CACTI AND OTHER SUCCULENTS struggle against drought

Succulents, so-called because of their juiciness (from the Latin sticcus meaning juice), are grouped together on the basis of their peculiar form, evolved in adaptation to the harsh drying conditions of their semi-desert habitat. This is an unnatural grouping as plants of succulent form, retaining stores of water within their swollen stems and leaves, are found in quite unrelated plant families. The Cactaceae, an almost exclusively American family containing many members of typically leafless succulent form, illustrate the wide variety of body shapes and blossoms peculiar to succulents; in other families some resemble stones and others weird sea animals or objets d’art. Though most cacti are succulents, there are some genera within the family, such as the genus Pereskia, which are non-succulent and have a ‘normal’ leafy habit. On the other hand, within some principally non-succulent families there is an occasional succulent member, such as the fleshy species of Senecio and Kleinia in the family Compositae and Agave in the narcissus family, showing that succulence is an opportunist mode, evolving in response to a parched environment, rather than a generic one. Other families which have some succulent members include those of the geranium, purslane, vine and milkweed. The two most important families of succulents besides the cacti are the Aizoaceae (mescmbryanthemums) and Crassulaceae, both centred on South Africa, although the latter include the hardy stonccrops and houseleeks widely spread in Europe.

Distribution

Succulents are widely but unevenly distributed. Originally most succulent plants other than cacti came from Africa, though some species are derived from southern Asia and some from America; all cacti however are American in origin, their presence elsewhere possibly resulting from the activities of man or birds. The spread of Rhipsalis in Africa and Asia is thought by some to have resulted from birds carrying its sticky fruit, while the prickly-pear cactus was brought to Australia by man to serve as range fence and emergency stock feed; before it was brought under control by means of an insect parasite it had taken over more than 24 million hectares (60 million acres). Some Opuntia species have penetrated north of the fiftieth parallel, which corresponds to the latitude of Stockholm; in South America some species grow even in the inhospitable climate of Patagonia. In Peru and Bolivia, cacti reach as far as the snow-line, 3,700-4,700111 (12,000-15,400ft), such as the mountain opuntias which bear woolly blankets of hair to protect them from the cold. Mexico is however the true home of the cactus—the state coat-ofarms bears an eagle holding a snake in its claws, perched on an Opuntia. In the valleys north of Mexico City, huge thousand-year-old globose cacti (Echinocactus ingens) and the profuse and strange white-haired Cephalocereus senilis (’old man of the desert’) give the impression of some vast purpose-built garden. Arizona has a characteristic appearance with open forests of candelabralike saguaro (Camegiea gigantea), cacti of 15m (49ft) and over in height, real cactus giants. The spherical species average im (3-3ft) in diameter and 2m (6-6ft) in height; these can weigh more than a ton! Some of the rarest cacti occur in Mexico and are miniatures of only a few centimetres in diameter; other rare species are still being uncovered in areas explored only recently such as Baja California.

Adaptation to drought conditions

Cacti and other succulents are true xcrophytes, adapted as they are to endure drought by having a very slow water loss in transpiration (the rate of transpiration of an Opuntia, a typical succulent, was found to be only onethirtieth of that from an equal area of a thin leaf), the result of a number of specialized adaptations. Within their swollen tissues these plants contain large quantities of mucilages, slimy substances able to retain large amounts of water by imbibition; by acting essentially as little reservoirs of water, these odd plants ! ave become able to obtain their essential nutrients even when there is a water shortage. This enables them to survive long periods of drought even though they appear dead. Cacti of the arid North Chilean desert, lying freely in the sand, can preserve a spark of life even if no rain falls for several years; and a laboratory-kept giant cactus, never watered, was found after six years to have lost only onethird of the moisture it originally contained. Succulents, then, are masters in the economy of water, enabling survival on the very threshold of death.

