A little water goes a long way

Succulence by itself would be of little use to plants facing drought conditions without specialadaptations for retaining such water as is collected. Many of the peculiarities of form and structure in succulents can be interpreted in this light. The following list includes a selection of these xeromorphic modifications.

Surface reduction. The surface of a succulent may be reduced to as little as Vjog of that of a comparable mesophyte. Leaves are the first to go. or at best are simple in outline and only very rarely pinnate or lobed. When leaves are present they are commonly overlapped in rosettes or like rows of roof tiles, whereby only part of the surface is exposed to drying winds. Often leaves develop only in response to watering (Senecio articulatus

Below (2.8]: Similar function from different organs: the five on the right are all leaves, the five on the left are modified stems (phylloclades)

15.2, Euphorbia milii 20.6) and drop as soon as drought ensues.

Twin-purpose stems. Where foliage is reduced, the stems are green and take over the role of leaves in photosynthesis.

Breath control. Water escapes from plants as vapour through the breathing pores (stomata), or directly through the walls of the surface cells (epidermis). The former is checked by the unique cycle of many succulents, in which, as already explained, the stomata open only at night, when evaporation is less. The number of stomata is also reduced, and they are commonly localized in shaded areas (the leaf undersides, the angles between ribs) or are overlaid with hairs, spines or other surface appendages. When stomata are sunken in pits (2.11), a column of still air retained above the pore has a similar effect. Transpiration through the epidermis is also kept down by a thick waterproof surface layer (cuticle) or the development of a whitish wax surface that also reflects some of the light ( 10.5).

Super-efficient roots. Many succulents have wide-spreading roots that never go far below the soil surface (3.10). These are especially suited to the rapid absorption of waterafter rainfall, and of moisture from the heavy night dews.

Telescoping of organs Many special features of succulents are explicable as a superimposing of organs that in a mesophyte are spread out on elongated growths. The cactus areole, a characteristic organ of the Family, is a contracted lateral branch, rather like the spur shoot

(2.9): Succulents show two main lines ol evolution from their hypothetical mesophyte ancestor (centre). To the right are types showing water storage in leaves:to the lelt a transfer of storage and photosynthetic activity to stems, with reduction or complete loss of foliage The end stage of both trends is a sphere having minimal surface to bulk

Leaf succulents

of an apple or cherry, with many growing points coincident at one level and leaves reduced to spines. The "fruit" of a prickly pear (1.7) is similarly an ovary sunken within a modified shoot. In Conophytum the ovary remains protected within a pair of leaves, and in Calymmanthium we have the most extraordinary development of all: the whole flower is engulfed by the stem and has to break its way out (2.10). Foreshortening of the main axis may lead to the leaves originating direct from the crown as in Lithops (2.12) or even to the entire plant being reduced to a flat flower head as in Crassula pageae.

Flower economy. The opening of a flower in a desert is like turning on the tap: precious water is lost unless economy is practised. Large flowers, such as those of Echinopsis (16.1), usually open at night and most have already wilted by the following morning. Small flowers are less of a drain on the plant's resources, but even for these the period of opening may be very limited (Anacampseros 12.5). A type of xeromorphy, where the petals are leathery and fleshy, is found in the Stapelieae (Chapter 19) and enables these blooms to remain open for several days without wilting.

Not all succulents possess all the above features, just as there are non-succulent xerophytes that have many such features but without the succulence. Annuals survive the dry season as seeds; hard-wooded desert shrubs such as the mes-quite (Prosopis) and the creosote bush (Larrea) root deeply. There are indeed many strategies for combatting water shortage; what might be called "the

Below (2 10): llower o/Calymmanthium substerile. which demónstrales an extreme example ol xeromorphy. Notice how the petals have to tear their way out ota solid green bud

succulent syndrome" is only one of them.

Succulent armature Prickles, thorns and spines are conspicuous features of many xerophytes (2.13).

Indeed, the spines of a cactus may be the first contact one makes with it, and the experience is often memorable. Each of the three terms has a special botanical meaning, and it is better to use a neutral word such as "arms" or "armature" in referring to them collectively.

Prickles are non-woody outgrowths of the surface that are not in direct contact with the conducting tissues (vascular system) of the plant. Familiar examples are the prickles of a rose or blackberry, which snap off cleanly when pushed. Examples among succulents are the leaf margins on some agaves (2.16) and aloes, and the randomly scattered prickles of Euphorbia milii (20.6).

Thorns are modified branches, woody and with their own vascular system. As evidence of their stem origin, traces of leaves and lateral buds can often be seen on them. Hawthorn and gorse provide well-known examples of thorns. In succulents. we find a great diversity of thorns in Euphorbia, some of which are obviously persistent inflorescences (20.4),

Be/ow(2.13): Types ol armature Leftto right prickles in Euphorbia milii. leal prickles in agave, thorns in Euphorbia schoenlandii, spines in a cactus and in Pelargonium spinosum. The presence ol some form ol armature is characteristic ol many xerophytes.

others more advanced and distinct from the flowering shoots (20.12).

