Life forms and survival

Many dwarf perennial succulents a especially adapted to pass the dormant season underground, and are hence to be seen only during the often brief rainy i periods, or when in flower. Indeed some, such as Albuca unifoliata (page 158). ai so retiring that their existence was unsus pected for decades, even in well-worked I areas. Bulbs and caudices are ideally I suited to tide plants over long periods of desiccation.HaworthiaandBulbineafford I examples of plants with contractile roots I — that is, roots whose strong fibrous core shrinks during drought and draws the crown of the plant deeper into the ground.

Small lateral roots tend to be replaced every year; they explore fresh terrain as the s becomes depleted of salts.

Flat-topped mamraillarias and neoport-erias achieve the same result by shrinkage of their turnip-like taproots; they may disappear from view completely as windblown sand and humus piles on top of them. In cultivation, such plants tend to grow much taller (16.43), although the shrinkage and retraction when dormant may still be evident. The difference shows up dramatically if the same plants are photographed during summer and winter (6.4. 5).

From rather limited samplings of root systems of succulents in the wild in America and Africa, the same general pattern emerges for cacti, euphorbias.

aloes, carallumas and others. The taproot is little developed, but the laterals radiate to considerable distances all around (3.10). A saguaro (Carnegiea) only 1.2m (4ft) high was found to have a spread of lateral roots up to 5m (17ft) from the main trunk. These laterals are never far from the soil surface: 2 to 8cm (% to 3'/4in) in Opunlia arbuscula. 1.5 to 3cm (Vs to 1 Win) in Echinocactus3. Root studies of succulents in East Africa« revealed that the fine feeding roots that grow out from the laterals sometimes turn upwards and approach still nearer to the surface, thus reversing the rule that roots all grow downwards. The roots of succulents are specialized for rapid absorption of whatever rain may fall, and they also utilize the heavy dews that condense overnight as a result of a sharp drop in temperature after dark. Profuse root development is noted near the crown of the plant —that is, in the area on which drips fall during rain. All this is in contrast with non-succulent xerophytes, whose roots generally run deep in the soil.

Succulents are usually only one component of the flora, sharing the habitat with other non-succulent xerophytes that have developed different ways of combatting drought (3.5, 7). Some are trees or shrubs, often spiny, with hard wood.

deep roots and leaves either early deciduous or small and leathery. They have developed to a high degree the art of wilting gracefully. Quick-growing annuals (drought evaders) pass the dry season as seed and burst rapidly into growth and flower as soon as the rains come. A few denizens of the semi-desert show hardly any xeromorphic features and nobody has yet satisfactorily explained their secret of survival. It has been shown, however, that certain small herbs, although not parasites in the sense of forming organic union with other plants, grow their roots close to those of cacti and imbibe water from the sheath of moisture surrounding the cactus root. When soil moisture falls below a given level, a cactus can reverse the flow and pass water down to the roots from the store in the stem.

Habitat as a guide to cultivation What can we learn from a study of succulents in the wild that will help us in their cultivation? The most obvious lesson is: don't attempt to imitate the natural environment. The much-repeated story of the lady who telephoned the weather bureau in Mexico City and watered her cacti only when told it was raining there may be apocryphal, but there are nevertheless many growers with exaggerated faith in trying to copy nature. Bedding displays of tall Cereus. Euphorbia. Fero-cactus and Gasteria may be the grower's ideal of "a slice of desert", as may the troughs of mixed Lithops neatly arrayed among matching pebbles, but these are pure works of art and have no counterpart in the wild. As sunlight passes through glass it loses much of its ultra-violet radiation (supposedly the reason why plants in glasshouses may burn during a heatwave), and the best-ventilated structure does not match the free air circulation out in the open. Yet none of this need discourage the cultivator, who can supply the basic needs of nutrition in other ways, knowing that a cactus is little different from a cabbage or a carnation in this respect. European-grown seedling cacti may never grow spines to match those of wild plants in size or colour intensity, but they can surpass them in shapeliness, freedom from pests, diseases and scars, and regularity of flowering. Some are unlikely to reach flowering size when pot-grown; others compensate by the splendour of their annual profusion. The only note of caution that should be sounded is to botanists who might be tempted to rely on garden-grown specimens in their systematic studies. Changes due to cultivation may be considerable (7.3), especially if the plants are grafted, and one must make allowance for increased size, reduction of protective devices and a less xeromorphic look.

