semillas la vida en cápsulas de tiempo - Clh.es

grow in muddy swamplands regularly inundated by the tides. Apart from their characteristic stilt roots, the mostsignificant morphological adaptation to their extreme habitat is their method of reproduction. Rather thanshedding ordinary seeds, most of which would be washed away by the tides and lost, mangroves are viviparous.Vivipary (live birth) in animals means that the embryo grows inside the mother (as in most mammals) not insidean egg (as in most reptiles and birds). Viviparous plants such as mangroves produce seeds that germinate whilestill attached to the parent plant. Once the zygote has been fertilised the embryo simply continues to grow. Byextending its hypocotyl, it soon penetrates the thin seed coat and breaks through the wall of the berry. A fullygrownembryo of the most common red mangrove (Rhizophora mangle, Rhizophoraceae) may be 25cm long.Eventually, the club-shaped “seedling” drops off and either plants itself immediately in the mud underneath orfloats in the sea until the next tide sets it down elsewhere. The advantages of vivipary in the mangroves’ habitatare clear: by supporting their embryos until they have grown into sizeable, well-differentiated plantlets, mangrovesgive their offspring a head start. Ready to go, and equipped with some reserves, mangrove seedlings root veryquickly once they touch ground.Ocean travellersThe seeds and fruits of many plants growing on or in the vicinity of coastlines eventually end up in the sea,where they are carried away by ocean currents. Fruits and seeds may be shed directly on the beach or drop intotidal pools and swamps from where the tide collects them. Those originating further inland reach the sea viastreams and rivers, in many cases accidentally. However, a number of plants, especially in the tropics, possessdiaspores specifically adapted to travel in sea water for months or even years; one example, the fruit of themangrove tree Cerbera manghas, has already been mentioned. Once such seaworthy diaspores reach the mainocean surface currents, their journey can take them thousands of kilometres away from their place of origin.Charles Darwin was enthralled by the idea that seeds from tropical countries could travel to Europe. Some peoplecollect exotic drift seeds and fruits, popularly known as “sea beans”, as a hobby.Fascinating though it may be, drifting with ocean currents is – like wind dispersal – a very haphazard andwasteful strategy. Many drift fruits and seeds lose their buoyancy and are likely to end up on the bottom of theocean or somewhere with unsuitable living conditions. For example, tropical fruits and seeds from South Americaand the Caribbean are regularly carried by the Gulf Stream to the rather inhospitable beaches of northernEurope. The most frequent arrivals from the New World are members of the legume family (Fabaceae), whichis probably why people call them “sea beans”. These seeds must have appeared strange to people throughouthistory, especially in the Middle Ages. It is not surprising that so many stories, legends and superstitious beliefsare woven around them. Christopher Columbus’s voyage of discovery was allegedly inspired by the exotic seaheart(Entada gigas) and the people of Porto Santo in the Azores still call the seed fava de Colom (Columbus’sbean). Today, collectors and creators of botanical jewellery value them for their beautiful shapes and colours. Apartfrom the sea-heart, the most famous sea beans are the true sea bean (Mucuna sloanei and M. urens), the sea purse(Dioclea reflexa), grey and yellow nickernuts (Caesalpinia bonduc, C. major), and Mary’s bean (Merremiadiscoidesperma). Most are legumes, although Mary’s bean belongs to the morning glory family (Convolvulaceae).Entada gigas grows as an enormous liana in the tropical forests of Central and South America and Africa.Its seeds are one of the most commonly found drift diaspores on European beaches. With a diameter of up to5cm, the heart-shaped brown seeds are themselves large but are also borne in the largest of all legume pods, upto 1.80m long. Sea- hearts and the large seeds of the related Entada phaseoloides from Africa and Australia werecommonly carved into snuff boxes and lockets in Norway and other parts of Europe. In England, the seeds wereused as teething rings and good luck charms to protect children at sea. Grey nickernuts were worn as amuletsby the people of the Hebrides to ward off the Evil Eye. The seeds were said to turn black when the wearer wasin danger. The most intriguing sea bean has to be the Mary’s bean. Produced by a woody vine that grows in theforests of southern Mexico and Central America, the black or brown, globose to oblong seeds are 20-30mm indiameter and 15-20mm thick. Their hallmark is a cross formed by two grooves, hence their name “crucifixionbean” or “Mary’s bean”. To religious people this seed had a symbolic meaning. Having survived the ocean, it wasbelieved to give protection to anyone who owned it. In the Hebrides, for example, a woman in labour holdinga Mary’s bean was assured of an easy delivery. The seeds were handed down as precious talismans from motherto daughter for generations.Apart from its resilience to sea water, the most important pre-requisite of a diaspore to successfully travelthe oceans is buoyancy. Most tropical diaspores do not float either in fresh or sea water. It is estimated that lessthan one percent of tropical seed plants produce fruits or seeds that drift in seawater for at least a month. Thosespecifically adapted to water dispersal possess various buoyancy devices. Seeds increase their specific gravity bynot filling the entire cavity of the thick, woody seed coat (e.g. kemiri nut, Aleurites moluccana, Euphorbiaceae), byleaving an air-filled gap between their two cotyledons (e.g. Entada spp., Mucuna spp., Merremia spp., Moramegistosperma), or by producing lightweight cotyledonary tissue (e.g. legumes like Dioclea spp.). Seaworthy fruitsmay or