Origin and diversity

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How succulence originated is not yet clearly understood, although we know that it has happened independently in widely dissimilar groups of plants, as Fig. 1.6 shows. Experiments have shown lence in Tradescantia by causing the cells in the leaf to elongate vertically, and a similar enhanced fleshiness can be induced in Kalanchoe by reducing the day length by periodic shadings. Not all succulents are comparable in habitat and behaviour. We find one type of succulence in plants of the seashore and salt marsh. A familiar example of these halophytes ("salt plants") is the glasswort or marsh samphire, Salicornia (1.5), which, unlike the xerophytic succulents, has a high rate of transpiration and wilts when allowed to dry out. But the same is true of Glotti-phyllum (11.13), from the hot dry karroos of South Africa, so there are exceptions to most attempts to define a succulent.

A classification into succulents and non-succulents helps the field botanist to distinguish one element of the flora from others, and certainly aids the gardener by grouping together plants of similar cultural needs, general aesthetics and collector appeal, just as he makes parallel groupings for alpines, epiphytes and aquatics. But botanists universally adopt a classification based on all characters of the plant, not just the degree of fleshiness, because it brings related kinds together and gives the maximum retrieval of information about them. This is the basis of Fig. 1.6, and of the layout of Part 2.

Difficulties of defining a succulent make the limits of our subject somewhat elastic. Some would include the halo-phytes, and some the yuccas, which show fleshiness of a sort in their stems although many are hardy enough to withstand frost. Others would include orchids (several of which qualify on all counts) and bromeliads — members of the pineapple Family now so popular as house plants. To include such plants would broaden the scope of this book considerably, and these two large and highly individual Families are more suitable for specialist treatment by themselves.

Another Family, the Zygophyllaceae.

is omitted for a different reason. Over large areas of South Africa Zygophyllum is a common sight, its plump watery foliage resembling that of the so-called ice-plant Family, the Mesembryanthem-aceae, with which it shares company. Yet who has ever seen any of its 100 species in cultivation? The reasons for its absence are easy enough to find. Transplants fail to establish, cuttings blacken and quickly perish, and seed, if it can be made to germinate at all, produces seedlings that languish and die. Such intractable plants pose problems for the conservationist, as explained in Chapter 7.

By now it will have become apparent that all cacti are succulent, but not all succulents are cacti. The single Family Cactaceae is of distinctive appearance but is confined to the New World (although naturalized elsewhere) and is only one of three major Families of succulents, each with over 1,500 species. There are also about 33 other Families in which succulence has evolved independently in one or more species-representatives of some 600 genera in all. Faced with such diversity, we are not surprised to find that succulents range in size from tiny annual weeds to towering trees, and that they occupy a vast range of habitats. Some require more heat than others, some partial shade; some are extremely sensitive to too much water, others tolerate it; growing seasons differ according to local conditions and to whether they live north or south of the equator. Thus anyone wishing to cultivate them must remember that they cannot all be treated alike in one glasshouse.

Uses of succulents If succulents have contributed little to human welfare to compare with the major agricultural crops, the catalogue of their uses is nevertheless long and varied. In regions where they are the dominant vegetation we find, not surprisingly, that they are locally exploited for all manner of uses: food, medicine, fibre and — in the case of tree-like species — wood.

Crops and food plants. The nearest to conventional crop plants are the agaves (Agave) and the prickly pears (Opuntia). The massive tough foliage of Agave sisalana contains long fibres that are extracted by shredding and made into ropes and twine, carpets and sacking. The best fibres are creamy white and rank second only to manila hemp in strength and durability. Brazil and Africa continue as the main sources of supply, despite competition from synthetic fibres. Agave is also the source of intoxicating drinks. One is obtained by decapitating the flower stalk as it arises and collecting the sticky sap that oozes freely from the stump. This contains 12-15 percent of sugar and is fermented to produce a rough brew called "pulque". Tequila is made by distilling the hearts of certain dwarf species of Agave.