Evolution of succulents

It is thought that all succulent plants evolved from other related plants growing in a normal environment by their gradual adaptation to

Right: Bizarre prickly pears (Opuntia) are very much at home in the extremely arid conditions at low levels on the Galapagos Islands, with volcanic ash to root in. the changing conditions of their habitat, in particular to the amount and regularity of rainfall, and forms which took to succulence became the forerunners of the present-day extreme forms. The cactus line evolved most probably from ancestors originating in the vast tropical forests in response to the changing climatic conditions of the areas in which they were established, or to which they had spread. In the cactus family, this adaptation to dry environments took the form of losing leaves and developing round or cylindrical bodies. They did this in order to reduce the loss of water by evaporation from the large leaf surfacearea.

At the same time the vital processes of assimilation and transpiration, normally enacted by the leaves, were gradually taken over by the green stem, the thin tissues of which came to store as much water as possible for periods of drought. (In many species, the stem tissues are about 90 percent water.) ‘Fat finger’ shapes were adopted as the best stem shapes for storing the maximum amount of water with the minimum surface area exposed, to cut down on evaporation.

To further this latter end, the stem skin became much thickened and wax-covered in many species, with the stomatal pores very widely-spaced and depressed in their positions in the skin. Often the cactus skin is ribbed and knobbly as well. This helps the plant to shrink in dry periods; by so doing, it can withstand the loss of as much as onethird of its total weight.

The root system also shows adaptations to desert conditions; many cacti have far-reaching fibrous or tuberous roots, located specially near the soil surface, to take up available water, in the form of dew as well as rain, over as wide a range of soil as possible. Since the rainy season in semi-desert regions usually consists of only a few downpours, the plants need to absorb as much moisture as they can during that period.

In humus-rich soils, which have better water-retaining properties, cactus roots form fine dense networks to exploit this condition. Quite a lot of cacti possess roots rather like turnips, often larger than the stem, which serve as underground water stores. Column-shaped cacti have thick round perpendicular roots in order to provide anchorage in soils which tend to be fine and unstable.

This evolution from ‘normal’ leafy plants into the forms of cacti so easily identifiable today happened very gradually. Sometimes the various stages of development may be observed in a brief show during the sprout and seedling stages of some cacti. In the very primitive cacti genera, the seedling produces fully-developed leaves like ordinary plants, while in more advanced genera, these appear only as traces in the first few days following germination, later to vanish almost completely. It is likely that the bushy Pereskia species which bear normal leaves, growing and flowering much like the wild rose, are very close to the original predecessors of the succulent cacti. Today’s leafless cacti can no longer produce leaves even if the climatic conditions are changed to provide an ample supply of water, so that when epiphytic cacti occurred in damp forests, their stems merely became flatter and broader until they resembled leaves, thus adapting themselves to the new conditions by increasing the surface area available for evaporation.

However, many non-cactaceous succulents have retained their leaves and here whole series of adaptations can be traced, from ‘normal’ to extremely fleshy leaves within a family, the most noteworthy example being the Aizoaceae. This family is a living textbook of evolution, the genera exhibiting gradations of form according to the amount of specialization imposed by the habitat. At one end of the scale are more or less woody plants, with well-spaced leaves arranged in opposite pairs; the fleshy leaves are the main indication of adaptation. Then there is a group with trailing stems but leaves more fleshy and more closely packed on the stems. These merge into the really fleshy forms, where stems are almost or entirely absent, with fewer and fewer pairs of leaves. The next stage comes when these leaves are reduced to a single pair, which may be partly joined or, in the most extreme cases, are converted into a single rounded mass or ‘plant-body’; the division between the pair is sometimes indicated by a groove or slit, or is sometimes not apparent.

It is remarkable that similar environmental pressures (long hot, dry periods typical of semi-desert or prairie climates) in quite separate parts of the world, caused all the different unrelated succulents-inthe-making to evolve along very similar lines, to adopt various expressions of the same strange physique. A good example is the amazing similarity, considering their such separate origins, between American cacti and African euphorbias. There are also similarities between certain cacti and agaves; for instance in some species of the cereus group, which have stems like those of agaves, the similarity is expressed in the name of the plant, as with Ariocarpus agapoides. This is a true case of parallel evolution.