may not combine these characteristics with air cavities in their pericarp, or a fibrous or corky coat. Withjust the latter, box fruits (Barringtonia asiatica, Lecythidaceae), common flotsam on the beaches of FrenchPolynesia, remain buoyant for at least two years.The coconut (Cocos nucifera, Arecaceae) combines a fibrous, spongy fruit coat with an air bubble in theendosperm cavity. The seed inside is protected by a thick, hard endocarp, making the fruit of the coconut a drydrupe rather than a nut. Although no one would call it a sea bean, the coconut is the classic example of an oceantraveller. Its excellent adaptation to sea dispersal has spread coconut palms throughout the tropics. The averagemaximum distance that a coconut can travel while still afloat and viable is 5,000 kilometres. When it finallybecomes stranded on a beach it will germinate slowly once rainfall has washed off the salt collected during itsjourney. Since sea-sand retains hardly any moisture, the liquid endosperm inside the coconut provides a crucialwater reserve from germination until the roots of the seedling reach fresh groundwater.The most enigmatic of drift fruits is the Seychelles nut, the fruit with the largest seed in the world. Althoughnot closely related, the Seychelles nut is similar to a coconut, hence its alternative names double coconut and coco demer. Unlike the coconut, the Seychelles nut is not adapted to ocean dispersal. It cannot float when fresh and doesnot survive prolonged contact with sea water. In the fifteenth century, long before the Seychelles were discoveredin 1743, the endocarps were washed up on the beaches of the Indian Ocean. Since most of them were found onthe Maldives the species was given the somewhat misleading Latin name Lodoicea maldivica. The true distributionof this extraordinary palm tree is limited to two islands of the Seychelles, Praslin and Curieuse. The Seychelles nutis famous not only for its size but also for the rather suggestive shape of its fruits, which gave rise to varioussuperstitions. Malay and Chinese sailors thought that the double coconut grew on a mysterious underwater treesimilar to a coconut palm. In Europe, it was thought that the highly-prized fruits had medicinal properties andthat their endosperm was an antidote to poison. Just how valuable the coco de mer was before the discovery of theSeychelles is described by Albert Smith Bickmore in his Travels in the East Indian Archipelago, published in 1869:“To early sailors in the Indian Ocean, the Seychelles nut was only known from drift seeds washed ashore throughout the region.The prince of Ceylon, who is said to have given a whole vessel laden with spice for a single specimen of the double coconut,could have satisfied his heart’s fullest desire if he had only known it was not rare on the Seychelles, north of Mauritius.”Self-dispersalRather than entrusting their seeds to wind, water or animals, some plants have developed mechanisms that enablethem to disperse their seeds themselves, at least for a short distance. Self-dispersal or autochory is either by activelycatapulting the seeds away (ballistic dispersal) or by burying the fruits in the ground (geocarpy).Ballistic dispersalMechanisms used by plants to expel their seeds from their fruits can be caused either by passive (hygroscopic)movements of dead tissues or by active movements that are due to high pressure in living cells.Passive explosivesDry, dehiscent fruits such as capsules and follicles open gradually along pre-formed lines as the pericarp dies anddries out. The shrinking of its tissues eventually causes the fruit wall to rupture. This is usually a slow, gradualprocess but in some fruits the pericarp is specifically adapted to build up a high degree of tension, which iseventually released by a sudden explosion that expels the seeds. The underlying principle of this mechanism isbased on the different orientation of elongated cells in neighbouring layers, which often consist of crossed, thickwalledfibres. As the cells dry out, they contract parallel to their longitudinal axis, causing the neighbouring layersto pull in different directions. The tension is finally released in an explosive torsion movement of the separatingfruit fragments, which usually correspond to whole or half carpels. In some legumes it is the two halves of thesingle carpel which separate along their dorsal and ventral sides. By twisting in opposite directions at lightningspeed, the carpel-halves fling out the seeds with great force. Common broom (Cytisus scoparius), gorse (Ulexeuropaeus), sweet peas (Lathyrus odoratus) and lupins (Lupinus spp.) are familiar examples displaying this behaviour.Usually the seeds are dispersed over a short distance. In gorse, they remain within the radius of the mother plantwith only about 2 per cent travelling 2-2.5m. As is often the case, the distances are more impressive in the tropics.The fruit of Tetraberlinia morelina, an African legume tree at home in the rainforests of west Gabon and southwestCameroon, aided by its great height, shoots its seeds up to 60m from the mother plant. This is the longest ballisticdispersal distance ever recorded.A plant family in which the typical fruit is an explosively dehiscent capsule is the Euphorbiaceae. Intemperate herbaceous members such as the petty spurge (Euphorbia peplus), sun spurge (Euphorbia helioscopia), dog’smercury (Mercurialis perennis) and annual mercury (Mercurialis annua) the fruit is composed of three carpels. Whenit explodes, the fruit disintegrates into six half-carpels, which churn out three seeds. The most remarkable examplein this family is native to the tropics of the New World. Not only has the gynoecium of the sandbox tree (Huracrepitans) from South America and the Caribbean many more (5-20) carpels than is usual in spurges. The ribbedfruit the size of a mandarin erupts much more violently, with the sound of a loud gunshot (Latin crepare = to burst).280 Semillas – La vida en cápsulas de tiempo