Spines are modified leaves, where the blade is undeveloped and the leaf stalk or stipules are stiff, woody and pointed (2.14). One special type of spine is the glochid, found in Opuntia. This is small, readily detached and barbed like a bee sting. Although no succulents have poison stings, these giochids are most irritant and discomforting, and the experienced cactophile always has forceps handy to remove them as soon as their presence is felt. In addition to the Cactaceae, the Didiereaceae and Sarcocaulon have spines, and there is a special sort (in Fouquieria and Pelargonium spinosum 2.13) where a leaf blade develops but soon drops, leaving its stalk to harden as

The above terms can be used precisely only if the origin of the armature is known, and that is not always the case. There isalso much diversity of opinion on why so many plants of dry places are heavily armed. First it must be stressed that an organ need not have a function: so long as it does not impede survival of the organism it may remain, just as does the human appendix. Thus one school of thought regards the armature as a mere relic—all that persists after streamlining the surfaceareas. Cactus spines are often rich in crystalline deposits (calcium oxalate, silica), and it has also been suggested that the armature might serve as a garbage dump for the waste products of the plant's metabolism.

Others see the armature as protective. Fat, juicy succulents would soon be eliminated in habitat by grazing animals if the plants had no means of defense. It is true that in times of drought some hoofed animals can kick open barrel cacti and devour the contents, and opuntias may also be grazed, but the giochids cause terrible inflammation of the mouth and stomach lining. Support for the view that armature is a defense mechanism comes from the revelation that among the many unarmed succulents, other deterrents are present: a milky irritant latex in Euphorbia and alkaloids in Lophophora. Ario-carpus, Echinopsis pachanoi. Seleni-cereus and the Stapelieae.

Hooked spines occur only in the cacti, and can be a means of propagation in smaller mammillarias where offsets detach readily and can become entangled on fur or feathers. Cylindrical-stemmed opuntias achieve the same result by shedding their spiny terminal joints: the dreaded jumping cholla (16.6) has the reputation of seeming to jump at you as you walk near it.

A good dense mantle of spines or bristles covering the stem of a cactus

Above (2 141: many while, spreading radial spines surrounding two brown, inclined, sic ~ ------ -ie whole arising Irom each

Above (2.15): A central spine o'Mammillaria seen under the scanning electron microscope, magnified200 limes Forms and patterns differ from species io species

Above (2 141: many while, spreading radial spines surrounding two brown, inclined, sic ~ ------ -ie whole arising Irom each

Above (2.15): A central spine o'Mammillaria seen under the scanning electron microscope, magnified200 limes Forms and patterns differ from species io species

maintains still air around the stomata and lessens evaporation. If the spines are white, some of the incident light will be reflected also.

Physiologists have suggested that the sharply pointed organs on xerophytes may act as nuclei for the condensation of night dews. Water droplets condensed on cactus spines run down to the felted areole, which soaks them up like a sponge. Whether or not actual reabsorp-tion directly into the stem takes place is, I think, unresolved as yet, but isolated experiments using markers added to the water suggest that it does. Otherwise, the water will run down to the soil at the stem base, where plentiful surface roots take it in. Reabsorption of water through the surface has been demonstrated in leaves of Crassula.

So far as the average cactophile is concerned, the function of the armature has never been in doubt. It is the crowning glory of any cactus, put there for his special delight as a mark of distinction from lesser vegetation. The seemingly endless variety in shape and form, texture and colour, provide him with the stimulus to collect. There are straight spines and hooked, plain and multicoloured, bearded and plumed, often several types at one areole, and most of them going through a cycle of colours as they age. In ferocacti the colours itensify dramatically after the spines are wetted. Bearing in mind the fact that cacti flower for only a few days, or at most weeks, every year, their popularity depends to a large extent on the beauty and bounty of the armature.

Flowers, fruits and seeds The way in which flowers function is the subject of Chapter 4; here we are concerned with establishing their structure and the names of the various parts (2.18). For the full story, there is no scarcity of

Left (2.16): Leaf prickles in agave The sloul marginal and terminal prickles make these plants formidable lo handle, and well protected against grazing animals.

separately in the centre of thi united when (used together The one or more carpels. Ire popular primers on botany; my present aim is merely to define those terms most commonly used by the botanist in describing succulents, and essential to an understanding of the systematic accounts of them in Part 2.