Without attempting to imitate habitats in a glasshouse, however, the cactophile does well to know something about how his plants fare in the wild, particularly in relation to how much rain they may receive and when they receive it. Despite contrasted climates and growing seasons, most South African succulents adapt quite happily to glasshouse culture, whether in Europe or in New Zealand, and grow when they are watered and rest when kept dry. For obvious reasons one encourages them to grow in summer, when light is at its maximum, and to rest in winter, when both light and heat are limiting factors. A few —mainly dwarf Mesembryanthemaceae — resist this treatment and grow out of character or die if watered at the wrong time (6.2). These test the skill of the specialist grower, but mostly respond to a correct watering routine. They are certainly worth the extra effort, because they include some of the most bizarre succulents.

Mean annual rainfall may be linked to ease of cultivation, too. It has long been recognized that certain globular cacti of North America are notoriously difficult to transplant, rarely surviving long as imports in Europe, and seed raising is equally difficult. Pediocaclus (16.39) includes several examples. Recently it has been shown5 that the "difficult" cacti all come from areas that have less than 25cm (lOin) average annual precipitation—that is, they are true desert cacti. The readily adaptable species are from regions of higher rainfall, although the intervals between rain may be long and erratic. Whether the same rule holds good with African succulents I do not know, although I suspect that it does, recalling the problems we have with Welwitschia. Zygophyllum. some stape-liads and dwarf Mesembryanthemaceae from South West Africa. But there are crassulas from the same area that acclimatize readily, visibly changing their form and structure in the process. Indeed, there can hardly be any one genus in all flowering plants with greater adaptability than Crassula, ranging from the limits of desert colonization to marsh and aquatic species, all within a single genus. Comparison of a "wild" and cultivated plant demonstrates how the adaptation takes place; internodes elongate, and leaves originally packed into a sphere draw apart and expose greater surface to the air. The resulting plants are atypical, and would draw a frown from a botanist and show judge, but they give an interesting insight into the close links between plant :s and the e

Right (3.I1): Carpobrot Mesembryanthemaceat is widely planted to consolidate sand dunes, some species thriving in temperate climates

Below (3.12): A coastal scene at Bu/iels Bay in the Cape Nature Reserve. South Africa. The succulent carpeting the ground is a species of Carpobrotus (Mesembryanthemaceae).

"The most important and appropriate subject of inquiry which arises in the science of botany is that proposed by

In plants', or whether there is a combination of the two sexes "

The terms "male and "female as used inclassical writingsabout plants bear little resemblance to present-day usage, for they were applied only by vague analogy with the animal kingdom. A clear understanding of the true function of stamens as the male organs and bearers of the pollen, and of egg cells (ovules) as the female organs, had to await discovery until the late seventeenth century, and further progress was possible only after the invention of the compound microscope.

Unlike animals, plants usually have both sexes in a single individual, which is referred to as hermaphrodite. Flowers are the means whereby a species perpetuates itself through sexual reproduction. Seed set involves the transfer of pollen from anthers, usually recognizable by the golden dust covering them, to the stigmas, the receptive tips of the styles down which a pollen tube must grow to fertilize the ovules in the carpel below. Following fertilization, each ovule ripens into a seed, and the ovary becomes the dry or fleshy fruit. Other parts of the flower, having fulfilled their attractive function, normally wither and fall off. The seed must then be dispersed away from the parent plant to take its chance of landing in a locale suitable for growth. We shall discuss these functions of flowers — pollination and dispersal —in turn.

