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Provides information on the names, characteristics, cultivation, and geographic distribution of more than 300 species of cacti and succulent plants.
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It has often been said that there can be no half measures in regard to cacti: people either love them or hate them. This assertion would appear to be borne out by the fact that on the one hand, many people find them unattractive, or indeed repulsive, because of their prickles, while on the other hand, there are innumerable associations of cactus lovers throughout the world -- for example, in New South Wales, Vienna, Zurich, Prague, Tokyo, Moscow and, naturally, Mexico City and El Centro, California.
The fascination that people feel for cacti is as multifaceted as the plants themselves and as mysterious as their origins, and it is a fascination that deepens as one's knowledge of the plants increases. Comprising over 2,000 species, Cactaceae is the largest of the many plant families -- such as the euphorbias, crassulas, agaves, mesembryanthemums, milkweeds and lilies -- that are known as succulents because they store water in their leaves, stems and roots. (In other words, to clarify a point that puzzles many people, all cacti are succulents, but not all succulents are cacti.)
The origins of the Cactaceae are thought to be very ancient in terms of the development of plant forms, although few succulent plants survive in fossilized form to tell us their history. The system of classification adopted by botanists for Cactaceae is based not only on the affinities between different genera but also on their presumed chronological development. It divides the cacti into four categories, as follows:
1. Plants that still produce leaves in spite of the special characteristics of the cactus family
2. Plants that produce leaves, but in most cases lose them very soon
3. Plants with rudimentary leaves similar to scales, or with no leaves at all
4. Plants with stems resembling leaves (cladodes), epiphytes or semi-epiphytes
This is a rational sequence based on the premise that succulent plants of families other than the Cactaceae lose their leaves because they have to adapt to dry climates, but it does not mean that cacti developed in precisely this way. Indeed they may well have been able to adapt contemporaneously to diverse climates. How and why the genus Rhipsalis, which belongs to a family native to the Americas, came to grow wild in equatorial Africa, Madagascar, the Mascarene Islands and Ceylon is a mystery, although it is likely that the plant was introduced into these areas by birds which carried its sticky seeds to the Old World.
All members of the Cactaceae are xerophytes in the broadest sense of the word: they are adapted for growth under dry conditions. In particular, they are designed to reduce moisture loss to a minimum, and they are capable of storing water in their tissues.
In addition, all members of the family -- no matter what their shape -- have a characteristic that distinguishes them from all other plant families: they produce areoles. These round to oval structures ranging from 1/16 in. (1 1/2 mm.) to more than 1/2 in. (13 mm.) across are found in widely varying positions on the cacti. They are composed of two perpendicular buds. From the upper bud come either the flowers and subsequent fruits or the new branches, which consist of segments -- often called "joints," like the upper and lower joints of a chicken leg -- that are knotted tightly together at the base. From the lower bud come the spines. These may resemble a cluster of small, wicked daggers (sometimes with barbed ends); or the dagger-like spines may be surrounded by tiny bristles or prickles and/or curly wool or hair. One of the peculiarities of the cacti is that the spines -- like the thorns on roses -- are not connected to the tissues below them; consequently no real harm is done to a plant when a spine is torn off. By contrast, when a spine is removed from a succulent such as Euphorbia, the tissue beneath it is damaged.
Cacti have numerous other distinctive features, but none of them is common to the entire family. They are simply features that enable us to distinguish one cactus genus from another. For example, roots may be quite superficial but very extensive, or they may be taproots, in some cases swollen and shaped like carrots, serving as a reserve for water and nutrients. Large taproots are particularly characteristic of small cacti growing in extremely dry areas. In some species the taproot resembles a dahlia's "foot," being subdivided so that some parts may continue to function even if others dry up or are damaged.