Of edible succulents, first place goes to the prickly pear. The popular name refers to the fruit, which is like a large plum or pear in appearance (1.7), full of sweet pulp and black seeds but covered with formidable spinesyA celebrated American plant breeder, Luther Burbank, set out at the start of the present century to select some for larger, less pippy and smoother fruits, and others for lack of spines so that the plant itself could be fed to cattle. Some of his large-fruited strains are grown commercially today in parts of California and Mexico and the fruits are eaten raw or made into jam, jelly and other confections6. Young stem sections stripped of their spines and diced can be eaten raw in salads or cooked and served as "nopalitos". As cattle fodder the prickly pear did not make the desert rejoice and blossom to the extent that Burbank hoped. It needed fencing to prevent the cattle from grubbing up whole plants, and proved to have a lower food value than other grazing plants for dry areas. But wild prickly pear still finds favour in times of famine, when the spines can be cut or burnt off and the juicy green stems fed to cattle.

Medicines and drugs. In the Old World the aloes have a long history of economic use parallel to that of Agave in the New World. Here the interest is in the bitter, slimy sap that oozes from cut leaves. It contains aloin, and when dried becomes the "aloes" of medicine. Taken internally it induces purging. Aloe leaves have long been credited with healing properties. They were sliced and laid on the skin to relieve itching and heal burns—a use that is occasionally revived today for the treatment of radioactive burns. The European houseleeks, Sempervivum, are said to have similar virtues.

Modest fame has recently come to the elephant's foot, Dioscorea elephantipes (21.8), and its allies by the discovery that the huge swollen caudex (basal storage organ) manufactures a precursor of

No account of the uses of succulents would be complete without mention of the peyote or peyotl Lophophora (16.34), the sacred cactus of Mexico. For over 2,000 years this has featured in ritual ceremonies of the Mexican Indians, who suck and swallow the dried tops of this dwarf, spineless cactus ("mescal buttons"). TTie resulting state of euphoria and colour visions are described in detail in a number of books, of which the best-known is Aldous Huxley's The Doors of Perception. The active ingredient is the alkaloid mescaline, but at least 14 other alkaloids have been identified in the same plant. In some countries possession of

Lophophora plants has now been made illegal because of this narcotic effect.

Other uses. In addition to food and medicine, other uses are many and varied. Columnar cacti, euphorbias and others make excellent hedges (1.8), impenetrable to anything but a bulldozer, and rampant creepers such as Carpo-hrotus (3.11) are good sand binders and have been planted to consolidate dunes in the South of England and elsewhere. They can likewise clothe fire breaks in forests. The virulent sap of some African species of Euphorbia is used as arrow and fish poison, and E. paganorum has ritual uses in West Africa, where it is planted around shrines.

Succulents in cultivation Horticulturally, succulents are more popular today than ever before. Periods of intense interest on the part of cultivators have in the past alternated with total neglect, during which time the

stage has been occupied by orchids, ferns, palms and other novelties as fashion dictated. Now, however, there are succulent plant societies (usually burdened with the tiresome tautonym "Cactus and Succulent Society of...") in America. Europe. South Africa. Australia. New Zealand and Japan. The demand for plants keeps many nurseries, large and small, in business—indeed, the drain upon habitats to supply imported plants has caused so much alarm to the conservationist that many countries now have strict legislation regarding the export and import of succulents. To the nurseryman and private grower this provides greater incentive to raise from seed, a topic I shall discuss in more detail in Chapter 7.

Right (I 7): Fruits ol Opurttia, the prickly pear ore ot the lew cacti ol economic value and raised commercially in America and S Africa

Below (t.8): Ocotillo, Fouquieria splendens, used as a hedge in Arizona Cut stems are merely planted and watered until established

Ldi.ii

of green tissue is usually in the seed leaves (cotyledons), which may number one or two. The flowering plants divide into two major groups: Monocotyledons (monocots for short) with one seed leaf, and Dicotyledons (dicots for short) with two. Even plants that have no leaves at maturity, such as cacti, have at least the rudiments of seed leaves recognizable as two bumps on the seedling.

The anatomy of roots Not all underground organs are roots, nor do all roots grow in the soil. Some stems grow underground. The surest distinction between stems and roots lies in their internal anatomy, but a more obvious

Right (2.1): Ferocactus diguetii. giant barrel cactus of Lower California. This specimen is nearly 2m (6% ft) tall Note the ribbed stem. Below (2.2): The rosette habit, as in this Echeveria. is common in plants growing in adverse habitats The leaves shade one another, and dead outer leaves protect the growing point during severe drought

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