characteristically scaly and woolly, with a prominent ring of stamens in the throat (Riha, 1981).
The genera Echinopsis, Lobivia, Trichocereus, Helianthocereus, Pseudolobivia and Soehrensiaare all very similar and no doubt closely related. Five characteristic features are commonly used as generic criteria to distinguish between the genera, and the number of species that are differentiated depends on which of these features is taken to be critical and given first priority. Thus, the genera defined can often include very different groups of plants which can result in a wide variety of names in use for these plants. L. arachnacantha, also named E. arachnacantha, is one such example, where a species exhibits few of the major distinguishing features and is therefore difficult to place in the former genus or subgenus (Cullman, 1987).
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