Patterns of distribution

To portray such a range of concentrations would require an enormous map and many fine shades of colouring, and to the best of my knowledge nobody has yet attempted one. If there were such a map. the heaviest shading would come around the horse latitudes. 30°N. and 30°S. of the Equator, and it is certain that South Africa and subtropical North and South America would score highest for numbers of speciesas well as for density of populations. America is the home of the cactus Family (Cactaceae) and the century plants (Agave), various Crassulaceae (Echeverioideae), some Portulacaceae (Lewisia. Talinum. etc). Euphorbiaceae and others. South Africa, the home of the widest range of all, lacks only indigenous cacti among the major Families, although even these are to be seen in the form of semi-naturalized prickly pears, originally planted near farms as emergency cattle fodder, and the widespread Rhipsalis.

Outside these heavily shaded areas of our map. much of the world's land surface, except the colder areas toward the poles, would have paler shadings. Crassulaceae occur wild in all five continents, notably as the tiny, moss-like annual crassulas (Tillaea), of minimal interest to succulent collectors but the most widespread of all. The native European succulents — Sedum (3.1) and Sempervivum mainly —have a special interest because they are almost the only succulents known to classical writers and hence featured in ancient literature.

Within the tropics, fewer succulent species occur, although even in the rain forests a few rather less fleshy types are to be found. Most conspicuous are climbers such as Hylocereus, Selenicereus and Epiphyllum in the Western Hemisphere, Cissus quadrungularis in Africa and Hoya in Southeast Asia, and epiphytes such as Aporocactus. Rhipsalis and Schlumbergera in Mexico and Dischidia in Southeast Asia. The epiphytes live as do the orchids and bromeliads, perched on the branches of trees or sometimes supported on rocks, but not as parasites: they derive their nutriment from humus transported by wind or rain. Their inbuilt stores of water profit them in times of drought. The tropics are also the home of certain relicts that seem to have been left behind in the advance into drier, more exposed habitats. They remain as "living fossils" that suggest an evolutionary link with the past: Pereskia (16.4) in Central America, for instance, or the leafy Caralluma frerei (19.5) in India. Some largeand adaptable genera such as the euphorbias, aloes and kalanchoes have many representatives within the tropics.

Recently much interest has been shown in types of succulents occurring in unlikely and isolated places such as oceanic islands. These are often curious caudiciforms with massively thickened (pachycaul) stems: Impaliens in Sumatra. Brighamia in Hawaii. Jatropha in Panama and Pelargonium in St Helena.

Man the distributor So far we have been considering indigenous succulents—that is, those growing wild before the arrival of man. The natural distribution patterns of succulents have been greatly upset by man's activities in transporting them from continent to continent. Rhipsalis, the wandering 'Mistletoe Cactus', has already been mentioned: it is found today in Africa (4.18). Madagascar and Sri Lanka, and is the only cactus genus apparently wild outside the New World, but it could well Right (3.1); Native European succulents: pennywort. Umbilicus rupestris. and stonecrop. Sedum reflexum growing on a I sfone wall in South-west England

Below(3.2): This seemingly desolate wilderness adjoining the Hoi River in South . Africa is one olthe richest hunting grounds lor discoveries often come to light.

have been an early introduction by man. The few succulents credited with medicinal or other uses have been carried around for so long that their original habitats—if indeed they had one. and are not ancient hybrids—are no longer known to us. Aloe barbadensis, the medicinal aloe, is one such (page 18). and purslane (Portulaca oleracea). long in use as a potherb, another.

The great adaptability of some succulents led to their becoming naturalized far from their native homes. Although it has few native succulents. Australia has the right climate for them, as was shown by the dramatic spread of certain species of Opunlia (16.7), introduced as ornamentals from the eighteenth century onwards. Over 1,500.000km2(60.000,000 acres) of agricultural land were overrun before control measures began to take effect. The dreaded prickly pear proved resistant to normal methods of cutting or spraying, and success at curbing its spread was achieved only by biological control by the moth Cactoblastis cac-lorum. whose larvae feed exclusively on the stems of Opuntia. Even today the danger of spread is not entirely past, as other species of cacti are unaffected by the predator.