The stems of cacti with persistent leaves are woody, since the leaves are the mechanism through which the plants transpire (give off moisture). By contrast, in cacti without leaves, transpiration as well as photosynthesis is a function of the stems, and these are especially adapted to the job in several ways. For one thing, they are generally compact cylinders, semicylinders or globes; therefore the surface area from which moisture can be transpired is reduced. In the second place, the stem is covered with a thick, waxy skin through which moisture cannot pass. Even the stomata are designed to slow moisture loss. Finally, transpiration is limited by the spines, bristles and small hairs, which, if fairly dense, insulate the epidermis and thus protect it from excessive cold or overexposure to ultraviolet rays. This protection mechanism is especially marked on the upper part of the stem or at its apex, where the tissues are more delicate and the flowers usually grow. This area is spectacular in the columnar cacti with heads of extremely dense bristles and hairs known as the cephalium. A particularly showy example is the Melocactus. It develops a special structure that appears to be superimposed upon the apex of the stem and is thickly covered with colored hairs and bristles from among which the flower buds appear.
The characteristic roundness of stems and joints, no matter what their length, ensures that no one part is at the mercy of the sun's rays for more than a short time in the course of a day, and that only a minimal part is permanently exposed to the north. In a great many species the base of the stem appears to lignify with the passage of time, but it does not become wood. The spongy tissues harden, but the water vessels continue to pass through them. The vessels are protected by an outer layer properly described as suberose, since it is far more like cork than true bark.
Sometimes stem tissues grow together to form an irregular shape; and joints may produce twin forms along the whole or most of their length, occasionally becoming distorted, opening out like fans, or bending over as they grow. This phenomenon, known as fasciation, results from various physical or bacteriological causes. It is not hereditary; affected plants or their offspring may indeed regress. It occurs in other succulents and gives rise to so-called "monstrosities" that are highly prized by cactus collectors. Monstrous shapes are generally grafted onto other plants, since -- except for unusually strong plants such as Cereus -- they are supported only precariously by their roots and may revert to normal.
Leaves are persistent only in the first category of cacti, consisting solely of the genus Pereskia, which is considered a transitional form between normal plants and xerophytes. The stem bears normal, often very prickly, areoles. The lower part produces leaves, petiolated to a greater or lesser degree, and new shoots spring from their axils.
The second category, composing the tribe Opuntieae, includes one genus with leaves that are more or less persistent. The leaves of the other genera are usually small, fall very soon, and perform no functions, the latter having been taken over by the stem. In all the genera of this tribe the areoles have groups of minute barbed bristles (glochids) that can be very troublesome and painful because they become detached at the slightest touch and penetrate the skin. Glochids are not found in any other members of the Cactaceae.
The cacti of the other two categories have rudimentary leaves reduced to often minute scales. Alternatively, leaves are absent altogether, in which case there are enlarged leaf bases that are fused together in various arrangements to form what are known as ribs or tubercles. The areoles grow on the ribs, often at the apex, though in some genera -- Mammillaria in particular -- both flowers and joints spring from an axillary areole at the base (or axil) of the tubercle, while the areole at the apex has no vegetative function. In other genera, the two areoles are still connected: although they appear to be separate, the one is in fact an extension of the other, connected by a very fine groove. When this is the case, new joints may also appear at the apex of the tubercle.
Cactus flowers are generally solitary, and there is no clear distinction between calyx and corolla in the perianth: there is a gradual transition from sepaloid to petaloid segments that are spiral-shaped and are often fused at the base or united to form a tube of varying shape and length. The segments (petals) may be oval, lanceolate, obtuse, acuminate, dentate or even laciniate; they may be white, yellow, red or violet, while the external sepals may be greenish or brownish. These flowers are mostly regular, and the perianth is inserted above the ovary, which is generally round or oval (in some species it becomes elongated at maturity) and often bears areoles, scales, spines or hairs. The stamens, which are always numerous, have long filaments. The pistil may be even longer, and the stigma is often stellate and sometimes colored. Some genera (e.g., Opuntia) have sensitive stamens: when touched by an insect or by a finger, they close over the top of the pistil, straightening up again a minute or two later. This experiment can generally be made only in bright sunlight when the flower is open, since the flowers of most day-flowering genera remain closed whenever sun is not shining directly upon them -- they close even when the sky clouds over, reopening when the clouds have passed by. The fruit of almost all cacti consists of a berry containing several or a great many seeds. In Opuntia, fruits are fairly large, but they are small and often tiny in other genera. The fruit of some genera becomes elongated and remains umbilicate, with a slight depression at its apex at the point where the perianth joined the ovary. In other cases this point was so small that all that remains of it is a small hole to which the residue of the dried corolla clings for a long time. Fruits are indehiscent in many cases. When they are more or less dehiscent, the seeds fall prey to ants, which to a certain extent help to disseminate them.