Protective spines

Apart from succulence and weird stem shapes, cacti are also noteworthy in appearance for their spines which serve as protection against desert-roaming creatures which might otherwise find a welcome source of food and drink in a juicy cactus. Spines vary greatly. They may be strong spikes of yellow, red, black or brown, iocm (4m) or more long, or long and silvery hairy outgrowths, as in Cephalocereus senilis. Some Ferocactus species have large and cruel fish-hooks such as Ferocactus latispinus, or the ‘devil’s tongue’, which has thick flat red-covered hooks up to o-5cm (o-2m) wide. Some Mammillaria are covered in delicate white ‘feathers’, while some Cereus species bear a fine ‘wool’ completely covering the stem to protect against the cold of the night and the heat of the sun, as well as a moisture-retaining net. Cactus spines are really modified leaves and not bristles which are formed from cutin, a substance produced by the plant’s skin; bristles are peculiar to the genus Opuntia, and they readily break off to cause itching of the skin.

Reproduction

However, the greatest attraction of the cacti is their glorious flowers. These brilliant blossoms, which range through all the colours of the rainbow with the exception of pure blue, can be as large as 40cm (i6in) across, among the largest in the entire plant realm, or be as tiny as those of the Melocacti. During the day-time these bright flowers attract visiting insects, sometimes even humming birds; when the day-light fails, the huge white nocturnal cactus blooms unfurl and, strangely luminescent in the dark hours, attract the night-flying pollinators, mainly moths and bats, with their strange odours. Flowers burst open at the start of the growing season, emerging from the growing points, generally as solitary blooms rather than clusters. Following pollination, the ovary turns into the fruit which, as with the flowers, can vary considerably through the species. Fruits tend to be more long-lasting ornaments than the flowers and most of them are edible as well.

Some fruits are minute, others reach the size of plums, eggs, even of oranges. In texture they may be fleshy or dry, and upon ripening they may either burst in various manners, or disintegrate in a characteristic fashion.

Within the fruit pulp lie the cactus seeds, microscopic or hailstone-sized, attached to the walls of the internal cavity of the ovary. Dispersal of the seeds depends on the nature of the fruit. Fleshy fruits are eaten by birds, the seeds passing un-harmed through the gut, whilst ants spread the seeds of ground cacti.

Most cacti can reproduce vegetatively from a number of planes on the body, though the chief centre of this activity is the crown. Opuntias can grow roots or shoots from any part of the body, even from tiny fragments of stem, and thus can multiply at an amazing rate. Should conditions be favourable, their stubborn growth can take possession over vast areas in a short space of time; hence the prickly-pear havoc in Australia. Many succulents of other families are equally fast in rooting from pieces of stem or even broken-off leaves.

Use to man

At one time, cacti served many useful and important purposes for man, but nowadays technology has largely overtaken these. In poorer parts, however, the large fruits of some Cerei, especially those of Opuntias, are regularly eaten by the native Mexicans as ‘tunas’ and ‘cactus figs’. These may be cither in a fresh state, preserved with sugar, or fermented into alcoholic drinks. Ripening as they do in the dry season, these fruits can be important when drought has caused other crops to fail. In parts of Brazil, spineless Opuntias are used for fodder, whilst in Mexico, some Cerei, notably C. marginatus, are frequently used to provide thick and impenetrable hedging.

When the giant cacti age, the lower parts and axial tissues turn woody, for these giants, which can weigh several tons, need a firm support. This ‘wood’, though very light, is tough enough to use for building houses, making household utensils, and for fuel in these otherwise barren and treeless localities.

Interesingly, a significant part in the Mexican economy was formerly played by the production of the red colouring material, cochineal, extracted from an insect bred on some host species of Opuntias. Nowadays, however, the cheap aniline dyes have largely replaced cochineal.

Many cacti have important medicinal properties. The massive-flowered Selenicercus grandi-florus, or ‘queen of the night’, for instance, yields a juice containing a glycoside valuable in the treatment of heart diseases. A well-known Mexican cactus, Lophoplwra williamsii, generally known as ‘peyotl’, contains alkaloids (mescalin, anhalonin, and pelotin) which produce colour and sound hallucinations, and at one time this cactus was the object of a widespread Indian cult.

It is not difficult to see why this strange group of plants, beautiful both in their glorious blooms and in their adaptations to their environment, have become worldwide objects of interest and cultivation. … *:

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