Self- and cross-pollination At first sight it seems a needless complication to attract insects or other visitors to the flower as unconscious carriers (vectors) of the pollen. Why not self-pollination? As many gardeners can testify, "selfing" is quite possible in many succulents when only one plant is in bloom. Some even set full pods without any outside agency at all. The first to supply an answer to this question was

Charles Darwin, who found by experiment that when primroses were forced to self-pollinate, they produced a lower set of seed and weaker progeny than when pollen came from another individual of the same species. "Inbreeding depression" is the term that plant breeders have for the ill effects of continued self-pollination. Very few succulents self-pollinate regularly in the wild. Annuals such as Portu-laca and Dorotheanthus do so to some extent, but not exclusively; they are attractive to insects and a measure of outcrossing (pollination from other individuals of the same species) takes place. Selfing tends to produce a stereotyped, true-breeding population that loses the ability to vary and hence to evolve. As a short-term means of increasing the population, it may have advantages, especially for annuals, but in the long run it is an evolutionary dead end.

Two extreme instances of self-pollination in succulenis are encountered in Anacampseros and Frailea. The Ana-campseros flower—the most ephemeral of all succulents—opens for only an hour or so, and sets an abundance of seed in

Above (4.1): In this Opuntia flower the anthers 1 have shed copious pollen, but the stigmas are still tightly closed and unreceptive Later they I will spread out and become sticky.

Right (4.2): Anthophorid bee on a tlower ot Opuntia. Most of the pollen has gone but note the yellow grains adhering to the green stigmas and to the body of the bee isolation without any insect visitors (12.5). In Frailea, we find a further stage: fruits gravid with fertile seeds ripen without any sign of an open flower at all. We call such behaviour cleistogamy. and there are many isolated examples among flowering plants. Frailea seems to get the best of two worlds: the cleistogamous fruits assure mass-production of individuals, whereas occasional fully expanded flowers set no seed with their own pollen and are adapted to outcrossing.

At the opposite extreme are the dioecious succulents, such as Euphorbia obesa (20.10), where two plants of opposite sex are needed in order to obtain seed. Those with yellow anthers are male, and those with greenish three-lobed stigmas

Below (4.3): A flower of Monanthes muralis 8mm Chin) in diameter, showing the large yellow, spoon-shaped nectaries between the linear petals and carpels. Although tiny, the flowers are clustered and the nectar drops sparkle in the sun

Above (4.4): A white-belliedsunbirdsipping nectar Irom the hanging flower of Aloe chabaudii in the Transvaal, South Africa Bird flowers are commonly red or yellow, robusL and secrete copious nectar.

are female. Dioscorea, the 'Elephant's Foot' (21.8), is also dioecious.

The sex-change flowers Many bisexual flowers are constructed and function in such a way as to discourage self-pollination. A fairly common arrangement is that shown by many cacti. When the flower first opens it is functionally male (4.1), the stamens brimming with yellow pollen but the stigma lobes held high and dry above them. Next, when most of the pollen has been shed, the stigmas open star-like and become receptive, making the flower effectively female. The stigmas provide an obvious landing place for insects attracted by the scent, colour or general form of the flower, and these will inevitably deposit on the stigma any pollen they carry. Finally, as the flower wilts, differential growth or shrinkage draws 1 the stigmas level with the brush of i stamens, where they can pick up any I residual pollen. When two kinds of pollen meet on the same stigma, the "selfed" pollen usually grows a pollen tube more slowly than the crossed and is hence less i likely to effect fertilization. Many plants have independently evolved this neat and efficient mechanism, which encourages ] outcrossing but, as a last resort, ensures j seed set if the flower is unvisited. The interested observer can learn much from a study of the blooms that open in his I collection, especially if he looks at them [I daily with a pocket magnifier.

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