NATURAL HABITATS AND DISTRIBUTION OF THE CACTUS
The greatest problem presented by the cultivation of any type of plant is the need for raising it under conditions similar to those of its native habitat. In point of fact, most plants have an almost incredible capacity for adaptation, but this flexibility inevitably brings about a change for the worse in their structure and appearance. This is true in the case of all succulents, including cacti. An increase or decrease in light, the wrong amount of moisture, or a dormant period that is too short or too long will, at best, result in feeble growth, lack of color, loss of the defense systems peculiar to every species, and an absence of flowers. At worst, the root system will be damaged, the tissues will rot, and the plant will eventually die.
A large number of disappointments and the death of many plants are due more to ignorance about geography or climate than to a lack of horticultural knowledge. For this reason, the descriptions of cacti given in this book include brief notes on their natural habitats. However, since it is impossible to describe the peculiarities of each plant's habitat in minute detail, and since cacti are very adaptable, we give a brief overview of their environmental distribution below. It is hoped that this will be a useful adjunct to the details given in each entry.
With one exception of no practical importance, all members of the Cactaceae family are native to the continents of North and South America. In this vast area there is a huge variety of habitats, from the tropical to the polar. Even the physical geography of the mountain chains differs markedly. Winds and ocean currents bring violent hurricanes to the essentially dry, mild climate of North America. Freezing air rolls off the high peaks of the Andes into the tropical forests of South America.
One of the chief adaptational achievements of the cactus is its tolerance to periods of drought. This tolerance is only relative, however, since it invariably depends upon the amount of water held in the substratum and, above all, on how long it takes for this to become stagnant. Strange as it may seem, neither the floods that occasionally devastate Texas and Colorado nor the hurricanes that strike Florida cause as much damage to cacti as one might imagine. This is mainly because cacti tend to grow on well-drained high ground, and also because such violent storms occur infrequently. Conversely, the dryness of the air in Arizona would in quite a short time prove fatal to Epiphyllum cacti, which grow in warm, damp, wooded areas.
On a purely practical, rather than botanical, basis, let us subdivide the members of the Cactaceae into four broad groups, according to habitat. The variants that occur in each group are ignored.
1. Plants from a desert or near-desert habitat
2. Plants from a mountainous habitat
3. Plants from steppes and grasslands
4. Plants from tropical or subtropical forests
These categories are not always applicable. For example, the many species and varieties of the genus Opuntia are so widely distributed as to make it appear ubiquitous. The flat-branched Opuntia polyacantha grows in the Canadian provinces of Alberta and British Columbia at a latitude of more than 50°N, while Opuntia australis, which has more or less oval or globular joints, and many other similar species grow in Patagonia (in order to survive there, all are of low and prostrate habit). Other species of Opuntia are to be found along the coasts of Florida and the Carolinas, in the Antilles, in the Galapagos Islands, in all the desert regions and in the Andes. Thus this genus, which is the largest of the Cactaceae, is able, through a variety of forms, to live in three out of four of the above habitats.
Desert or near-desert regions in which periods of complete aridity often alternate with torrential rain are found largely in the southwestern United States (although they extend southward into Mexico). This huge expanse of country reaches from Montana and Utah in the north to the Mexican border and from California in the west to beyond the Rocky Mountains, across the whole of the Texas plateau. Climatic conditions vary widely. The area of true desert is relatively small: it lies mainly to the west of the mountains, stretching eastward beyond the Great Salt Lake, along the Colorado River and the Rio Grande. Desert cacti are found in great abund...Language Notes:
Text: English, Italian (translation)
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