Unlike the vegetative features, which contribute a highly individual "desert facies" to succulent plants, the reproductive organs have changed little during the advance into dry habitats and show few features that we can point out as typical of succulents. Each retains the characters of its Family, as set out in Chapters 9-21.1 should like to stress one overall truth about succulents; they all flower, regardless of what the prophets of doom would have us believe after having tried to grow the wrong sorts under the wrong conditions. If they did not bloom and set seed, they would have disappeared from this earth long ago.

Botanists interpret a flower as a transformation of a shoot in which the calyx (sepals), corolla (petals), andro-ecium (stamens), and gynoecium (carpels) are derived from modified leaves. The order of parts is always constant, with the female organs at the centre sur rounded by males and then the protective outer whorls of petals and sepals that make up the perianth. The whole arises from the more or less flattened tip of the axis, which is known as the receptacle. Pollen from the anthers is required to reach the surface of a stigma, where a pollen tube grows down into the ovary and the pollen nucleus enters an ovule and effects fertilization (2.17).

Following pollination and fertilization, the sepals, petals, stamens and styles wither and usually drop off, leaving the ovary to enlarge and form a fruit— a dry capsule (2.20), or a juicy berry (2.21), or rarely something half way between, as in several cactus genera. The tiny ovules ripen into seeds, which may be minute and dust-like (Dinteranthus. Crassu-laceae) or larger than peas (Jatropha). Cactus fruits are peculiar in often containing areoles on the outside, and appear to have originated from an ovary sunk in a modified shoot. In opuntia and some pereskias the unripe fruits proliferate to grow further flower buds and eventually even whole chains of fruits, one suspended from the other. In certain opuntias such a chain eventually drops off and roots to form a daughter plant beneath the parent.

Flower colours in succulents cover most of the spectrum except blue, which is curiously absent from all except Sedum coeruleum, a handful of semi-succulent Labiatae, and Cyanotis. The dazzling reds, magentas and pinks of those Families included in Caryophyllales (see page 130) are due to betacyanin (2.5), a nitrogenous pigment that differs chemically from the anthocyanin found in all other plant Orders.

Some succulents flower in a few weeks from seed: the annual portulacas (5.3, 12.1) and Dorbtheanthus (11.10). for example. Many popular dwarf cacti (Mammillaria) and Mesembryanthe-maceae bloom in the second year from seed or even sooner. Some take many years to attain maturity; the tree cacti. Didiereaceae and Agave (14.1), for example. The name "century plants" for the last is an exaggeration: seven to 70 years is more typical of the lifespan. Then the rosette rapidly puts up a huge inflorescence, after which it dies, to be replaced by seedlings, offsets, or viviparous buds falling from the old

inflorescence. In Melocactus the onset of flowering after perhaps ten years of vegetative growth is the development of a cephalium, or crown of wool and bristles (16.28). after which the plant is fully mature and ceases to grow vegetatively. The name "Turk's Cap' is descriptive of this large central cephalium. Other cacti produce cephalia laterally (Espostoa) or lopsidedly (Cephalocereus).

Most flowers are more complicated than the simple Crassula (2.19). with its ring of free parts in fives. But it is always possible to relate them to the same basic plan through transformations such as the fusion or splitting of parts, or alterations in number or function. In place of radial symmetry like the spokes of a wheel (actinomorphy) a flower may be symmetrical about one vertical plane only (zygomorphic 9.1).

A special subject by itself is the form of pollen grains, seeds, spines and other plant surfaces as revealed under the microscope, especially the recently developed scanning electron microscope (SEM), which enables us to obtain superbly three-dimensional pictures at a wide range of magnifications (2.15). Every species is found to have its own peculiar patterning and shapes, which, like finger-prints, can be a valuable means of identification. A whole new field of study has opened up, and taxonomists are increasingly using characters of pollen and seed in their classifications.

Above (2201 Immature trulls ol Agave. Eventually each will ripen to a dry, brown capsule which sheds its Hal winged, black seeds through three vertical splits

Right (2.21): Cyphostemma juttae. a relative ol grape like Injits The plant grows to a height ol about 75cm (30in).

Fleshy fruits

Dry fruits

Fleshy fruits

The berry of Cissus (15.5) and Cyphostemma (2.21) is grape-like and derived from a simple, superior ovary In Cucurbitaceae, the berry is a miniature gourd derived from an inferior ovary, hence including tissue of the supporting

In Cactaceae Ihe same is true- even vegetative areoles may be present on the berry (1.7).

Dry fruits

A follicle is a fruit derived Irom a single carpel that splits longitudinally, as in Crassulaceae, Asclepiadaceae (19.8) and Apocynaceae.

carpels united in a ring In Liliaceae and Agavaceae it dehisces by vertical splits: in portulacas by a circular lid. A very special type of capsule with valves actuated by water is described in Chapter 11 for Mesembryanthemaceae A schizocarp is a capsule that breaks up at maturitv into part-fruits (mericarps) - three in euphorbias (4.16).

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