Succulents in habitat My first encounter with cacti in their native habitat shattered some preconceived ideas about them gained from years of tending potted specimens in a London glasshouse. It took place on the slopes of the Rocky Mountains near Denver, Colorado, to which I had been led by a friend with the enthusiasm of so many American cactophiles, who love to show off their unique flora. However, the site seemed so unpromising that I wondered who would ever go cactus hunting in this sort of terrain? It looked more like a rough English meadow than the anticipated desert of Western movies. But.

sure enough, many cacti were there nestling in the grass, and so hidden that one almost tripped over them before seeing them. So two illusions were dispelled at once: succulents do not grow alone in solitary possession of the desert, and the smaller ones (those dearest to the heart of the connoisseur), although so conspicuous in a pot, are often very well camouflaged in nature.

These facts are true not only of the American cacti but of many South African Stapelieae, haworthiasand other genera. Further surprises were in store. The most frequently encountered soil type turned out to be a form of weathered granite, quite unlike anything a conscientious gardener weaned on commercial potting mixes would consider fit to bring inside his glasshouse. Although the soil appears to be short of organic matter, analysis shows that there is usually no lack of the elements essential for growth. Water, which makes the nutrient salts available for absorption, is the limiting factor, and its sparsity slows down the decay of humus. It would be a mistake to collect soil along with the plants and assume that it would be the ideal medium for their cultivation. Succulents grow where they do often because competition excludes them from other habitats, not because conditions are ideal there.

Natural variation. But the most far-reaching of all revelations on studying succulents in their natural environment is their variability (5.1). In cultivation we see little of this: every plant of one species in collections may even be the same

Right (3.3}: Aloe dichotoma on level ground in Soulh Africa lorming an unusual type ol lorest. The plants reach 9 m (30ft) in height and their wood is light and librous.

Below (3.4) Aloe macrocarpa growing in Northern Ghana. The roots lind anchorage and sustenance in the rock crevices

individual derived by repeated vegetative propagation from a single import. This gives us a stereotyped image of what that species looks like, so that when we see a slightly different fresh import we jump to the conclusion that it is a new species. Field collectors further distort the picture by tending to go for anything that looks different, sampling the extremes of the population rather than average specimens. Thus the collector can get a narrow and totally false picture of the species. A species is not one individual, whether living or preserved on a herbarium sheet. It is the sum of all its populations, and may be as full of minor variations as is the human species. Homo sapiens.

A succulent paradise. In a few places in South Africa, almost the only plants to be seen are succulents (3.2), from the tiniest of annuals to tree aloes and euphorbias that may dominate the landscape (3.3). I have wandered bemused in parts of the South African Richtersveld and Little Karroo, which together form the ultimate paradises of the succulentaddict, trying to assimilate all the species as one after another comes into view, some familiar to the point of being taken for granted, others excitingly new. Instinctively my hand lifts the branches of shrubs as I walk by. Hiding under them I have found up to ten different species of dwarf succulents, representative of several unrelated plant families. Photographs, regrettably, do scant justice to such floral surfeit. Time stands still as you attempt to overcome human limitations and take it all in. To anyone reared on the relative poverty of the flora of Great Britain the wealth of such areas is unbelievable.

Semi-deserts. That "cacti grow in deserts" is a favourite piece of dogma. The word "cacti" is misused to cover anything from a prickly pear to a carrion flower, and "desert" also needs careful defining. For this we turn to the ecologists, who arbitrarily define a desert as an area that averages less than 25cm (lOin) of rain a year. Now the life forms of succulents, with their closely guarded internal store of water, are ideal for conferring upon them a camel-like disregard for periodic droughts. But to store water they must receive water in the first place, and periodic drought is not the same as perpetual drought. The succulents most popular with collectors flourish in areas where the annual precipitation may be quite a bit more than for true desert, as defined above. Indeed, some get as much as 50 to 80cm (20 to 32in). Hence it is more precise to speak of these as semi-desert plants.

Mountain slopes, screes and rough rocky places generally are favourite locales for succulents. Inclines and precipices are especially favoured by

Above(3.5): Chaparral-type vegetation in the Mojave Desert California made up o/Oountia with yuccas and smaller xerophytes Note the wide spacing and absence of ground cover.

