Dispersal

The second function of flowers, following cross-pollination and fertilization, is the dissemination of seed. Efficient dispersal requires that daughter plants grow up sufficiently far from the parent to avoid direct competition, but not so faraway as to lessen chances of cross-pollination. Long-distance dispersal provides the means of founding new populations and increasing the range of a species.

Dispersal mechanisms have been classified under five groupings6: wind, water, mechanical means, animals and man. As we examine these insofaras they relate to succulents, it will be convenient also to mention vegetative means of dispersal as well as seeds.

Wind. The globular, easily detached offsets of the houseleek (Sempervivum 10.19) provide us with an example of what is called a tumble-weed, detaching and being caught up in the breeze to roll

around and root where and when conditions are favourable. This is a simple means of establishing daughter colonies, and in flat country surprisingly large distances can be covered.

Many succulents bear capsules that dry and split open at maturity, allowing the seeds to fall out or catch in the breeze (Aloineae; Agave 2.20). Such seeds are often winged, shaped like a sail or covered in sail-like outgrowths. A refinement is the development of something akin to a pepperpot. In Anacampseros a basketlike structure remains after shedding the floral parts, and by swaying in the breeze it releases a few seeds at a time (4.17). Pterocactus and Dioscorea have seeds with conspicuous wings, and the large winged seeds of Welwitschia (not strictly a succulent, but a unique desert inhabitant) are about 3cm (l!4in) across. Despite the bulk of these seeds, I have seen clouds of them blowing around in the strong onshore winds that sweep their level habitat in South West Africa.

Plumed seeds or fruits are common in succulents and highly efficient for transport, being found in some of the most widespread genera: Senecio, whose parachutes float over all five continents: the Stapelieae, which extend from India to the Cape of Good Hope; Ceropegia, which

Above (4. t5): Seed dispersal in Pelargonium carnosum. The style is just splitting, each awn carrying one seed with its own parachute

BelowC4.t6): Thethreelobedcapsulesot euphorbias (this is E. virosa) fly apart with an seeds a distance ot some metres.

has reached the Canary Islands from Africa; Pachypodium (18.3-5); and Adeni-um (18.1,2) whose seeds have two parachutes. one at each end. and whose range includes East Africa and Socotra. A special type of plumed dispersal organ is produced by Pelargonium (4.15). The long style in the centre of its bloom splits into five springy strands (atuns,) at maturity, and each curls up like a spring, carrying part of the ovary with a single seed. The plume of hairs assists in wind transport. On falling to the ground, the spiral awn responds to changes in humidity, curling this way and that, and effectively screws the seed into the ground.

Equally efficient for long-distance dispersal are tiny dust-like seeds, as shown by the wide distribution of the Crassu-laceae. Although richest in genera and species in South Africa, this Family occurs in all five continents, and Crassula has reached isolated islands as well: the Azores, the Prince Edwards, and Ker-guelen. Its tiny seeds are also carried on the fleeces of sheep, and plants turn up in dock areas where wool is imported.

Below (4.17]: Stages in the developing Iruit ol Anacampseros. from wilted bloom (left) to a basket of fibres (right], from which seeds are intermittently blown by the wind.

Water. The classic example of seed dispersal by water is in the Mesembryan-themaceae. A few genera have capsules that open when dry, and Carpobrotus has a fleshy berry, but the remainder have capsules with 4, 5 or more valves that open star-like in response to water and usually close again as the fruit dries out, repeating the process several times. The open capsule has an almost flower-likeappearanee, givinga novel look to the plants in a collection after they have been watered. At the base of each valve is special hygroscopic tissue, which expands when wetted and causes the valve to open. The biological significance of this is plain: the seed is released at the one time when there is water enough for germination, and the seedlings become established before the next drought. It has been observed that the open capsules are what we call "splash cups"—that is, they are so designed that raindrops striking the conical centre flush out the seeds from the chambers below. In this way seeds may be shot 1.5m (5ft) or more from the parent plant.

Conicosia capsules open once only on wetting and do not close again. The loose seedsare then shaken out overa period of time as from a pepperpot. Subsequently the light, buoyant capsule breaks off and rolls along the ground, scattering further seeds over greater distances. Finally it decomposes into segments, each composed of a wing-like membrane that divided the cell chambers. In this are two tiny pouches, each trapping a single seed. These seeds have a long viability (I have had good germination after five years) and so they ensure perpetuation in time as well as space. Three different dispersal mechanisms from one fruit must constitute something of a record; indeed, the fruits of Mesembryanthemaceae are among the most complicated structurally of any plant.

Mechanical means of dispersal. Under this heading we can include "expansive dispersal" in which plants send out long runners or stolons (Crassula sarmentosa) or creep and root along the ground (Sedum lineare, Ceropegia woodii), so that a single individual spreads and, after its death, survives as daughter colonies. Some kalanchoes (such as K. suarezensis) dip their long leaves to the soil and produce new plantlets from the tip; others, the familiar 'Mother of Thousands', K. daigremontiana (6.23) and K. tubifloru. develop adventitious buds along the leaf margins, and these drop off and generate new plants all around the parent — rather

too readily for comfort in a crowded glasshouse. Some agaves achieve a similar local spread by dropping bulbils from the inflorescence. The inbuilt water reserves of succulents make them especially well adapted for mass propagation by detached portions of the plant body, even by single leaves.

Turning to seeds, we find explosive mechanisms well-developed in Euphorbia and Dorslenia. When the three-lobed capsule of a Euphorbia has ripened, it bursts into three part-fruits, each containing a single seed. The bizarre flat green receptacles of Dorstenia are covered in many minute unisexual flowers and edged with tentacle-like bracts (4.19). Each female flower ripens a single seed, which is shot out some distance from the receptacle by differential contraction, much as the soap slips from one's hand in the bath. In the process, the cup-like hollow housing the fruit turns inside out, so the bumps perceived on the surface of the receptacle are not seeds, but evidence that the seed has already gone. Collecting seed from these plants is a problem. The best way is to isolate the specimen before its fruits are ripe and to screen it with a glass shade or fine net—a lady's stocking

Dispersal by animals. Just as many flowering plants rely upon animals for transfer of the pollen, so do many depend on animals for transport of their seeds, either externally or internally. Fleshy berries are the predominant fruit type in cacti, ranging from the tiny mistletoe-like berries of Rhipsalis (4.18) to the massive, highly coloured fruits of Hylocereus. as big as oranges. Sometimes the fruit becomes even more conspicuous by splitting and offering a souffle of seeds mixed with white or coloured pulp. Opuntia produces the edible prickly pear (1.7), attractive to many wild creatures as well as to man. Once prickly pear had been introduced to South Africa, baboons and lemurs ate the fruits and rapidly spread it. In Arizona over 50 species of birds have been recorded as eating the fruits of the giant saguaro (Carnegiea). The seeds of all these plants are small and thick-skinned. so that they pass undamaged through the digestive system and end up germinating on a ready-made compost heap. Some seeds, indeed, do not germinate readily fresh from the fruit, and if they do not pass through an animal's digestive system they need chipping or chemical treatment with acid to assist.

Cissus (15.5) and Cyphostemma (2.21), relatives of the grape vine, have large pips, and these are probably not swallowed but rejected in situ, or stick to the bill of a bird and are wiped off some distance from the plant that bore them.

The minute fruits of Peperomia. borne on long green spikes (9.2) are sticky and probably transported on the wings of birds. This would account for the wide dispersal of this large genus, centred in South America but reaching many islands of the tropics.

Dispersal by man. Man, by far the most disruptive of all influences on the natural distribution of plants, spreads them throughout the globe in a number of ways, intentionally and unintentionally. Seeds travel from one place to another attached to clothes and cartwheels, trains and aeroplanes. Others become mixed in with seed crops, drugs, bird seed, fodder, vegetable packing and fibre, wool, ballast and road-building material. Human activity has altered the range of some species for so long that it is no longer possible to be certain where they originated. But by carrying plants to new environments we at least ensure their survival if the original habitat is destroyed.

Right(4.18): Rhipsalis baccifera. here growing in Ghana as a tropical forest epiphyte is the only cactus found apparently wild outside the New World. Tiny flowers give way

5: GENETICS AND EVOLUTION

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