Fog zone vegetation rosette types such as Aeonium and Aloe (3.4), which if planted with the rosette horizontal in a pot will often grow over sideways to face the light. The mountaineering skill of the would-be collector, nil in my case, is tested to the limits. More obliging species inhabit level ground, particularly those hot, dry plains and mountain plateaux, sparsely dotted with thorny shrubs, that are known as chaparral in Mexico and the American Southwest (3.5) and veld[t] in southern Africa. They utilize every bit of shade,

Below (3.6): Fog bell vegetation in Northern Chile In the absence of rainfall, plants can survive if they make use of atmospheric condensation as fog rolls In from the sea

Elevation(m)

and species that grow out in full sunlight, such as Ariocarpus and Lithops, are commonly half buried in the soil. The immensity of such areas makes one realize how much there is yet to be explored. It is not surprising that new species are coming to light every year.

It must be remembered that, unlike the aerial parts of the plant, the roots of succulents have little protection against water loss, and if roots dry out beyond the threshold of survival, plant growth is checked until new roots form. Hence

Elevation(m)

there is a tendency for roots to insinuate into clefts in rocks where water collects, making the plant impossible to remove without damage. Anyone who has tried to dig up a caudex-forming Pachypodium from its bed of compacted rock chippings will realize the enormous pressure exerted by the turnip-like expanding root, which sometimes reaches one metre (39in) in length. In cultivation such plants not infrequently burst their pots. Seedling succulents require a measure of shade, and it is probable that all species in the wild begin life under the protection of rocks, other shrubs or the parent plant.

Incidentally, there are other sorts of desert than those typified by the Sahara and Gobi. The icy. barren wastes toward the Poles are also deserts, because water is withheld from plants by being frozen. No' succulents have ventured into these areas. Another kind of desert exists on the western slopes of the Andes, in Chile and Peru, where precipitation is negligible but heavy coastal fogs provide just enough moisture to foster the growth of xerophytes' (3.6). Under these conditions grow Oroya (16.17), Borzicactus and other dwarf globular cacti. Collectors cherish such plants because they are easy to adapt to normal glasshouse culture away from their habitat—another instance of how needless it is to attempt to duplicate the habitat in glasshouses, even if it were practicable to do so. A very similar environment to that of the Andes is found at lesser altitudes along the Atlantic coast of southern Africa in the Namib Desert, where a unique succulent flora has built up in the narrow coastal belt within reach of dew-laden onshore winds.

Temperate habitats. Although succulents are a less conspicuous feature of the flora outside semi-desert areas, some compete well with non-succulents and their inbuilt store of water allows them to colonize specialized habitats. Thus in the mountains of Europe and Asia rosette succulents such as Sempervivum and Rosularia are at home in alpine regions alongside saxifrages and other xero-phytic non-succulents. The high evaoora-tion rate during alpine winters when the soil is frozen needs some form of water conservation. Lewisias occupy similar niches in western North America. A few succulents even compete in moister habitats: the annual Sedum villosum inhabits fresh-water marshes, and at least one Crassula can grow wholly submerged in water, although it flowers only when the branches break the surface. A native of New Zealand, C. helmsii has become a popular, if invasive, aerating plant for pools and aquaria. Indeed, there are few environments from which succulents are completely absent. Ironically, instead of calling them desert plants, we should be almost nearer the truth to say that deserts are the one place where they do not grow.

The influence of climate As we have seen, the habitats of succulents are exceedingly diverse. The only features they have in common are periodic drought and temperatures generally above freezing. Even freedom from frost is not universal; cacti in the high Andes, the northern USA and southern Canada freeze annually, though protected from the lowest temperatures

Bekw Over an area as large as South Africa, climatic factors vary widely and influence our ideas on cultivation Top map (3.8) Rainfall areas of South Africa Bottom map (3.9): Expected annual rainfall in South Africa by a blanket of snow. A glance at rainfall maps of South Africa* (3.8, 9)-the centre of maximum diversity and density of succulents—reveals big differences in the season when rain is expected and in the total amount, from almost nil to more than the annual average for the British Isles. (Note that the seasons are reversed relative to the Northern Hemisphere.) If we are seeking cultural assistance from the homeland, then, it is not sufficient merely to know that a plant comes from South Africa. Closer localization is needed for us to decide how much rain it normally receives and. equally as important, at which time of the year.

South Africa—Rainfall areas

South Africa—Rainfall areas

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